In this article we will discuss about:- 1. Anatomy and Physiology of Sheep 2. Reproduction of Sheep 3. Birth and Lactation 4. Growth and Development 5. Nutrition 6. Development Programmes.
Contents:
- Anatomy and Physiology of Sheep
- Reproduction of Sheep
- Birth and Lactation of Sheep
- Growth and Development of Sheep
- Nutrition Required for Sheep
- Sheep Development Programmes
1. Anatomy and Physiology of Sheep:
The bodies of animals are composed of millions of small units called cells. Cells which are similar in shape and the work they do are grouped together into tissues, like nerve tissue, connective tissue, bone tissue and muscle tissue. Organs consist of cells of different tissues grouped together to make a body unit which performs a certain function. The stomach, the eye and the kidneys are body organs.
The sheep belongs to the family of grass-eating mammals called the Bovidae, and, like the cow, chews its cud and walks on paired hooves.
The sheep’s body is a good one to study because it is small, is very similar to the cow’s body, and in many ways resembles the bodies of other farm animals. In the sheep the head is joined to the body by the neck, while the front and back limbs are joined to the body at the shoulder and the pelvis.
The soft parts of the body are supported by the bony skeleton. All animals must have adequate supplies of calcium in their diet because bone is formed from calcium carbonate and calcium phosphate. A typical long bone consists of a “head” or joint surface at each end, and a shaft in between.
The ends of such a bone are made of thousands of interlacing bony threads called spongy bone, but the shaft is made of very dense or compact bone. Blood vessels and nerves enter the marrow inside a bone through many small holes called foramina. The outside of a bone has attached to it a close-fitting layer of bone-forming cells called the periosteum.
The skull consists of a bony box—the cranium—which contains the brain. The face and jaw bones are attached to the cranium. In sheep and cattle the lower jaw contains incisor or biting teeth in the front, and molars or grinding teeth further back.
The upper jaw has only a bony pad instead of incisors and the sheep tears off grass by gripping it between its lower incisors and this pad. The upper lid is divided, allowing the teeth to graze very closely.
The bones of the spinal column run down through the body, there being seven cervical, thirteen dorsal and six lumbar vertebrae. The four vertebrae of the sacrum are fused together, and the tail bones number sixteen to eighteen. The thirteen dorsal vertebrae have ribs attached to them. All the ribs are joined to the sternum or breast bone except the last, which is a floating rib.
The front leg is joined to the body by the scapula, or shoulder blade, at the lower end of which is the humerus or bone of the upper leg. The bones of the foreleg are the radius and ulna, usually joined together, and below them are the six carpal or knee bones. The shin is formed from the two fused metacarpal bones or cannons, but the smaller cannon is often reduced to a ridge on the larger one.
Beneath the shin are the two digits on which the animal walks. Each digit is formed from two small pastern bones and one pedal bone which fits inside the half hoof. There are also three small bones called the sesamoids at the back of the pasterns.
The pelvis is a complete girdle formed from three bones on each side. The hip bones project to the side, while the two pin bones point backwards. The two pubic bones join together to make the floor of this girdle. In females the young animal passes through the girdle as it is born.
The hind limb consists of the femur or thigh bone, then the tibia, the lower end of which meets the six tarsal bones forming the hock. Below the hock are the two fused metatarsals or shin bones, while the bones of the two digits are similar to those of the front leg.
The heart of a sheep or cow is found low down in the part of the chest which lies between the front legs. When an animal is ill, a veterinary surgeon puts his stethoscope onto this part of the chest to listen to the beats of the heart. The hearts of sheep and goats beat a little faster (75) than human hearts (70), but the heart rate in cattle is slower (53) and that of the horse very much slower (40).
The chief artery of the body is the aorta which leaves the heart and passes up and backwards close to the backbone. The aorta branches off into smaller arteries which carry blood to all parts of the body. Veins collect the blood from every part of the body and return it to the heart. The blood has a number of important functions.
They are:
1. The red blood cells take oxygen to all the cells of the body;
2. The blood conveys nutrients from the intestine to all the body cells;
3. Carbon dioxide from the body cells is carried to the lungs;
4. Waste substances containing nitrogen are taken from the body cells to the kidneys;
5. The blood may help to cool the body by taking some of the “core heat” to the outside of the body under the skin;
6. The blood is used to transfer nutrients from one part of the body where they may be stored to another part where they are used;
7. The blood distributes hormones or “chemical messengers” which are put into the blood by the ductless glands;
8. The blood carries antibody substances and white blood cells which may combat bacteria and other foreign substances which invade the body.
This system consists of millions of small tubes which are found in every part of the body. Their function is to carry away bacteria or poisons which may have entered the body through wounds. The lymphatic vessels are like a great drainage system, one function being to take unwanted materials to the lymph glands where they are destroyed. You will often notice lymph glands as small oval grey lumps in slices of lamb.
A butcher can tell if an animal is diseased because he notices the swollen lymph glands. Meat inspectors are very careful to look at the lymph glands of carcases for signs of the bacteria causing tuberculosis.
Air enters the sheep through the nostrils and passes through the voice box or larynx into the trachea. The trachea divides into two bronchi which take air to the lungs. In most four-footed animals the lungs lie high up in the chest pressed against the ribs and backbone. The function of the lungs is to absorb oxygen gas from the air, and to pass out into the air carbon dioxide gas, which is a waste product.
In animals which do not sweat through the skin, the lungs also serve to rid the body of excess heat. Thus sheep will be seen to breathe very rapidly in hot weather. Sheep normally take about twenty-five breaths a minute when resting, cattle about fourteen and humans about fifteen.
As a sheep takes a bite of grass, it chews briefly with the molar teeth at the back of the jaw, at the same time mixing the grass with copious amounts of saliva. The sheep swallows the food which then passes down the oesophagus or gullet. The sheep and cow have stomachs which are quite unlike those of the pig and hen, for they are divided into four separate compartments.
The food first enters the paunch, or rumen, or first stomach. The rumen is so large that it occupies almost the whole of the left side of the abdomen. While the sheep is grazing the rumen stores food, and this is acted on by millions of bacteria and protozoa which are called the flora and fauna of the rumen.
After grazing for a while the sheep may lie down and chew the cud. The food returns to the mouth in small lumps and is chewed thoroughly a second time. When swallowed it returns to the rumen, where the finer well-chewed material settles to the bottom of the rumen. Strong contractions of the rumen cause some of the contents to overflow into the second stomach called the honeycomb or reticulum.
The second stomach then passes the food into the omasum or third stomach. The walls of this stomach have many thin leaves of tissue attached to them, and for this reason the third stomach is also known as the “bible”. The “leaves” of this stomach extract 60 per cent of the water from the food, which is then passed on to the fourth stomach or abomasum. In this organ gastric juice is added to the food at a rate which makes up for the removal of water by the omasum.
The gastric juice contains hydrochloric acid which immediately kills the rumen organisms and begins to disintegrate them. The food which these organisms contain will later be digested and absorbed by the sheep. Food leaves the abomasum via the pyloric valve and enters the duodenum.
Bile from the liver and digestive juices from the pancreas are added to the food while it flows through the duodenum. Food then passes on to the “runners”, or small intestines, which are 24 metres long. It is here that the digested foods are absorbed into the bloodstream of the sheep.
The caecum or blind gut is a tube 25 cm long which holds the food while fermentation goes on. As the undigested part of the food enters the colon, water is absorbed from it. By the time the undigested food reaches the rectum it has been turned into the semi-solid faeces.
Figure 36.2 shows the shape and position of the sheep’s digestive tract as it would appear looking down on the sheep from above. Notice the great size of the rumen, which takes up all the left side. When cows have bloat, it is always the left side of the abdomen which is blown out, because that is where the rumen is found. The arrow shows the place where the trocar tube is pushed into the rumen to release the gases causing bloat.
To understand why digestion in the ruminant animals is so different from digestion in one-stomach or monogastric animals, we must try to understand what happens in the rumen. The walls of the rumen do not produce any digestive juice. Instead the rumen is like a large fermentation chamber in which millions of bacteria and protozoa are able to live and multiply. It is these millions of rumen microbes which produce digestive enzymes which attack the food in the rumen.
The rumen microbes have the following effects:
1. They are able to digest the cellulose skeleton material of plants. This cellulose is turned into simple substances, such as acetic acid, most of which are immediately absorbed through the rumen into the sheep’s blood. This is the reason why ruminant animals like the sheep and cow can make some use of dry plant material like straw which is useless to animals like the pig and the fowl.
2. They are able to absorb certain materials from the sheep’s diet and use these to make the B vitamins which may have been absent from the diet. This building of molecules by microbes is called biosynthesis. When the rumen microbes are later digested in the sheep’s intestine the B vitamins which they contained will be available for the benefit of the sheep.
3. They are able to improve the quality of the protein in the diet. The rumen microbes digest some of the proteins in the diet breaking the protein molecules into pieces called amino acids. The microbe cells then absorb these amino acids in order to rebuild them into their own proteins.
But the microbes also manufacture some other amino acids of a kind which were not in the diet proteins. The microbes’ protein therefore contains a richer variety of amino acids than was to be found in the protein of the diet.
We can see that the ruminant really has two digestive systems—the digestion by microbe enzymes in the rumen and digestion by the sheep’s enzymes in the small intestine. Of course when the rumen microbes reach the small intestine they are digested by the sheep’s enzymes. Any nutrients contained in the microbe cells will then be absorbed by the sheep. In one sense the sheep lives not so much on clover but on digested rumen microbes.
It is clear, therefore, that because it has a rumen the ruminant animal has the following advantages:
1. It can make use of “fibre” in plants.
2. It does not need to be supplied with B vitamins.
3. It does not have to be supplied with protein of good quality. In fact the rumen microbes can make amino acids from almost any substance containing nitrogen, such as urea.
A muscle consists of thousands of small muscle cells grouped together into small and large bundles. These bundles are grouped together and held by wrappings of connective tissue. The ends of a muscle change into strong tendons by means of which the muscle is joined to the bones.
As a muscle contracts, its tendons pull the bones together, and this makes movement. If a muscle does not do much work it is soft and tender, and is easily cooked by roasting; but if a muscle does a lot of work then it contains much tough connective tissue, is hard to cook, and is therefore cheaper meat.
Nearly all the bones of the body have muscles attached to them, but only a few of the chief meat muscles need to be mentioned.
The “eye” muscles (or longissimus dorsi) are long muscles which lie on top of the transverse processes of the vertebrae on either side of the dorsal spines. These are considered to be very important because not only are they tender and easily cooked, but their size is a good indication of the general fleshing of the animal.
The width of these muscles does not change very much, but their thickness has been shown to be due to two things, namely:
1. The breeding of the animal – The thickness of the eye muscle is inherited from the parents.
2. Nutrition – On poor feed the eye muscles will not fill out, nor will they be covered with much fat. This shrunken area will allow you to feel the spines of the backbone easily.
The tender loin, or fillet (properly called the psoas muscle) runs along the loin region under the transverse processes of the vertebrae. In cattle this muscle is big, very tender and expensive. It does not work hard and does not contain much connective tissue.
The groups of muscles around the buttock are also tender and are called the “round” in cattle and the leg in sheep.
In contrast to these valuable muscles, those of the front end of the animal are low priced, especially in cattle, for they contain a lot of tough connective tissue. In young sheep the connective tissues may not have had time to toughen, and the meat from the front end may be roasted easily.
Messages are sent to various organs and parts of the body in two ways. The nervous system conveys messages from one part of the body to another along the nerves. The endocrine or ductless glands send messages by making and releasing into the blood chemical messengers or hormones.
Thus the pancreas releases the hormone insulin which allows the body cells to use blood sugar. The thyroid gland produces the hormone thyroxin which governs the rate at which cells use energy.
The most important of all these endocrine glands is the pituitary gland, for it governs the activity of all the other endocrine glands. Thus is makes a hormone thyrotropin which governs the action of the thyroid gland, and the gonadotropin hormones which govern the activity of the reproductive organs.
2. Reproduction
in Sheep:
If we expect farm animals to multiply as often as possible, it is very important for us to learn as much as we can about what happens in reproduction, and what steps the farmer can take to make his animals breed more successfully.
Let us study sheep as an example, for the reproductive changes they undergo are similar to those in other animals. In sexual reproduction what happens is that one sex cell or sperm from the male unites with one sex cell or ovum from the female. The single cell so formed multiplies many times and grows into a young animal.
The main reproductive organs of the ram are the testes, in which the male sex cells or sperms are made. The testes lie outside the body in a sac called the scrotum, and are placed in this position so that the sperms will have a temperature slightly lower than that of the body in which to develop. The sperms are collected from the testes by long coiled tubes, each called an epididymis, which are closely attached to the testes.
These tubes join two other tubes, the spermatic ducts, which take the sperms into the seminal vesicles, reservoirs located in the abdomen. When required, the sperms leave the seminal vesicles and become mixed with a fluid made by a gland called the prostate. This fluid causes the sperms to commence swimming as they pass down the urethra to the outside.
The main organs of reproduction in the ewe are the ovaries, lying inside the body near the hip bone. The ovaries make egg cells, or ova, which, being released, find their way to and enter the fallopian tubes. The number of eggs released varies in different animals, but in the ewe the number is one or two.
If the egg is not fertilised it passes through a large muscular organ called the uterus, and through the vagina, or birth canal, to the outside. If the egg is fertilised by a male sperm, it moves down to the uterus where it becomes attached to the wall and begins to turn into a young animal.
Animals may be divided into two groups according to whether they are seasonal breeders, like sheep, or non-seasonal breeders which will mate at any time of the year, such as dogs, cats and cattle.
In the case of seasonal breeders like the sheep, we might well wonder what causes them to begin breeding at a particular time of the year. In the case of sheep we know that the breeding season is brought about by changes in the length of the day. In autumn, when daylight hours are decreasing, the sun shining on the eyes and faces of rams and ewes causes a small gland at the base of the brain—the pituitary—to make chemical substances or hormones called gonadotrophins. These hormones are carried in the blood, causing the testes of the ram to make sperms and the ovaries of the ewe to make and shed eggs.
In the ram the testes then make the male sex hormone, which causes the ram to develop male features such as bigger bones and more muscle, and which also causes him to seek a mate.
The reproductive changes in the ewe are very complicated, and to understand them we must know what happens in the ovary. The eggs or ova can be seen lying inside small sacs called follicles. When these follicles have grown to full size, they break through the skin of the ovary and rupture, and then the eggs are set free. This is called ovulation, and while it is happening the ovary has been making the female sex hormone called oestrogen, which causes the ewe to come on heat and seek a mate.
This hormone also causes the ewe’s body to have a female shape and stimulates the growth of the uterus and udder. The ruptured follicle heals over and begins making another hormone called progesterone, which stops the heat periods. If the egg is not fertilised, then the ovary stops making progesterone after about sixteen days, and the ewe has another heat period.
So in the case of the ewe, we see that the breeding season consists of short periods of heat which only last one and a half days, alternating with periods of quiet lasting sixteen days. However, if an egg is fertilised, the ovary then continues to make the progesterone hormone, which stops all further heat periods until after the lamb is born. Progesterone also stimulates the growth of the uterus and the udder.
Table 38.1 gives condensed details of the breeding of the four chief kinds of farm animals.
It is important to mate animals at a time which is as close to ovulation as possible. If this is not done, the sperms may die before the egg is shed and ready to be fertilised. Horses and cattle should be mated as late as possible in the heat period of the female, whereas pigs and sheep may be mated at any time in the heat period.
Highly fertile animals are those which reproduce themselves efficiently; all the processes of reproduction run smoothly, eggs are shed and fertilised, and the young are born and reared successfully. Most of the factors which influence fertility are under the control of the stockowner, and therefore a knowledge of these factors will help in reducing infertility.
Reproduction will be most successful if animals are mated at the time of the year when their breeding activity is most efficient. Thus, although some breeds of sheep will mate at other times of the year, autumn matings produce the highest lambing percentages.
There are breed differences which influence the efficiency of reproduction. This is well known in sheep, where some breeds like the Border Leicester have a higher percentage of twins than other breeds like the Merino.
One of the biggest causes of infertility is disease. Any disease which produces a fever will reduce the breeding ability of male animals, since the sperms must have a temperature lower than the normal body temperature to develop properly.
Probably the worst disease is brucellosis or contagious abortion, caused by a germ which makes cows drop their calves before they are fully formed. This germ also causes epididymitis in rams as well as other diseases in livestock. Stock must be kept healthy if they are to reproduce themselves efficiently.
Because of the harmful effects on sperms of high temperatures, rams may not be fertile if they are used for mating during the hottest part of the summer.
One of the commonest causes of low fertility is lack of good feed in the dry season. Flushing is the practice of putting animals on to good feed for some weeks before they are due to be mated, in the hope of increasing fertility. On the other hand, animals which are too fat will not breed well, and the ideal is to have sheep in what is called “forward store condition” when they are mated, that is, neither too fat nor underfed.
Reproduction can be made more efficient by such methods as artificial insemination. In this process, semen is only taken from healthy males of outstanding value. The semen is examined under a microscope to see whether the sperms are active or dead. It is usually diluted with a fluid and introduced into females at the time most likely to lead to fertilisation. Of course, many females may be inseminated with one lot of male semen.
3. Birth and Lactation
of Sheep:
Once an ovum has been fertilised by a sperm, it commences to grow into a young animal. For a time it grows slowly, living on the yolk which was stored in the ovum. It then moves down the fallopian tube and comes to rest in the uterus.
The inside of the uterus is soft and spongy, and the embryo sinks into this tissue and absorbs food from it. Later on the embryo grows so much larger that it can no longer absorb enough food in this way. It then begins to make a special filter called the placenta, which becomes attached to the uterus wall.
The embryo is then called a foetus. Its food passes from the mother’s blood through the placenta and along a cord into the blood of the foetus. The foetus at this time is wrapped in membranes and is protected from injury by fluid in the membranes acting as a cushion.
As the foetus grows bigger, the uterus grows and stretches greatly. Since the foetus makes extremely rapid growth towards the end of pregnancy, it is necessary during this period for the mother to be given high-quality food.
Finally the foetus reaches full size, is pushed along the vagina or birth canal, and is born onto the ground. The cord which joins the young animal to the placenta is broken and the mother clears away the membranes from the nose of her young one so that it can breathe properly. She then helps to dry the wet skin of the young animal by licking it all over.
Once the young one is properly dry and has had its first drink, it has much better chances of living, but cold, rain and winds may cause the death of a young animal in the first few hours of its life. Sometime after the birth has occurred, the placenta is expelled from the uterus of the mother. This is called the afterbirth.
The udder begins to grow in size long before birth, and is fully formed when the young animal is born. At first it does not contain milk but a rather sticky fluid which is called colostrum. This is a special strengthening food for the newly born animal, but it rapidly changes into milk after a day or so.
Milk is the ideal food specially designed to aid the growth of young animals until they are weaned. If we take the young away from the mother before weaning, or if the mother fails to produce enough milk, the growth and development of the young animal will be affected.
This is one of the reasons why dairy and beef cattle have different shaped bodies—we leave beef calves on their mothers, but take dairy calves away so that we can use the mother’s milk. The growth of calves is faster in those beef breeds in which the cow makes a lot of milk.
The udder is a large organ consisting of milk-secreting tissue, fibrous supporting tissue and fat. Materials for making milk are carried to the udder in the blood, and the cells of the many small milk glands, or alveoli, make the milk. Some substances like water and vitamins pass unchanged through the alveoli and into the milk.
Other substances such as carbohydrates, fats and proteins are changed by the cells of the alveoli into special forms. In milk the sugar is called lactose, the protein is called casein and the fat is butterfat.
Milk is conveyed from the alveoli by fine ducts down to large openings in the udder called the cisterns. The milk passes from the cisterns down the teat canal to the outside. Unless the skin of the teat is stimulated by the sucking of the calf or by the massaging effect of the hands of the dairyman, the milk will not be “let down” properly from the alveoli where it is made and stored.
After birth the quantity of milk secreted increases until a peak is reached. In ewes, maximum milk secretion occurs three weeks after lambing, but in cows the peak may be from three to six weeks after calving. The length of lactation varies with the kind and breed of animal, but in a good dairy cow it may last for nine or ten months, becoming less and less until she goes dry. The cow will then remain dry until she has had another calf. Dairymen who sell whole milk usually try to spread matings throughout the year so that cows are coming in at intervals.
The amount and length of milk secretion in any female animal depends upon a number of factors, the most important of which is nutrition. If cows are poorly fed before they calve, the lactation will be shorter and lighter than it should be.
Since dry cows are often put into the paddock which has the poorest feed, it is not surprising that some of them do not have long lactations. “Steaming-up” is the practice of giving pregnant cows improved nutrition before calving. This has been found to increase and lengthen the lactation.
If cows are poorly fed after calving, the lactation period may be shortened by as much as 12 weeks. If feed is likely to be short after a cow calves, she may be made to milk longer by giving her a special ration of fodder in addition to what she may get from the pastures. In some experiments in New Zealand it was shown that milk production could be increased by almost 50 per cent by feeding a supplement of nine kilos of silage daily eight weeks after calving.
The amount of milk produced and the length of lactation also depend on breed. It is well known that dairy cows usually make more milk than beef cows. In the dairy breeds, those which produce the most milk usually have the lowest fat content and vice versa.
The quantity of milk produced at each lactation increases with age until maturity is reached, but the quality of the milk remains unchanged in any cow. The importance of lower milk yields from young females is shown by the fact that lambs from young ewes grow more slowly than those from mature ewes. Litters from young sows are not as heavy at weaning as the mother’s litter of the following year.
It is normal for cows and mares to be pregnant and producing milk at the same time, and pregnancy has an effect on the lactation. If mating occurs soon after birth, then the lactation will be shortened and lighter in quantity. In dairy cows, milk yield begins to drop away rapidly after five months of pregnancy, but there are some cows which can be milked until they are almost due to calve.
In all female animals careful handling and quietness are essential if long lactations are desired. Excitement and noise have the effect of reducing the quantity of milk produced and the length of lactation. Heat periods often reduce the quantity of milk produced, probably because females tend to become excited at this time.
4. Growth and Development
of Sheep:
As a young animal gets older, two kinds of changes happen to it:
1. It puts on weight and becomes bigger – This is called growth.
2. It changes its shape – A young animal has a big head and a small body, but a mature animal has a relatively small head and a big body. These changes are called development.
Scientists have discovered that all animals grow in much the same manner. If we watch them and measure them, we find that they all follow certain laws of growth and development.
The first law of growth concerns the animal when it is still in its mother’s body. From this law we find that the growth of the young unborn animal or foetus becomes faster and faster near the time it is to be born, but its growth in the early stages of pregnancy is very slow. Because of this law, we see that it is more important to feed the mother well in the later stages of pregnancy when she has to build up the rapidly growing body of the young one inside her.
The second law of growth is that, when the young animals are born, their size and weight will depend on three things:
1. The number of young born – Twins are not usually as heavy as if only one animal were born. When sows have big litters, the piglets are smaller than when there are few piglets in the litter.
2. The nutrition of the mother – If the mother has been starved, it is likely that the young animal will be smaller at birth.
3. The breed – The young of some breeds are always bigger at birth than the young of other breeds. Thus Corriedale ewes usually have bigger lambs than Merino ewes.
The third law of growth explains that after birth, growth in all animals occurs in the same manner. It is slow at first; then growth occurs at a very fast rate; and finally growth slows up again as the animal approaches maturity. We say that growth after birth follows an “S” curve.
If we are selling animals for their meat, we should sell them at the end of their period of fast growth. If we keep them longer than this they are eating just as much grass or food per day, but are only putting on weight slowly.
According to the fourth law of growth, lack of sufficient food will slow up growth, but this is seen much more in young animals than in older ones. If starving occurs early in the life of a young animal, its growth may be so stunted that it will never reach mature size. In a time of drought the feeding of young stock should receive first attention, the older animals may be able to recover from a period of starvation.
The fifth law of growth explains that many other factors besides lack of feed will hinder growth, and these factors affect the growth of young animal’s more than older ones.
These factors are:
1. Disease;
2. Parasites such as worms, lice and flies;
3. Low temperatures;
4. Lack of shade in hot climates.
This is a most important law. It is essential to know whether young animals are putting on weight as fast as they should, but it is very difficult to judge this just by looking at them. There is only one sure way of discovering the weight gain of animals, and that is to weigh them. Of course we do not weigh each animal, but we mark and weigh a few representative animals in each flock or herd at intervals of a fortnight or a month.
If they fail to put on weight as fast as they should, we know immediately that something is wrong and we can look for the cause. Thus Merino weaners should gain about 700 grams each a week. If they fall to a gain of only 450 grams each, it may be because they have too great a worm burden and should be drenched. On the other hand cold weather or any of the other factors mentioned could also be the reason for slower growth.
The fourth and fifth laws of growth tell us that an animal’s growth may be retarded if its external conditions are not ideal. The sixth law states that growth is also affected by internal factors—the heredity of an animal.
The inheritance of an animal sets an upper limit to its possible rate of growth and mature size, whereas its nutrition and general environment will determine its actual rate of growth and mature size. In other words, the parents of an animal will determine how big it might grow, but how big it does grow is governed by its food supply and the general care which the farmer gives it.
The laws of growth may be summarised as follows:
1. Growth of the foetus is most rapid in the period just before birth.
2. Birth weight depends on breed, nutrition of the mother and number born.
3. Growth after birth in all animals follows an “S” curve.
4. Lack of feed retards growth, especially in the early stages of life.
5. Disease, parasites, shade, shelter and climate affect growth in a manner similar to the effects of nutrition.
6. The growth of an animal is affected by its heredity.
Different parts of the body grow at different rates. If you look at a newly born calf or a kitten, you will notice that it seems to have a big head, long legs and a small body. A mature cow or a grown cat seems to have a small head and legs with a large body.
Now of course the head and legs have not grown smaller, but what has happened is that they have grown first and are early-developing parts, whereas the body, loins and hindquarters have grown last and are late-developing parts.
The last part of the body to develop properly to full size is the loin region. Knowing this law we can look at an animal and tell whether it is fully mature, or whether it is still light in the flank. Some breeds of animals never develop properly in the flank, and if we find such animals among pigs or beef cattle we should cull them.
The animal’s body is built up from various tissues such as muscle tissue, nerve tissue, fat tissue, bone tissue and so on. According to the second law of development some of these develop early and some are late-developing tissues. The order in which tissues develop is – brain—bone—muscle—fat. Apparently what happens is that in the young animal, the brain and bone cells take most of the food or nutrients coming into the body.
This idea of tissues competing with one another for the food is a most important one, and is illustrated in Figure 40.2 where we can see that more arrows go to the brain and bone tissues than to the meat and fat.
From a study of this law we find that:
1. Young animals will have a higher percentage of waste parts—bones and brain—than older animals.
2. Anything that hinders growth will affect late-developing parts and tissues first, especially the fat. In Figure 40.2 the effect of lessening the food supply can be seen if we remove one arrow from each of the tissues. Bone still has two arrows and meat one arrow, but fat is left with no arrows, that is, the fat formation ceases.
3. By using hormones, scientists now find that they can alter the tissue competition in animals and cause food to be directed more to one tissue than is normally the case. Thus the hormone hexoestrol may cause more meat and less fat to be formed in beef cattle. When more research is done it may be possible, for instance, to cause sheep to turn much more of their food into wool, but at the present time not enough is known about the possible uses of hormones in controlling development.
The third law of development states that the organs of the body grow at different rates. Some organs like the heart, lungs and intestines are early- developing organs and make rapid growth first. Other organs, like the udder and the sex organs, are late-developing. Thus we see that the carcases of young animals will contain a lot of intestine and other viscera, which are useless parts.
According to the fourth law of development, the sex of an animal influences its body development. In male animals the bones and muscle tissues grow strongly and the fat tissue less. Females have shorter, thinner bones, less meat and more fat than males.
Referring to Figure 40.2, it is the chemical substances or hormones from the sex glands that alter the number of arrows going to the various tissues. In castrated males there are no male sex hormones, and so they develop more like a female with more fat and lighter bones than males.
The heredity or parentage of an animal affects its development. In some breeds and strains of breeds, development of tissues, organs and body parts goes ahead at a faster rate than in other breeds and strains. We say that these animals are “early maturing”. What this means to the practical farmer is that some breeds and strains will reach mature shape and tissue development at lighter weights and earlier ages.
Such animals can be put on the market much earlier, and the cost of feeding them is much less than with late-maturing breeds and strains. It should be noted that some animals never reach mature shape and the right proportions of meat and fat—some of the Asian breeds of cattle for example. Such breeds are used because they possess other qualities such as resistance to heat or diseases.
The nutrition of an animal affects the development of its parts, organs and tissues in an orderly way. Late-developing parts, organs and tissues are encouraged most by good feeding, and stunted most by underfeeding. This means that underfeeding will not stunt early-developing parts so much, and this is seen in starving stock which appear to have big heads and legs with stunted bodies.
By applying this law and controlling the food supply of an animal, it is possible to produce the kind of carcase desired. This can be seen in relation to the production of bacon carcases. If a pig is fed well right up to bacon weight the carcase tends to have far too much fat. If we underfeed a pig early and late, the carcase lacks sufficient meat and fat. If a pig is fed well in the early stages and less later on, there is plenty of meat formed but not too much fat.
The next law refers to fat development and it shows that fat is always laid down in the following order:
1. First, near the kidneys and around the intestines. (In cattle this is called suet fat.)
2. Second, over the outside of the body. It is possible to tell when this stage is being reached because the layer of fat outside the muscles fills out the shape of the body, and begins to hide or cover over the bony projections such as the spines of the backbone.
3. Last of all fat begins to be formed in amongst the muscle or meat fibres. This appearance is called “marbling” in beef meat, and “streaky” bacon in pig meat. Meat which has the fat distributed through it like this is supposed to be tender and easier to eat than meat from a carcase which has only reached the second stage of fat development. However, research work recently done throws some doubt on the tenderness of “marbled” meat. Other factors may be more important.
The eighth law of development concerns the kind of fat which is formed. Most people prefer a hard white fat to a yellow oily fat. Hard white fat is only formed when the animal has been grown quickly with no setbacks, and when it has had to make the fat from starchy foods.
The undesirable yellow oily fat is formed:
1. When the animal has been fed on fatty or oily foods. This may happen when pigs are fed on much waste from hotels which contains scraps of fatty meat.
2. When the animal has had setbacks in its growth. A yellow pigment called carotene which occurs in green leaves is laid down with the fat tissues. If an animal passes through a hard time, it uses up some of its own fat, but the yellow carotene is left behind and becomes concentrated.
Dairy cows and laying hens regularly go through a hard time when they are in full production, so the fat of these animals tends to be yellow and soft. In a young quickly grown cockerel or pullet, the fat may be white and firm, and the same is true of quickly grown beef.
The principles or laws of development may be summarised as follows:
1. Different parts of the body grow at different rates.
2. Different tissues of the body grow at different rates.
3. Different organs of the body grow at different rates.
4. Sex influences body development.
5. An animal’s heredity influences its body development.
6. Nutrition affects development. Late-developing parts, organs and tissues are retarded most by underfeeding, and stimulated most by good feeding.
7. Fat is formed first in the gut, then over the muscles, and finally amongst the muscles.
8. The kind of fat formed depends on the conditions of growth.
5. Nutrition
Required for Sheep:
Food of the right quality and quantity is the greatest single need of the animal industries of Australia, for no matter how well-bred an animal may be, it can only produce properly if it is given enough raw materials for making its product. A barrel can only hold as much water as the height of the lowest stave in its side.
The chief factors which help to keep up high animal production are good nutrition, good breeding, freedom from disease, suitable climate and good management. If we call the barrel “animal production”, we can then imagine the factors of production as staves making up the sides of the barrel. In Australia nutrition or feed is usually the lowest stave in the production barrel, and until nutrition is improved, production cannot increase much.
Foods are substances which can be digested, absorbed and used by the bodies of animals. Some foods like starch contain only one kind of food substance, but usually the farmer is dealing with foods like lucerne hay in which mixtures of food substances are found. It is most important that a student beginning a study of feeding should clearly understand the chief nutrients or kinds of food substances and what uses each one serves in the animal’s body.
These are substances like the sugars, starches and the celluloses which are composed of atoms of carbon, hydrogen and oxygen. All animals can use sugars and starches as food, but only sheep and cattle can make full use of cellulose, which is the substance forming the cell walls of young plants. Some foods like maize and sorghum grains, and the cereal grains like wheat, oats and barley, contain a high percentage of starch and are a concentrated form of carbohydrates.
The animal uses carbohydrates to provide it with energy, so that it can maintain the working of its organs, keep up its body temperature and be able to move about. It is most important to remember that carbohydrates alone do not stimulate growth or produce more meat or wool, but merely keep the body going and maintain it.
When animals are very weak, as in a drought, what they need most is energy to keep going, and at such times carbohydrates are needed most. If an animal is supplied with too much carbohydrate it will usually turn a lot of it into fat.
These are oily, greasy substances like linseed oil, peanut oil, lard, tallow and mutton fat. They also contain atoms of carbon, hydrogen and oxygen, but there is much less oxygen than in carbohydrates. Fats are also energy foods and are a much more concentrated form of energy than carbohydrates. Some fat is desirable in the diet of all animals, but too much fat in the diet will cause the fat of the animal to be soft and oily rather than firm.
These are very important substances in foods, containing about 15 per cent of nitrogen atoms as well as atoms of carbon, hydrogen and oxygen. The molecules of protein are formed from many smaller molecules called amino acids. Proteins are the most important substances found in each animal cell, but the cell cannot make its proteins unless it is given the pieces or amino acids.
The animal must eat proteins, and its digestive juices will then take the protein molecules to pieces, as if undoing a string of beads. The amino acid pieces are then carried in the blood and taken into the body cells which can then put the pieces together again, in a different pattern, to make their own protein.
Proteins are needed to make cell substance or protoplasm, such as muscle, wool and hair. Proteins are so important to young animals that the mother puts a protein called casein into the milk. Unless a ewe, cow or sow is fed on enough protein it cannot make large quantities of milk. Most foods contain some protein, but some do not contain nearly enough to provide for growth and production.
Young leafy pasture and protein concentrates like linseed cake, meat meal and fish meal contain high proportions of protein and can be used to feed animals living on a low-protein diet. Since foods rich in protein are so important, and since they are usually scarce, they are the most expensive of all foods to buy.
These are substances which are needed by animals in very small amounts. If vitamins are absent from food, animals will become disordered and ill. Farm animals which live in sunlight and have access to some green feed do not usually suffer from a shortage of vitamins in Australia. But in some cases, as with hens housed in battery cages inside a shed, or pigs fed on maize grain alone, vitamins must be supplied in the feed to prevent trouble occurring.
Vitamin A is needed for proper growth and health of an animal, but since it can be obtained from green feed, and enough of it stored in the liver to last a long time, it is not often a cause of trouble. Vitamin A can be supplied to stock in fish oils such as cod-liver oil.
Vitamin D is needed by animals to help them make proper use of calcium, and lack of this vitamin can cause a disease called rickets in which the bones are deformed.
Vitamin D is contained in fish oils, but can also be formed in the skin if it is exposed to sunlight. Deficiencies of this vitamin may occur in sheep of the southern parts of Australia if winter days are consistently cloudy, or in poultry which do not receive enough sunlight.
Minerals are chemical substances needed by all animals for proper growth and development. Calcium, phosphorus, magnesium, sodium, potassium and chlorine atoms are needed in large quantities by all animals, and nine other kinds of atoms are needed in smaller amounts.
In general, animals obtain sufficient of these elements in their normal diet. A calcium deficiency may occur in drought-affected sheep which are fed on wheat alone, since wheat grains contain only .04 per cent of calcium. This lack of calcium can be corrected by putting some ground limestone into the feed.
Although sheep and cattle seem to like salt licks, scientists have shown that grazing stock can live quite normally without salt, and that there seems to be no useful purpose in supplying it.
Water is essential to all animals, for it forms a large proportion of the body weight, and animal products like eggs and milk have high percentages of water.
The stockowner must know the food needs of his animals, and the foods he should provide to supply these needs.
Young animals are growing fast, making new muscle and other tissues, and using large amounts of energy. Therefore they need large amounts of protein and sufficient energy-giving carbohydrates in their diet.
Animals like the laying hen and the dairy cow are daily putting protein into the form of milk and eggs, and will need a high proportion of protein in their food, as well as enough carbohydrate for energy. On the other hand, old bullocks or horses not doing hard work will not need nearly as much food of any kind.
The protein needs of animals can be expressed in several ways. For instance we can make a list of different kinds of animals and write down the percentage of protein they need in their feed.
We can then make a list of some common foods and write down the protein they contain as a percentage.
From Table 41.1 we can see that sheep can be kept in the store condition with about six per cent of protein in the feed. From Table 41.2 we can see that if we try to keep them on old dry native grass, they will not get enough protein and will lose weight. We can see that a little lucerne hay might give these sheep the protein they need.
You will remember that protein molecules are made of many smaller molecules called amino acids. Some of these amino acids are more important than others. Nine of them are so important they are called the “essential amino acids”.
Animals will not grow properly unless they have enough of these amino acids. Ruminant animals such as cattle, goats and sheep can make all amino acids including these special amino acids in the rumen. The rumen microbes make these amino acids for the animals.
Monogastric animals like pigs and chickens have no rumen, so the essential amino acids which they need must be in their feed, or they will not grow properly. This is why when we are working out which feeds to give pigs and chickens, it is not enough to know the percentage of protein in their feed; we must also make sure that the feed contains enough of the essential amino acids.
If we are making up a special diet for animals there is an interesting method, called the cross technique, for working out how to mix different feeds to get a certain protein percentage. Suppose we need a diet which has a 20 per cent protein content, and suppose the two feeds to be mixed are wheat with a 10 per cent protein percentage and meat meal with 50 per cent protein.
The two feeds are written on the left, one under the other.
Then crossed lines are drawn as in the diagram and the wanted protein percentage is put in the middle. Then we subtract 10 from 20 and get 10; we also subtract 20 from 50 and get 30. On the right we see that by mixing 30 parts of wheat with 10 parts of meat meal, we have a mixed diet with a 20 per cent protein content.
The energy needs of animals are now expressed as megajoules of energy. If we take any sample of a feed and dry it, we can find out the total or gross energy it contains by burning it and measuring the amount of heat it produces. But this is not a useful measure of energy.
It is better if we take a sample of the feed and give it to an animal such as a pig or chicken. Then we can collect the manure, dry it, and find out how much energy has gone right through the animal without being used. So we can find out how much of the feed is digestible energy.
But if we feed an animal on this food, some of the digestible energy will be lost because some of it goes into the urine or some of it is lost as gas. So what we really want to know about a feed is how much of its energy can be actually used by the animal. This energy is called metabolisable energy. Table 41.3 is a list of some feeds with measures of metabolisable or usable energy they contain in megajoules per kilogram of the feed.
The energy needs of animals can also be expressed as megajoules of energy needed per day, but this is not used in pigs and chickens because they are usually allowed to eat as much of a ration as they want.
The energy needs of animals can be calculated, but they will depend on many factors such as:
1. The body weight of the animal. Thus a pig of 25 kg body weight needs 14 MJ of metabolisable energy per day. This could be satisfied by feeding it 1 kg of wheat a day (see Table 41.3). But a pig which weighs 50 kg will need 26.27 MJ metabolisable energy per day.
2. Whether the animal is in full production or not. For example a dairy cow producing 9 litres of milk a day will need more energy than one which is only producing 3 litres a day.
3. The energy needs of the animal will increase if the temperature falls too low, because more heat must be produced to keep the animal warm.
4. If an animal is under any stress, e.g. by bullying from other animals, its energy needs will be increased.
We are interested in the energy content of feeds and the energy needs of animals for two reasons:
1. In working out diets that are suitable for certain animals we need to know the energy content of the feed including the amount of energy supplied by protein in the feed.
2. In times of drought we need to be able to buy the feed which is the cheapest per megajoule of energy.
Choosing the right kind of a food for an animal’s needs is not only a matter of finding a food with the right proportions of protein and carbohydrate, but also making sure that the food is palatable and that the nutrients in it are digestible. The palatability of a food depends on whether it is fresh and free from bad odours, sharp tastes and mildew.
The digestibility of a food largely depends on the amount of fibre it contains. When a plant is young, the carbohydrates it contains are in the form of easily digested sugars and starches, and also cellulose which can be digested by sheep and cattle. As the plant ages, the sugars and starches disappear, and the cellulose walls of the cells become coated with tough indigestible substances called lignins.
Not only does a plant become less digestible as it grows, but the amount of it which stock can eat also become less. We can say therefore that as a plant ages, its digestibility and its indigestibility become less.
When animals are being entirely hand fed, two procedures are open to us. In the case of poultry, it is usual to choose foods which have the right proportions of protein and carbohydrate, and to allow the birds to eat as much as they will.
In the case of hand fed dairy cattle, it would be too wasteful and expensive to allow stock to eat at will, and so rations have to be made up for them. In making up rations we allow a certain amount of food for the body maintenance of the animal, plus an extra amount depending upon how much milk is produced.
Since the great majority of Australian animals are not hand fed, but gain most of their food requirements from pasture, we must try to find a way of valuing a pasture. This cannot be done exactly, because the food value of pastures changes from day to day, but we can form an approximate idea of a pasture’s value by learning about the changing food value of a grass at different stages of its growth. Figure 41.2 shows us stages in the growth of a grass like paspalum.
In early spring we see that the first shoots are very rich in protein and have an energy value equal to that of the wheat grain. As the plant grows older, the energy values and protein percentages fall. Since dairy cows in full milk need about 14 per cent of protein in their diet, we see that paspalum ceases to be an ideal food after a while, and with its low protein content in late autumn is not nearly enough to keep up high milk production.
We can also see that the grass will not supply the energy needs of cows because it becomes indigestible in autumn. Moreover, the cows cannot supply their energy needs by eating more and more grass in late summer and autumn, because the feed has low indigestibility and they just cannot eat large quantities of it.
There are two ways in which the farmer can improve the food value of his pastures. The protein content can be raised by introducing legumes, such as white clover, sub clover or vetches into the grass pasture, since these plants are richer in proteins than any of the grasses.
One reason why pasture plants lose their food value as they age is that much of the food they contain passes into the seeds when they are formed. If the farmer can stop a plant from seeding, and by mowing make it put out new shoots all the time, then its protein content, energy value, indigestibility and digestibility will be kept at a high level for a much longer time.
6. Sheep Development Programmes:
India stands sixth among the countries of the world in sheep population. There were 39.24 m. sheep in 1956 and 42.01 m. in 1966. In 1972, the number declined to 40.39 million. Sheep provide not only wool but also mutton, sheep skins and manure. The largest number of sheep is found in Andhra Pradesh followed by Rajasthan, Tamil Nadu, Karnataka, U P., Maharashtra, Gujarat and Jammu and Kashmir.
Wool production has increased in India during the last few years. Wool production was about 43.6 million kg. in 1991-92 while it was about 38 million kg, in 1984-85. Wool produced in India, is mainly of carpet quality. It has been estimated that 57 per cent of the total wool production is carpet wool while only 43 per cent is finer wool. This has necessitated import of fine wool every year. The table given above shows the production of wool in the country. It is clear from the table that wool production is increasing in India.
There is a wide variation in the types of sheep found in different parts of the country. In the temperate Himalayan sheep region and dry western region, sheep are of woolly type. The main breeds being Gurej, Karnat, Bhakarwal, Gaddi and Rampur Bushan in the former region and Lohi, Bikaneri, Marwari, Kutchi and Kathiawari in the latter region. In the southern and eastern regions, sheep are mainly of the hairy type and are bred for mutton production, the main breeds being Deccani, Nellore, Bellari Mandya and Bandur.
The average wool and mutton production of Indian sheep is low. For instance, Rajasthani sheep give on an average about 1.4 Kg. of wool a year, while in countries like U.K., Australia and New Zealand, the fine breeds produce annually 5 to 6 Kg. wool. The quality of wool compared to Indian wool is also inferior. The Indian sheep are Inferior in size to many exotic breeds.
The rams and ewes of Indian breeds of sheep weight between 27 and 36 kgs. and between 18 and 27 kg. respectively, whereas pure bred rams of many exotic breeds weigh between 60 and 113 kg. and ewes between 54 and 74 kg. respectively. Therefore, the yield of mutton from Indian sheep is low in comparison with that of the exotic sheep.
Therefore, there is good scope for effecting genetic improvement for increased production of wool and mutton. Such improvement can be brought about through adoption of a sound selective breeding programme. Several breeding programmes so far undertaken in the country adopting selection and/or cross-breeding have shown that under proper care and management conditions, the Indian sheep has potentiality for improvement.
The sheep development programme for the small and marginal farmers and agricultural labourers should aim at improving the production capacity of the flocks already owned by them and /or introduction of good quality sheep with the farmers as a mixed farming system. It is estimated that, we have about 24 breeds of sheep of which only 5 are considered as medium or fine wool breeds, 14 as coarse carpet quality wool breeds and the rest as mutton breeds.
Increasing the productivity of sheep could be attempted in three ways viz.:
(i) Improving the yield and uniformity of carpet wool;
(ii) Improving the yield of mutton and meat type indigenous breed particularly in the south; and
(iii) Developing new fine wool and mutton breeds adapted to the different agro-climatic regions in the country through cross-breeding of indigenous sheep with exotic breeds.
Some trials have been made to study the result of cross-breeding indigenous breeds with exotic fine wool breeds for improving wool production. Breeds such as Australian Merinos, Russian Merinos, Spanish Merinos, Rambouillet, Polworth, etc., have been used for cross breeding.
Some fine wool breeds have been evolved from such crosses; as Hissardale (produced by inter-breeding 7/8th Australian Merino and 1/8th Bikaneri sheep); Kashmir Merino (through inter-breeding Merino with Deccani crossbreeds) and Nilgris (through inter-breeding crosses of Cape Merino, South Down and Cheriot rams with Coimbatore).
So far as cross-breeding of sheep for mutton production is concerned, research programmes should be initiated in different regions of India to study the suitability of a number of exotic breeds of sheep for cross-breeding with local types. Experimental studies may also be taken up in a number of large sheep breeding farms of the Animal Husbandary Departments.
The programme for developing sheep should be taken up through small and marginal farmers and agricultural labourers in such SFDA/MFAL districts located in sheep breeding tracts or in those districts which can be easily linked up with Wool Grading and Disposal Centres and/or large mutton consuming markets in cities and large towns.
In the northern temperate regions, where sheep are reared mainly for wool production, progressive cross-breeding with fine wool exotic breeds should be popularised; as with Rambouillet, Merinos and Corriedale; while in the northern plains and western arid zones, crossbreeding with breeds like Rambouillet and Merinos for production of quality wool should be adopted particularly in Rajasthan, Gujarat and Haryana, where Chokla/Nali or Patanwadi breeds are reared.
In areas where cultivation of crop is more intensive and where irrigation facilities are comparatively better (as in South Eastern Rajasthan where Sonadi and Malpura sheep predominate) cross-breeding with Corriedale breed could be adopted to produce dual purpose sheep with improved wool, higher growth rate and better body weight.
In good quality wool producing areas, the policy should be to encourage selective breeding for improvement in body weight and wool yield of uniform quality. In the southern regions, breeds like Deccani, Bellary and Coimbatore could be crossbred with Corriedale breed for improving both wool and mutton. In other areas, selective breeding or grading up local sheep with breeds like Mandya, Nellore, Madras and Red should be carried on.
Other Measures for Sheep Development:
Special attention should be given to improve quality of grass lands used for grazing sheep. For this the village Panchayats should be given adequate financing assistance and technical guidance by the State Governments for developing grasslands through fencing, receding and application of fertilizers.
Further, timely detection and control of sheep diseases must also be given due attention by the extension veterinarians. The farmers should be given training not only in breeding and management practices but also in improved methods of shearing classification and grading of wool for the market. Equipments, machinery and development of intermediate technology for utilization of improved wool would be arranged by the State departments.
Sheep Breeding-cum-Wool Marketing Cooperative Societies should be organised at the primary level to cover every phase of activity of sheep production and marketing of wool and mutton.
Government Action:
A large sheep breeding farm has been set up at Hissar, for this assistance in the form of breeding stock, equipment and experts have been provided by Australia. Three large sheep breeding farms have also been set up in Andhra Pradesh, Jammu and Kashmir and Karnataka.
In 1960-61, there were 14 sheep breeding farms in the country. The number increased to 24 in 1965-66 and to 32 in 1968-69. At the end of 1973-74, it was expected to reach 45. During this period, the sheep and wool extension centres increased from 305 to 461 and 503 and by 1973-74 it may reach 553. The production of wool during Fifth Plan will be about 31.0 m. kg. The programme of sheep shearing, wool grading and marketing was initiated with the assistance of UNDP in Rajasthan. It is now working in 8 centres.
Production of mutton during the Fifth Plan would be 258.4 m. kg. by 1978-79 as against 114.84 m. kg. in 1973-74.