In this article we will discuss about the agronomy of irrigated cropping systems.
Tillage:
Establishing crops after cereals is somewhat difficult without adequate land preparation due to large quantities of residues left behind by the cereals. The problem is much more complicated after lowland rice crop.
Thorough land preparation requires long time which limits multiple cropping systems. No till or minimum tillage methods which have become easy and effective to practice with the advent of modern herbicides are suggested to reduce the time between two crops.
Different techniques may have to be adopted for different crops that follow rice. Pulse crops can be established on heavy soils by broadcasting presoaked seed 7 to 10 days before rice harvest. Crops such as maize and cotton requiring low populations can be dibbled by hand.
Sorghum and large seeded legumes can be drilled. Depending on the soil conditions and time available for sowing/planting the second crop, appropriate tillage systems should be followed in intensive multiple cropping systems.
Varieties of Crops:
Short duration high yielding photo-insensitive cultivars that can fit in different multiple cropping systems are available. Now it is possible to raise more than two or three crops in a year in the same piece of land, provided adequate irrigation is made available.
Fertiliser Needs:
Nitrogen fertilisers, generally, produce little residual effect. However, continuous application of even moderate doses of phosphorus builds up significant residual phosphorus.
Fertiliser requirement of two or three crop systems is greatly increased compared to a single season cropping. However, consideration of residual fertility decreases total sequential system fertiliser needs compared with recommended fertiliser schedules for each of the crops in the cropping system.
A general guideline for fertilising sequential system could be to fertilise those crops that respond most in the system and allow others to make use of residual fertility. On this basis, nitrogen and phosphorus should be applied to cereals, phosphorus to short season legumes, potassium to cereals and tuber crops, zinc to rainy season cereals and sulphur to the oil seed crops.
Water Management:
In irrigated multiple cropping systems, the crops are grown in sequence. As such method of irrigation and irrigation schedules are not different from that of only one crop in a season/year. However, cropping systems vary depending on the duration of water availability. If water is adequate although the year, rice-rice-rice is the usual system. Rice-rice-pulse crop could be ideal if water is a limiting factor during summer.
Cropping systems should be planned depending on the duration of water availability after the harvest of rainy season crop in November/December. If water is available upto April a short duration rice can be raised as second crop under the climatic conditions of south India.
A second crop of groundnut could be ideal if water is available upto the end of March. A second crop of sunflower can be grown if water is made available upto second fortnight of March. Any short duration pulse crop could be a second crop if water is available upto the end of February.
Weed Management:
Weed management practices should be planned for the cropping system as a whole rather than for individual crops due to the problems associated with herbicide residues in the soil. Crop-weed associations should also be kept in mind while planning for weed management. Inclusion of lowland rice in the sequence greatly reduced the weed problem in succeeding crop.
Similarly, pulse crops with smothering effect can reduce weed seed production leading to less weed problem in succeeding crop. Care should be taken to avoid residual toxicity of herbicide on succeeding crops.