In India, as in many other countries, grasslands abound in shruby and herbaceous weeds that are unpalatable and, many a time, harmful to the animals. These reduce the area under palatable grasses which, in turn, lowers the grassland productivity. Good grassland management practices like deferred grazing; fertilizer application, reseeding of the gaps, and soil and water conservation practices boost the growth of forage grasses and thus, indirectly help in suppressing the weeds.
Sometimes, a grassland may be so weedy that it may be desirable to plow it down and resow it with new, superior grass species. Herbicides can be used successfully to complement these ‘good grassland management practices’ to improve their quality and productivity.
A wide choice of herbicides is available for the control of herbaceous broadleaf weeds, grasses, and woody and semiwoody brush weeds in grasslands. It is quite possible that certain herbicides may sometimes injure temporarily the forage species but these recover soon with vigour, provided good grassland management practices are integrated with herbicidal control. In each case, prescribed waiting periods should be followed before feeding the treated forage to the animals.
2,4-D (or MCPA) ester (0.5-2.0 kg ha-1) is widely used to control a number of herbaceous, dicot weeds (and sedges) in grasslands. Some of these weeds succumb better to 2,4-DB (or MCPB) than 2,4- D or MCPA. Against the perennial dicot weeds and semi-woody and woody brush weeds, a mixture of picloram and 2,4-D (1: 2) has been used successfully. Picloram is the present day versatile herbicide to control difficult, perennial, broadleaf weeds and brushes in grasslands.
When weeds are distributed evently, it can be applied either as 0.25% blanket spray or spread as dry granules (7-10 kg ha-1). But when only a few’ scattered brushes are to be treated, picloram granules can be spread around each brush in about 30 cm diameter area. In either case the herbicide should be applied in rainy season.
Glyphosate is better translocated in perennial weeds to reach their deep roots and rhizomes than many other herbicides. It has, therefore, been widely used for foliar treatment of a deep rooted brush like Lantana camara. It is better to first manually cut the lantana shoots for fuel and then spray its regrowth with glyphosate. A 0.25% spray concentration is considered effective for spot treatment of the brush.
Silvex (2-8 kg ha-1 or 0.2-0.4 % spray), dicamba (0.25% spray), and amitrole (0.8% spray) are some other common herbicides used to deal with specific perennial broadleaf weeds, and brushes in grasslands. Dicamba can be applied also as 25-50% solution by the stem injection method.
Of recent, tebuthiuron has been added to the list of brush weed killers in grasslands. It can be either applied as foliar spray (0.1- 0.2%) or as soil treatment in the form of pellets (0.40 kg ha-1). The palatable grasses regain ground in due course after the destruction of the brush weeds.
Dalapon (4-8 kg ha-1), amitrole (2-2.5 kg ha-1), diuron (1-2 kg ha-1), prometryn (3-4 kg ha-1), and bensulide (4-8 kg ha-1) are common herbicides used in the western world to control annual grasses in grasslands and ranges. For their spot treatment, 2% spray of dalapon or 0.2% of bromoxynil can be used in grasslands and ranges.
Spot treatment with bromoxynil will also control a few scattered patches of perennial grass weeds. But when the grassland is widely infested with perennial grasses, it is better to renovate it by plowing it down and resowing it with some desirable grass species.
Proper agronomic management and herbicides should be integrated to prevent its reinfestation with perennial weeds. In case a grassland was to be renovated without tillage, one could spray 0.1% paraquat to destroy the existing vegetation and plant the new grassland species after proper fertilization.