A major cause of unsatisfactory forage crop performance is poor plant nutrition. Nitrogen deficiency is a common problem on many soils. Forage sorghum and millet are very heavy users of nitrogen especially, and when grown in favourable locations or with irrigation for high yields. Topdressing with nitrogenous fertilisers between grazing or cuts is often required to sustain high yields. There must also be an adequate supply of phosphorus, sulphur, potassium and other essential nutrients at sowing to carry the crop right through.
The rate of nutrient removal from the soil can be huge if dry matter yields are high.
For example, One tonne of dry matter of sweet sorghum will remove approximately the following amounts of nutrients:
One tonne of dry matter of sweet sorghum will remove approximately the following amounts of nutrients: Nitrogen 28 kg
Phosphorus 3.5 kg
Potassium 20 kg
Sulphur 2.5 kg
Calcium 3 kg
Magnesium 3 kg
Calculations using the above figures indicate that a highly productive irrigated hay crop yielding 20 t/hectare dry matter removes approximately the following:
700kg/hectare single super
800kg/hectare muriate of potash
High yielding crops obviously require high fertility soils, but the soil must also have generous moisture-holding capacity and favourable structure and texture to at least 1 metre depth. These characteristics are even more essential for maximum yields in rain-grown crops.
Paddock history, previous crop performance, nutrient budgeting and reliable soil testing can help to determine likely fertiliser needs for sustainable forage cropping.
As plants get older and taller, fibre content rises and animal intake declines. However, with sweet sorghums like Pioneer® brand Mega Sweet and Pioneer® brand SSS the sugar content reaches a peak in the older growth, so they can be efficiently used for fattening at a later stage.
Sudan grasses like SSS have finer stems and leaves and better leaf-to-stem ratio than sorghum.