L. 't MANNETJE and J.P. EBERSOHN

Abstract

Grazing intake and animal production are related to herbage present, herbage allowance, legume content, canopy structure, digestibility, protein and mineral content and grazing behaviour. The predictive value of these variables is limited over a range of pasture conditions because intake depends on a complex interaction of these attributes and animal factors. Intake and animal production are asymptotically related to total dry matter in the pasture where the herbage consumed is a constant proportion of herbage present or allowance. Where the pasture contains a seasonally variable amount of dead material the relationship applies only to the green fraction of the pasture.
Tall tropical and sub-tropical pastures have a lower digestibility and bulk density than short or temperate pastures. Grazing animals are able to maintain intake to some extent by increasing grazing time and rate of biting at reduced bite size when the amount of the preferred fraction of the herbage decreases. Digestibility, leafiness and protein content of the herbage are inadequate predictors of intake because they do not account for animal preferences for different species, leaf over stem or for different ages of plant material.
It is concluded that there is a need for a multi-factor approach to study intake and for a new generation of tropical and sub-tropical pasture cultivars with reduced senescence, greater stress and grazing tolerance, higher leaf to stem ratio and greater intake. The mineral concentration of legumes adapted to low soil P levels limits their intake and further studies are required to overcome this problem.