DIET COMPOSITION OF CATTLE GRAZING STYLOSANTHES-GRASS PASTURES IN THESEASONALLY DRY TROPICS - THE EFFECT OF PHOSPHORUS AS FERTILIZER OR SUPPLEMENT

The effect of phosphorus (P) on diet selection in cattle grazing pastu res based on Stylosanthes hamata cv. Verano and S. scabra cv. Seca in the seasonally dry tropics was studied in 3 grazing trials on soils wi th inherently low P status (less than or equal to 5 mu g/g bicarbonate -extractable P). The contribution of Stylosanthes (stylo) to diets was estimated at regular intervals (approximately monthly) using carbon i sotope analysis of faeces. Feeding P supplement increased the proporti on of style in the diet but the effect was seasonal and variable, bein g most pronounced during the late wet and early dry seasons when dieta ry style proportions are usually highest. The results indicated that P supplement influenced diet selection through its effect on dietary P status such that cattle with a deficiency of dietary P select a lower proportion of style than those with higher P intakes. The effect of fe rtiliser P on dietary style content was also variable. The dominant ef fect was via fertiliser-induced changes in pasture botanical compositi on. Higher style proportions in the diet of heifers grazing unfertilis ed pasture compared with those grazing fertilised pasture were associa ted with increasing style dominance in unfertilised pastures as oppose d to grass dominance in fertilised pastures. At the same time there wa s evidence of a fertiliser treatment effect that was consistent with d ietary P status influencing diet composition. In the grass-dominant pa stures, unsupplemented cattle on low P pasture selected less style tha n those grazing pastures of higher P status due to more frequent ferti lising but there was no difference where P supplement was fed. A stron g seasonal preference for grass early in the wet season, reinforced by a generally reduced preference for style in cattle with diets deficie nt in P, probably hastened the development of style dominance in unfer tilised pasture where P supplement was not fed.