Diet composition of cattle grazing Stylosanthes-grass pastures in the seasonally dry tropics: the effect of phosphorus as fertiliser or supplement

The effect of phosphorus (P) on diet selection in cattle grazing pastures based on Stylosanthes hamata cv. Verano and S. scabra cv. Seca in the seasonally dry tropics was studied in 3 grazing trials on soils with 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 isotope analysis of faeces. Feeding P supplement increased the proportion of style in the diet but the effect was seasonal and variable, being most pronounced during the late wet and early dry seasons when dietary 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 fertiliser P on dietary style content was also variable. The dominant effect was via fertiliser-induced changes in pasture botanical composition. Higher style proportions in the diet of heifers grazing unfertilised pasture compared with those grazing fertilised pasture were associated with increasing style dominance in unfertilised pastures as opposed to grass dominance in fertilised pastures. At the same time there was evidence of a fertiliser treatment effect that was consistent with dietary P status influencing diet composition. In the grass-dominant pastures, unsupplemented cattle on low P pasture selected less style than those grazing pastures of higher P status due to more frequent fertilising but there was no difference where P supplement was fed. A strong seasonal preference for grass early in the wet season, reinforced by a generally reduced preference for style in cattle with diets deficient in P, probably hastened the development of style dominance in unfertilised pasture where P supplement was not fed.