Effect of phosphorus as fertiliser or supplement on the forage intake of heifers grazing stylo-based pastures

Abstract

The effect of phosphorus (P) fertiliser and P supplement on the forage intake and liveweight gain of yearling Droughtmaster heifers grazing tropical legume based pasture was determined. Forage intake was calculated from estimates of faecal output using slow release, chromic oxide capsules and the predicted in vivo dry matter digestibility of forage selectively grazed by steers fistulated at the oesophagus. Faecal output, forage intake, and liveweight gain increased with increasing rates of fertiliser or with P supplement. Average gross forage intake (kg/day) of supplemented heifers grazing unfertilised pasture during the growing season was nearly 50% more than that of unsupplemented heifers grazing similar pastures, and liveweight gain over the growing season was increased by 65%. Similar, but greater, increases in forage intake and liveweight gain occurred with heifers grazing pasture fertilised annually with 10 kg P/ha as superphosphate. Dietary P concentrations on pasture that had received just 1 application of fertiliser (10 kg P/ha) 5 years before the study were only marginally higher than on unfertilised pasture, but forage intakes were higher and not significantly increased by supplementation. However, the liveweight gain of unsupplemented heifers grazing pasture that was fertilised on 1 occasion was depressed compared with heifers receiving supplement or grazing pasture fertilised more frequently.