Supplementation of steers grazing Stylosanthes hamata pastures at Katherine, Northern Territory

Abstract

Native perennial grass pastures were oversown with Stylosanthes hamata cv. Verano at 3 fertility levels, i.e, without fertiliser or with suboptimal levels of superphosphate or rock phosphate. Brahman steers, of varying ages, grazed the pastures and their responses to fertility level and direct supplementation with phosphorus and sulfur were determined over 4 years. Phosphorus supplementation doubled liveweight gains during the early and late wet season periods to about 0.9 and 0-7 kg/day, respectively. Pasture fertility level did not affect gains in the early wet season in most years but superphosphate increased the late wet season gains by about 0.1 kg/day. Small weight losses usually occurred in all treatments during the early dry season, with losses increasing with animal age. Weight losses were highest during the late dry season, particularly for the older, phosphorus supplemented, steers. This response was attributed to size rather than to age or supplementation per se. Sulfur supplementation had no effect upon liveweight gain or upon the pastures at any time. In unfertilised pastures, Verano increased to about 25% of the dry matter composition whilst pastures fertilised with superphosphate and rock phosphate became legume dominant after 2 and 3 years, respectively. Fertiliser also increased the nitrogen, phosphorus and sulfur concentrations in the forage, particularly the Verano, with superphosphate more effective than rock phosphate. In pastures where steers were supplemented with phosphorus there was a higher proportion of the naturalised legume Alysicarpus vaginalis and a lesser quantity of forage on offer. Blood inorganic phosphate and rib cortical thickness were reliable indicators of responsiveness of steers to phosphorus supplementation when measured during, or immediately after, the period of active growth.