Influence of fertiliser and grazing management on North Island moist hill country 2. Pasture botanical composition

Abstract

Abstract A grazing trial was conducted on 99 ha of steep, low fertility hill country in southern Hawke's Bay, during 1975–81. There were 2 fertiliser treatments: low (LF) and high (HF) superphosphate application (plus lime on HF), and 3 grazing managements — rotational grazing by sheep (RGS) or cattle (RGC), and set stocking by sheep (SSS). As part of a larger measurement programme, botanical composition of pastures was monitored over the 6-year period. HF pastures had a greater content of ryegrass (Lolium perenne L.) and legumes than LF pastures, and a smaller content of low fertility tolerant (LFT) grasses (e.g., browntop, Agrostis capillaris L.; sweet vernal, Anthoxanthum odoratum L.) and weed species. Ryegrass content of pasture under the 3 managements was in the order RGC > RGS > SSS. RGC pastures had a smaller content of LFT grasses, and a greater content of legumes than sheep-grazed pastures. SSS pastures were more weedy than those rotationally grazed. Slope and aspect of measurement site also influenced botanical composition. As the trial proceeded, legume content rose then fell in all treatments. The decline was attributed to increased competitiveness of associated grasses as symbiotically fixed N was cycled and soil N availability increased. This phenomenon places limitations on the use of fertiliser P to promote and maintain legume dominance.