The effect of stocking rate on the population dynamics of siratro in siratro (Macroptilium atropurpureum)-setaria (Setaria sphacelata) pastures in south-east Queensland. I. Survival of plants and stolons

Abstract

The persistence of plants and stolons of the tropical forage legume Siratro (Macroptilium utropurpureurn) was measured in Siratro-setaria (Seluriu sphacelala) pastures at Samford, south-east Queensland. The pastures were sown in 1968. Three pastures were set stocked and continuously grazed from 1969 to 1985 at 1.1, 1.7, and 2.3 heifers ha-1. A fourth pasture was stocked at 3.0 heifers ha-1 from 1969 to 1973 and at 2.0 heifers ha-1 thereafter. In 1972 the density of Siratro crowns was similar in the four pastures, although crowns were smaller and stolon density was considerably less at the highest stocking rate. Persistence of plants at a moderate stocking rate (1.7 heifers ha-1) and a heavy stocking rate (3.0/2.0 heifers ha-1) was followed in fixed quadrats from 1971 to 1986. Stocking rate had a marked effect on the recruitment of new crowns and on crown survival. Total numbers gradually declined at the heavy stocking rate and no plants were recorded after 1982. This decline was associated with poor survival of plants (half life of only 6 months) and with reduced input of new plants, particularly after 1980. In contrast, Siratro density at 1.7 heifers ha-1 remained constant from 1971 to 1978. and then increased to 1982. This was associated with a longer half life of plants (c. 20 months) and sustained input of new crowns. However, there was a drastic decline in Siratro density from 1982 to 1985. Siratro was clearly unable to persist under heavy grazing, but there was no single reason why it suddenly declined in a moderately grazed pasture where it had persisted for 14 years. However, the advent of Siratro rust, together with a run of years with below average summer rainfall, are likely major contributory factors. Both these factors increased the grazing pressure on Siratro under the set-stocking and continuous grazing that was imposed.