The effect of stocking rate on pasture & animal production from native pasture & native pasture oversown with Indian couch (Bothriochloa pertusa)

Summary

This project compared a Bothriochloa pertusa dominant pasture with a species-rich native pasture dominated by the tufted perennial species Bothriochloa decipiens, Heteropogon contortus, Themeda triandra, Chrysopogon fallax and Eulalia fulva. The native pasture formed an understory to a Eucalyptus woodland growing on a solodic-solodised-solonetz soil with an available phosphorus level of 7.5ppm. Twelve paddocks were created. These varied in size to accommodate three stocking rate (SR) treatments of 0.3, 0.6 and 0.9 steerslha for each pasture.

There were two pasture treatment blocks, each containing three stocking rate treatments in a split plot design. Blocks were replicated giving a 2x2x3 layout = 12 paddocks. The areas destined to become Indian couch pastures were heavily grazed prior to, and after, sowing 7kg seedha without any cultivation. The Indian couch established well and paddocks were stocked with the experimental SRs 13 months after sowing in March 1988. The experiment terminated in June 1993. Despite some atypical seasons and the need to destock for various periods, the objectives of the work were largely achieved.

Stocking rate increases reduced both pasture yield (by 3-5t/unit increase in SR) and steer gains (by more than 100kg unit increase inSR). Differences between pastures were only apparent at the medium and high SR where, over time, Indian couch tended to give higher pasture yields and steer gains. The better gains with Indian couch at these SRs was more of academic interest than of practical significance, since at these rates gain/ha was similar to or lower than the gains/ha at low SR. Native pasture remained fairly stable botanically at the low SR, but the tufted perennial species declined at both the medium and high SR. Sowing Indian couch hastened the botanical changes due to stocking rate, and it became the dominant species at these higher SRs. At the low SR, the contribution of Indian couch declined from initial values indicating that this is not an invasive species in the area at low SR. Contribution of Indian couch to pasture yield was linearly related to SR. Quality of the Indian conch was similar to the other native perennial grasses. Increased steer gains were related to higher yield on Indian couch pastures at the higher SRs. Maximum liveweight gainlha was achieved between 0.3 and 0.6 steers/ha. Stocking at 0.9 steers/ha was not sustainable and scalds appeared in the pastures. Even at low SR, steers would need to spend 2.8 years on the pastures after weaning to reach 500kg liveweight.

An unexpected outcome from the experiment was the effect of age of steers grazing together on annual liveweight gains. Weaners gained more than yearlings, and yearlings more than 2yr olds, at all stocking rates. In some years these differences were greater than the main treatments imposed.