The effect of paraquat desiccation in spring on plant and animal production from annual pastures

The effects on pasture yield and composition and on sheep performance of paraquat applied during spring to annual pastures were studied in three experiments. One experiment was on clover dominant pasture and the other two were on pure stands of ryegrass. Spraying reduced dry matter at the beginning of summer in only one of the three experiments, In this experiment, spraying at head emergence of the ryegrass caused the greatest yield loss. Dry matter was lost from untreated pasture following anthesis of the grasses as they senesced but sprayed pastures had proportionately the same losses. Spraying at head emergence increased the proportion of grass leaf in the mature dry ryegrass pastures, but later applications were much less effective. The effects of paraquat on nutrient concentrations in the mature herbage varied between experiments. On the clover dominant pasture, N, K, Ca, and Mg were higher in the clover on unsprayed plots but not in the grass component. With pure ryegrass pastures, spraying resulted in significantly higher nitrogen concentration but had little effect on other nutrients. Spraying at head emergence gave a higher yield of nitrogen per hectare of ryegrass but otherwise paraquat tended not to affect or to give slightly reduced nutrient yields. The in vitro digestibility of annual ryegrass decreased rapidly after head emergence. Spraying at this time reduced the rate of this decline, whereas later sprays had little effect on the decline. Paraquat had no effect on losses of nutrients from the pastures that occurred over the summer through weathering. Following paraquat application, the liveweight gain of Merino weaner sheep was improved in one of the three experiments. Higher wool production was recorded in one treatment in one experiment. Paraquat at 0.21 kg ha-' active ion reduced the number of grass seedlings regenerating in the season following spraying.