Effect of chemical removal of grasses from pasture leys on pasture and sheep production

Abstract

Pasture production, pasture composition and quality, and liveweight, body condition and wool growth of Merino weaner wethers were monitored over 2 seasons and at 2 stocking rates (4 and 8 sheep/ha) on a mixed annual grass-legume pasture sprayed with propyzamide to control grasses. Propyzamide virtually eliminated annual grasses from the pasture (less than 5 kg/ha of grass dry matter in spring v. 403 kg/ha on untreated pasture in 1981) and this carried over into the second season (61 kg/ha v. 647 kg/ha in spring 1982). Propyzamide treated pastures had less total dry matter (P = 0.05) throughout 1981: however, except for a single sample date, there was no effect of propyzamide on total available pasture in the second season. Increased growth of clover and capeweed compensated for the absence of the grasses. The higher stocking rate reduced available clover, capeweed and total dry matter (P = 0.05) throughout both years. Sheep grazing grass-free pastures had lower liveweights during winter in both years, but made compensatory gains during late spring and summer consistent with the higher quality (1.6 v. 1.07% nitrogen when sampled in January) of propyzamide treated pastures. In 198 1, wool growth rates were reduced at the higher stocking rate and total clean wool production was reduced from 4.55 kg/sheep at 4/ha to 3.65 kg/sheep at 8/ha. Pasture treatment had no effect on wool production in either year. The implications of using selective herbicides to remove the annual grass component of legume-based annual pastures in south-western Australia are dis cussed in relation to pasture and sheep production.