Grazing sheep on cereal crops in winter has become widely adopted in medium–high-rainfall zones of Australia. Interest in this practice has spread to the lower rainfall parts of the cereal–livestock zone where it is being applied to shorter season crop varieties. A farm-system modelling study was conducted to investigate the value of deferment of annual pastures by grazing spring wheat in their place. The biophysical simulation model, based on a representative wheat and sheep farming system in the wheatbelt of Western Australia, involved two grazing-management scenarios and used climate data for the period 1962–2011 for three locations in Western Australia representing low-, medium- and high-rainfall cropping regions: Merredin, Wickepin and Kojonup. The grazing-management policy of the main scenario, ‘crop grazing’, placed livestock on the crops only until the crop reached Zadoks growth stage 30, provided the green biomass of the farm’s annual pastures was <800 kg/ha. A second ‘shadow-grazing’ scenario was run in which a group of ewes identical to the main ewe flock was used to graze annual pastures simultaneously with the main ewe flock whenever the main flock grazed wheat crops. The difference between the two scenarios represented the pasture deferment value associated with grazing wheat crops.
Pasture deferment had little effect on total pasture production during the period when crops were grazed. However, there was a small benefit to feed supply through the accumulation of pasture during the period of crop grazing. This feed was available at a time of year when feed is scarce. This was reflected in improved animal production, with the weight of lambs at weaning being higher in the crop-grazing scenario than the shadow-grazing scenario. These results suggest that although increases in pasture productivity and feed supply associated with spring crop grazing are only marginal, grazing of spring wheat crops can still lead to changes in lamb production because this enterprise is sensitive to the feed supply in winter.
