Effects of shearing and lambing dates on the predicted pasture requirements of sheep in two Tasmanian locations

Abstract

A computer simulation model of growth and production was used to assess the effects of a variety of lambing and shearing dates on the pasture requirement of sheep at two Tasmanian localities, Swansea on the east coast and Bothwell, inland at 650 m above sea level. Time of shearing was predicted to have little effect on the total annual pasture requirement of sheep maintaining a conceptus-free empty body weight of 37.0 kg at either locality. Although temperatures were higher at Swansea than at Bothwell, and during the summer months at both localities, correspondingly greater wind speeds counteracted the differences in temperature, so that the heat loss following shearing was predicted to be similar irrespective of the time of year or locality. Breeding ewes were predicted to require about 1.35 times more, and a ewe plus one lamb growing to 35 kg in 135 days about 1.65 times more, pasture annually than a non-breeding sheep for the conditions simulated. Shearing, however, determined the time of the year when pasture requirement was increased and had a substantial effect on pasture consumption during the winter months. For example, with non-breeding sheep, a change in shearing date from October to June was predicted to increase feed requirements for the three winter months by 328 g day-l or 66%. Because of the greater normal heat production in pregnant and lactating sheep, winter shearing had a smaller effect on the winter pasture requirements of sheep lambing at this time of year. A change from October to June in shearing date for a ewe lambing in July was predicted to increase the pasture requirement during winter by 203 g day-1 or only 18%.