Effect of stocking rate, time of lambing and inclusion of lucerne on prime lamb production in north-east Victoria

Abstract

Results of a grazing study involving Border Leicester x Merino ewes and their progeny are reported for a 4-year period 1969 to 1972. The study was conducted at Rutherglen in north-east Victoria. The 1260 ewes observed throughout the study were allotted to 42 treatment plots with 30 ewes per plot. Thirty of the plots were located on annual pasture (subterranean clover, barley grass) involving three replicates of ewes stocked at 7.4,9.9 and 12.4 ewes ha-l for ewes joined over 7-week periods to commence lambing about May 5, June 23 or August 8. Additionally, an extra three plots with spring lambing were grazed at 14.8 ewes ha-1. A further 12 plots contained lucerne on one third of the plot area. The ewes on these plots also lambed for a 7-week period commencing August 8, and provided three replicates of 7.4 9.9, 12.4 and 14.8 ewes ha-1. First services were concentrated early in the joining period in the spring-lambing ewes, there were few returns to services and few ewes failed to mate. On the other hand, among ewes joined to lamb in autumn, first services were distributed over the entire joining period, and many ewes returned to service or failed to mate. The mating pattern of ewes joined to lamb in winter were intermediate between the autumn and spring lambing groups. Although the liveweight of spring-lambing ewes was lighter at mating, the number of lambs born per 100 ewes joined was greater than for winter lambing, and these in turn were greater than for autumn lambing ewes. The response to an additional 1 kg liveweight at mating was 1.7, 2.0 and 2.8 additional lambs born per 100 ewes joined for autumn, winter and spring lambing, respectively. Wool production per hectare increased linearly with increase in stocking rate; it was marginally affected by time of lambing and was greatest where lucerne was provided. The number of lambs reared per ha increased linearly with increase in stocking rate, albeit there was some decline in carcase weight and a small decline in numbers of lambs reared per ewe. Lambs born in autumn and winter were generally slaughtered in prime condition at 31 kg liveweight, except in the drought year of 1972, but more lambs were produced from a winter lambing than from autumn lambing. When lambing was in August-September there was a slight increase in the numbers of lambs reared above that of winter lambing, but the proportion of lambs marketed at less than 31 kg was greatly increased. Where lucerne was provided to spring-lambing ewes the greatest weight of meat per hectare was produced, and few lambs were less than 31 kg. In descending order of magnitude, the major factors affecting carrying capacity were season, year, stocking rate and time of lambing. Seasonal shortcomings in nutrition were alleviated by the provision of supplementary feed. At the lighter stocking rates of 7.4 and 9.9 ewes ha-1 the amounts of supplements fed were very low, even in the drought year of 1972. These two lowest stocking rates could be maintained indefinitely without excessive use of supplementary feed. Discernible changes in botanical composition with partial loss of productive grass and clover species, reduction in pasture growth rate, increased use of supplementary feed and lowered wool production of ewes, occurred with spring lambing without lucerne, particularly at 12.4 and 14.8 ewes ha-1. Pasture composition and growth details were very similar with either autumn or winter lambing.