Lambing date and its effect on the production and utilisation of grass-clover and lucerne pastures

Grass-clover and lucerne farmlets stocked at 15 and 20 ewes/ha respectively had similar yields of saleable products. From grass-clover the liveweight of lambs weaned was 583 kg/ha and the ewes produced 70 kg/ha of wool. On lucerne the liveweight of lambs weaned was 552 kg/ha and a wool clip from the ewes of 95 kg/ha. Lambing performance was relatively high from both pastures however on lucerne a significantly lower (P<0.05) lambing percentage (114%) was recorded than on grass-clover (166%). This was attributed partly to high coumestan levels in lucerne at the time of mating and partly to the occurrence of "border disease" in only the lucerne farmlets. Net pasture production recorded over the 12 month period was higher for both pastures than what was considered normal for the environment, 13 040 kg DM/ha from grass-clover and 20 120 kg DM/ha from lucerne. This was due mainly to the above normal rainfall over the trial period but in addition, net production from rotationally grazed pastures had not been previously recorded in this environment. Although annual DM production from lucerne was 54% higher than grass-clover total animal production was similar. This was due to; the greater seasonal variability of lucerne growth compared with grass-clover making it more difficult on lucerne to meet ewe requirements, especially over autumn winter and early spring. The in vitro digestibility of the total herbage present before grazing over the year was significantly higher (p <0.01) on grass-clover (69%) than lucerne (55%) pasture. The lower digestibility of the lucerne was identified as a major factor affecting apparent intake whereas on grass-clover D.M. yield was more critical. The mating of ewes early (March 24) or late (April 28) had no effect over the year on the growth or composition of either pasture. Only minor effects on animal performance were observed which on grass clover were associated with the climatic conditions prevailing or with more complex interactions between previous levels of pasture utilisation and the current animal requirements and occurred over relatively short periods of up to 2 weeks at the end of the flushing, pregnancy, and lactation periods. The effect observed of greatest practical significance, was that lambs born from the late group on grass-clover had a lower liveweight gain (6.8 kg vs 7.7 kg) from 8 weeks of age to weaning at 12 weeks than earlier born lambs. The decline in growth was attributed to a decline in pasture quality associated with an accumulation of dead material. Post-weaning, a similar effect was observed but this time caused by dry conditions resulting in a rapid decline in pasture growth. On lucerne, the only effects of lambing date observed were: flushing could not be maintained on lucerne alone for the late lambing group and a higher amount of supplementary feed was fed in late pregnancy to the early lambing ewes. From the trial it is concluded that in the Lincoln environment a farm based totally on lucerne would not improve production efficiency from a ewe/lamb system. Ewes mated to commence lambing on August 20 was satisfactory on grass-clover pasture but on all-lucerne, September 5 would be more desirable than either of the two dates selected in the trial to commence lambing. Four-paddock experimental farmlets proved satisfactory for comparing two management systems on two quite different pasture types and on no treatment was paddock number considered a limiting factor. The capacitance meter and the "paddock mean" method of selecting sampling sites was an efficient technique for measuring pasture yield both before and after grazing in grass-clover or lucerne pastures.