Livestock and pasture production from dryland lucerne and two lucerne/grass mixtures over four years in Canterbury

Abstract

The productivity of a lucerne monoculture, lucerne/brome and lucerne/cocksfoot binary mixtures grown on shallow soils at Ashley Dene, Canterbury, were compared under grazing over four years. Pastures were grazed with separate mobs of ewes & lambs during the spring, weaned lambs during the summer, and hoggets during autumn. The live weight gain of sheep was the ultimate measurement of pasture performance. In turn, this was related to seasonal pasture yield and pasture composition. The live weight production of the lucerne monoculture averaged 28% more than the binary mixtures, which were not different from each other. During the first year annual live weight gain was 780 (±26.2) kg/ha for all treatments, because lucerne dominated their composition. Thereafter, lucerne made up >70% of pasture yield in the lucerne treatment and ~37% in the binary mixtures. Live weight gain was 550-800 kg/ha on the lucerne treatment and 400-550 kg/ha on the binary mixtures over the next three years. Annual pasture yields were similar among treatments each year. These ranged from 6.3-10.8 (±0.28) t DM/ha with soil water availability during the growing season being a strong predictor of yield. The growth response to thermal time, while it appeared soil water was not limiting, averaged 6.1 (±0.21) kg DM/ha/°C day during the first two years and 4.3 kg DM/ha/°C day during the last two. The difference among years was related to higher pre-grazing pasture covers during the first two years which probably increased interception of photosynthetically active radiation. The onset of a soil water deficit, which limited pasture growth, was in mid-December during the first two years and in early/mid-November during the final two. This occurred at comparable times for each pasture treatment suggesting soil water accessibility was independent of this, on these shallow soils. This is consistent with 184 mm of soil water accessibility.

The higher live weight gain of the lucerne treatment eventuated through up to 30% higher metabolisable energy intake compared with the binary mixtures. The shoot structure of lucerne appeared to facilitate enhanced discrimination between more and less palatable portions during grazing relative to sown grass species. This may be complemented by the lower levels of neutral detergent fibre in lucerne which results in faster digestion and therefore increased pasture intake.