Feeding lambs proportional mixtures of lucerne (Medicago sativa) and forage brassica (Brassica napus) grown under warm and dry conditions

Context Metabolisable energy (ME) and crude protein (CP) concentrations of forages are key determinants of lamb performance during pasture-based finishing. However, forage species often have unbalanced proportions of ME and CP, which can lead to below-optimum performance of lambs. This may be a problem with maturing lucerne (Medicago sativa L.) swards during dry summer conditions, and is a major challenge faced by grazing industries in Mediterranean environments. Aim. We investigated whether two summer forages, lucerne and forage brassica (Brassica napus L. cv. Winfred), with differing ME and CP concentration could improve the growth of maternal-composite lambs when offered in mixtures compared with either forage fed alone. Methods. A randomised complete block experiment with five treatments and 10 replicate animals per treatment was conducted in an animal house at Hamilton, Victoria, from February to March 2014. The treatments were 100% lucerne, 75% lucerne : 25% brassica, 50% lucerne : 50% brassica, 25% lucerne : 75% brassica, and 100% brassica on a drymatter basis and fed to lambs. The lucerne and brassica were harvested daily and chopped to similar to 3 cm length before mixing and feeding. The lambs were adapted for 4 weeks before the start of the experimental phase. Key results. Fasted liveweight gain of lambs increased linearly with an increasing proportion of brassica and a decreasing proportion of lucerne in the diet; no mixture performed better than brassica alone. Conclusions. Under the warm and dry conditions of this experiment, the ME of lucerne was more limiting to lamb growth than its CP concentration.