Lamb growth rates and carcass characteristics of White Dorper and crossbred lambs grazing traditional and novel pastures during spring in southern Australia

Abstract

Context Finishing lambs on pasture and specialist forages is a commonly used production system in southern Australia; however, there is little information properly comparing multiple pasture options for lamb production. Aim We aimed to compare growth rates of lambs when grazing traditional pastures, novel legumes, or a herb–legume mixture. Methods Six pasture and forage treatments were sown in three replicate 0.4-ha plots and with access to irrigation. Treatments were French serradella (Ornithopus sativus Brot.), bladder clover (Trifolium spumosum L.), forage brassica (Brassica napus L.), lucerne (Medicago sativa L.), lucerne + phalaris (Phalaris aquatica L.), and arrowleaf clover (Trifolium vesiculosum Savi) + chicory (Cichorium intybus L.). The last two treatments were sown in a 1:1 alternate sowing-row (tyne) arrangement. All plots were grazed concurrently from 15 October to 2 December 2014, each plot with a mixed-sex group of 13 weaned lambs: five White Dorper, three White Suffolk × White Dorper and five White Suffolk × Merino lambs. During the grazing period, lambs were weighed each week following an overnight curfew. Starting liveweight (mean ± s.d.) was 32.4 ± 5.5 kg. At the completion of the grazing period, lambs were slaughtered and carcass traits recorded. Key results Lambs grazing arrowleaf clover + chicory had the highest average daily growth rates (ADG) (245 g/lamb.day) and those grazing lucerne + phalaris the lowest (119 g/lamb.day). There was a lag in ADG for the forage brassica treatment over the first 14 days of grazing, and lamb growth plateaued for the bladder clover treatment toward the end of the trial. Overall, ADG for the forage brassica and bladder clover treatments was similar to that for lucerne. Lambs grazing French serradella had similar ADG to 18 November compared with those grazing other treatments, except arrowleaf clover + chicory; however, lambs grazing French serradella were withdrawn from the experiment on 26 November after losing weight. The genotype × treatment interaction was significant for final liveweights, with White Suffolk × White Dorper lambs having higher predicted final liveweight on the arrowleaf clover + chicory pasture (45.2 kg) than White Dorper (43.8 kg) and White Suffolk × Merino (44.0 kg) lambs, and higher liveweight on lucerne than White Suffolk × Merino lambs (42.6 vs 41.4 kg). White Dorper lambs had a higher overall sale value owing to their higher dressing percentage. The proportion of lambs reaching the finished weight target was higher for arrowleaf clover + chicory (0.96) than for any other treatment. Conclusions and implications Using a forage option that maintains high levels of production and quality later in spring can increase the proportion of lambs reaching target weights and the final sale weight of lambs.