Sodium deficiency in lucerne (Medicago sativa) forage in southern Australia and the effect of sodium and barley supplementation on the growth rate of lambs grazing lucerne

Abstract

Context Liveweight gains (LWGs) of lambs grazing lucerne (Medicago sativa L.) in the mixed-farming zone of Australia commonly do not match predicted rates of growth. It has been suggested that this could be due to sodium (Na) deficiency or an imbalance in the crude protein:metabolisable energy ratio (CP:ME). Aims This research aimed to determine whether sodium concentration in lucerne is below requirements for growing lambs and whether supplementation with salt (NaCl) and/or barley grain would increase LWG of lambs grazing lucerne. Methods Pluck-samples were collected along a transect in 65 paddocks containing lucerne pasture in southern Australia. Herbage that most likely represented the animal diet was ‘plucked’, avoiding the less digestible plant parts. Two replicated grazing experiments using different cohorts of lambs compared LWG of unsupplemented lambs with lambs given supplements of salt and/or barley. Key results The survey of lucerne pastures found that Na concentrations were below published requirements (0.07 g Na/kg DM) for growing lambs in 85% of samples collected from dryland lucerne pastures, but were above lamb requirements in 95% of samples collected from lucerne pastures gown under spray irrigation, which suggests a degree of salinity in the irrigation water. Supplementation with salt or barley did not affect LWG of lambs in Experiment 1, and residual feed on-offer did not differ among treatments. Salt increased LWG by 14% in Experiment 2 (292 g/lamb.day vs 256 g/lamb.day in those without access to salt; P = 0.03), with a return on investment of 1220%. Supplementation with whole barley did not increase (P > 0.05) LWG of lambs grazing lucerne. Barley supplementation resulted in higher residual feed on-offer in Experiment 2, suggesting a substitution of barley for lucerne. Mean LWG in Experiment 1 (192 g/lamb.day) was lower than in Experiment 2 (274 g/lamb.day), suggesting that the lack of response to salt supplementation in Experiment 1 may have been due to lamb growth rates being restricted by other factors such as reduced leaf availability and diet quality. Conclusions and implications Results of the survey and grazing experiments confirm that Na concentration of dryland lucerne in southern Australia is commonly below lamb requirements. Producers should consider Na supplementation for lambs grazing dryland lucerne pastures as a way to increase lamb growth rates.