Effects and interaction of lucerne and subtropical legumes in a Sorghum almum pasture

Abstract

An experiment was carried out at Lawes in south-east Queensland to examine the behaviour of subtropical legumes (mixture of Glycine wightii and Phaseolus atropurpureus), of lucerne, and of all three legumes together in a Sorghum almum pasture. Two grazing regimes were imposed, both averaging almost one beast an acre a year, but one involving longer breaks between grazings. A final year of common grazing was used to obtain a better assessment of treatment effects. There was no evidence of any difference in persistence between the Nunbank and Crooble cultivars of S. almum. Initial growth of legumes was poor due to strong competition from S. almum, but in the second year they made a substantial contribution to total yield of dry matter and nitrogen, and in the following two years they also increased significantly the yield of S. almum. In general the sub-tropical legumes yielded more than lucerne, and were also more effective in increasing S. almum yields. There were only minor effects on the nitrogen content of S. almum. Lucerne and subtropical legumes were strongly competitive when sown together, in that each yielded less and had a smaller effect on S. almum than in separate mixtures with the grass ; the effects of lucerne were particularly marked. These competitive effects were less pronounced with less frequent grazing. Soil nitrogen in the 0-12 inch soil layer declined during the first two years of the pasture but increased during the following two, the accumulation occurring mainly in the surface 3 inches of plots containing legumes. Over the total pasture phase (four years), there was a small nett loss of nitrogen from the 0-12 inch layer. Volunteer species associated with a follow-up crop of Sorghum vulgare showed positive responses to residual nitrogen from lucerne but no response to subtropical legumes. It is suggested that this was associated with a slower turnover of nitrogen from lucerne root systems. There was no response in the crop itself.