Comparative effects of annual medics (Medicago spp.) and nitrogen fertiliser on the herbage yield and quality of subtropical grass pastures in southern inland Queensland

Abstract

A comparison was made between the winter annual legumes Medicago scutellata cv. Robinson (snail medic) and M. truncatula cv. Jemalong (barrel medic) and nitrogen (N) fertiliser as a means of increasing herbage dry matter yields and N contents, and extending the growing season, of introduced winter-dormant tropical grass pastures in southern inland Queensland. Pastures containing either the forage sorghum hybrid cv. Silk, buffel grass or green panic were grown under dryland conditions on high and low fertility soils at 3 sites (Warwick, Gayndah and Roma) with decreasing winter rainfall respectively, and under irrigation on the high fertility soils at Warwick and Gayndah. Medics were adapted to all soils selected except the low fertility soil at Gayndah. Under irrigation or adequate rainfall, DM yields of medics were 8-10 t/ha at each site. Under dryland conditions medic yields varied with winter rainfall, reaching half of their apparent potential over 4 years at Warwick and one third of their potential at Gayndah and Roma (4 years and 6 years respectively). The yield of grass in irrigated grass-medic pastures was over twice that of unfertilised grass and similar to that of grass fertilised with 100 kg N/ha.year. Under dryland conditions medics increased associated grass yields by 25-100% at Warwick and Gayndah, the grass yields being equivalent to N fertiliser at 50 kg/ha.year on grass alone. At Roma where summer rainfall was low, medics had no effect on grass yields. Total yields of grass-medic pastures were up to 5 times greater than those of unfertilised grass. Medics increased the N concentration in summer grass growth and extended the growing season by producing high quality dry matter in winter and spring. The results indicate considerable potential for expanding the use of annual medics in dryland pastures.