R.M. JONES1, C.K. McDONALD1, R.J. CLEMENTS2 and G.A. BUNCH1
1CSIRO Tropical Agriculture, Brisbane, Queensland
2ACIAR, Canberra, Australian Capital Territory, Australia
Five tropical legumes were compared in terms of persistence, productivity and effect on cattle liveweight gain. The experimental site in sub-tropical, subhumid Queensland had a long-term average rainfall of 703 mm, but during the 10-year study the average was 530 mm and there were 4 consecutive years with below 500 mm. The site had previously supported improved pastures of buffel grass (Cenchrus ciliaris). The legumes, grown with buffel grass, were: Wynn cassia (Chamaecrista rotundifolia); siratro (Macroptilium atropurpureum); stylo — a mixture of Fitzroy and Seca shrubby stylos (Stylosanthes scabra) and fine stem stylo (S. hippocampoides), and Bargoo jointvetch (Aeschynomene falcata). Bargoo was continuously grazed at only 1 stocking rate but the other legumes had 3 or 4 continuous stocking rates ranging from 0.54–1.45 yearling heifers/ha.
There was no effect of legume species on live-weight gain. This was largely attributed to the dry years and low levels of legume, but also to the fact that the experimental site had enhanced soil nitrogen status from the previous experiment where buffel grass was grown with a productive legume or fertilised with nitrogen.
Siratro failed to persist over the 10-year period because there was inadequate recruitment to compensate for the loss of the original plants. The original plants persisted for up to 4 years but siratro was unable to build up seed reserves, which reached a peak of only 80 seeds/m2 in one treatment in one year. Persistence of fine stem stylo, although poor, was slightly better than that of siratro as there was more seed set and seedling recruitment. Cassia plants were short-lived, very rarely surviving for 3 years, but cassia was able to persist because it set large amounts of seed in good rainfall conditions and, on some occasions, small amounts under low rainfall. Seed reserves reached a peak of > 4000 seeds/m2. Shrubby stylo and Bargoo had much better persistence of individual plants than cassia, some plants persisting for over 6 years. They were able to build up reasonable seed reserves, reaching over 1000 seeds/m2 in some years. The highest plant density of cassia, shrubby stylo and Bargoo was recorded in the last year of the experiment.
In general, stocking rate had much less effect on legume persistence than rainfall. However, grazing management can be used to assist legume persistence and control legume:grass balance. Some suggestions for management of the different legumes are given.