Some aspects of competition between a tropical grass and a tropical legume : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science in Plant Science at Massey University
Verano stylo (Stylosanthes hamata c.v Verano) and Guinea grass (Panicum maximum c.v Coloniao) are two tropical pasture species reported to be superior in performance to many other species in the northeast of Thailand. A mixed pasture of these two species, therefore, has a potential to produce a high herbage yield in terms of both quantity and quality. Little is known, however, about the compatibility of these two species. A glasshouse experiment was set-up to establish competitive situations between these two species. The experimental design was based on the de Wit model (Replacement series principle). Dry matter yield per plant of both species decreased markedly when the grass proportion increased. The reduction in dry matter yield was not proportional to the increase in grass proportion. Branch number in legume and tiller number in grass was the yield component most sensitive to plant competition. The results of relative replacement rate analysis indicated that during early stages of growth verano stylo was very sensitive to competition from grass. Verano stylo appeared to compete with the guinea grass more successfully after it had approximately 20 leaves. An increasing grass proportion had no affect on the shoot/root ratio of the guinea grass but decreased the shoot/root ratio of legume plants Plant height of guinea grass was decreased by increasing plant competition while legume height was not affected. In contrast, leaf area distribution of legume was affected by increasing plant competition while that of guinea grass was not affected. Increasing plant competition decreased herbage quality in both species as measured by the leaf/non-leaf ratio. In addition, under severe competition legume plants also showed a reduction in leaf nitrogen concentration. Flowering time of verano stylo was markedly affected by competition from guinea grass. Flowering occurred after 7 weeks in the monoculture. In association with grasses flowering was delayed on average 11 weeks in two treatments while in the mixture containing the highest proportion of grass the legume plants remained vegetative throughout the trial. The results demonstrated that there was no yield advantage from any of mixtures between these two species over the monoculture under the conditions of this study. One of the posssible reasons for this severe suppression of verano stylo from the guinea grass plants could have been associated with a consequent reduction in the legume capacity to fix nitrogen. The legume monoculture appeared to produce a higher yield than the other combinations in terms of protein content. Management strategies to help overcome legume suppression are discussed. Options such as reducing grass population relative to legume, establishing the legume before the grass, earlier defoliation, and/or the strategic application of fertilizers.