Agroecological studies of Desmanthus: a tropical forage legume

The use of forage legumes in tropical regions to improve the efficiency of animal production from grazing has been limited, largely because of the lack of economic incentives. There is a clear need, therefore, to investigate the existing gene pool of tropical forage plants to assess their potential for pasture improvement. Therefore, the present study evaluated the agronomic and ecological aspects of plant development in a set of genotypes of the genus Desmanthus and their relationships with the components of the surrounding environment, at different stages of growth, in a series of laboratory and field experiments. Accessions of the genus Desmanthus formed permanent soil seed banks that ranged from 281 to 1303 seeds/m2, with a large variation between genotypes, in experiments on a duplex soil on the Douglas Campus of James Cook University, Townsville. Genotypes originally collected in Argentina had larger seed banks than those of other tested genotypes, but a small number of surviving plants. Fire increased seedling recruitment in almost all observed genotypes. Temperatures observed during controlled grass-fires reached a maximum of 300 ºC at the soil surface, 80 ºC at 10 mm depth, and around 30 ºC at 30 mm depth suggesting that all seeds located at soil surface were killed, those at 10 mm depth were probably softened, and those at 30 mm or more in soil had no alteration in their seed-coat permeability. Changes in strophiolar structure and germination, in response to the variation of oven temperatures ranging from 25 ºC to 120 ºC were observed in seeds of nine genotypes of Desmanthus. There were two groups with different patterns of responses: genotypes in which strophiolar structures were not significantly affected by temperatures below 80 ºC; and genotypes with significant changes in the strophiolar structures when temperature rose to 60 ºC. Seedlings of 8 accessions of the Desmanthus complex, growing directly under trees in open savanna woodland had higher values of means for number of leaves/plant, height of plant, and number of plants surviving than seedlings growing between trees. Three years after sowing, all plants from the between-canopy environment had died, while many plants of accessions TQ88, CPI 79653, and CPI 91162 were thriving under the tree canopy. Plants of D. virgatus CPI 78382 and D. leptophyllus TQ 88 growing in soils collected from under and between canopies had significantly increased their seedling emergence, by increasing shade levels and watering frequency. A low number of seedlings died in both genotypes, growing in soil from under the canopy but, plant deaths drastically increased in seedlings grown in soil from between the canopy. Growing in soil collected from under-canopies, plants allocated most of their dry matter to the production of aerial, rather than the underground parts, however, when grown in soil from between-canopies environment the largest proportion of the total dry matter was diverted to the underground parts. This diversified behaviour of biomass allocation for shoot and root in the two soils is thought to be controlled by the contents of nutrients in soil. Seven accessions of the Desmanthus complex, sown into a pasture as seeds or seedlings, under two levels of competition with the natural vegetation, showed to have differentiated behaviour according the different treatments. Plant establishment and dry matter yields of plants sown by seed into unaltered vegetation were significantly reduced by competition. The effect on liveweight changes and wool growth of Merino sheep of 200 g hay of four different forms of the Desmanthus complex included as a supplement to a diet of 600 g Mitchell grass (Astrebla spp.) was compared with 200 g hay of Stylosanthes hamata cv. Verano. Verano and D. virgatus CPI 79653 supplemented diets had the highest dry matter digestibility (46.52% and 44.94% respectively). All the legume supplemented diets produced significantly more wool than the control. Clean wool growth was significantly correlated with nutritional parameters. The levels of nitrogen and sulphur present in some Desmanthus genotypes shows the potential of these plants in promoting wool growth