Relationships between age and biomass of individual plants and seed production in two grazed tropical legumes. 1. Derivation of relationships

The distributions of individual plant age and biomass of Chamaecrista rotundifolia cv. Wynn and a mix of Stylosanthes scabra cvv. Seca and Fitzroy in grazed grass–legume pastures were examined to determine their effect on seed production. The effects of enhanced soil water conditions and severe defoliation on seed production were assessed in ungrazed plots. These experiments were part of a larger study to develop a demographic model of perennial forage legumes. The distribution of individual plant age and biomass was highly skewed towards a large number of young/small plants, with fewer old/large plants. Lack of seed set when stem length was less than approximately 200 mm, and in most small plants (<2 g), resulted in older/larger plants contributing far more to seed production and, to a lesser extent, legume biomass, than they did to legume plant numbers. C. rotundifolia seed production was linearly related to individual plant biomass but was highly varaiable and was greatly reduced in swards containing >3000 kg/ha of grass. Using log-transformed data, plant biomass accounted for 74% of the variation in seed production (SP), but together with grass biomass accounted for 91% of the variation [ln(SP) = 6.01 + 0.91*ln(BIOMASS) – 0.28*ln(GRASS BIOMASS), P < 0.001]. Total legume biomass accounted for only 44% of the variation in seed production. S. scabra herbage allowance (kg legume/head) had a major impact on seed production. Total legume biomass and individual plant biomass alone accounted for less than 40% of the variation in seed production. Using herbage allowance (HA) as well as individual plant biomass improved the prediction of seed production (SP) to account for 74% of the variation [ln(SP) = 0.11 + 1.14*ln(BIOMASS) + 0.24*ln(HA), P < 0.001]. Enhanced soil water conditions increased the biomass of individual plants of both species and increased the seed production per gram of plant in S. scabra but not in C. rotundifolia. Severe defoliation in early summer or autumn can greatly reduce or even eliminate seed production by some plants by removal of flowers, reducing individual plant biomass, or allowing insufficient time for plants to reach minimum stem lengths. The different factors affecting seed production in the 2 species highlight the complexity of legume seed set in grazed pasture systems, and some implications for grazing management and modelling are discussed.