Relationships between age and biomass of individual plants and seed production in two grazed tropical legumes. 2. Validation of models

Abstract

Relationships predicting legume seed production from total legume biomass and from age and biomass of individual legume plants in a grazing trial at CSIRO Narayen Research Station, in south-eastern Queensland, were compared with observed soil seed measurements over 9 years. This was part of a larger project to develop a demographic model of tropical perennial forage legumes in grazed pastures. Suitable data were available from Chamaecrista rotundifolia–Cenchrus ciliaris pastures stocked at 0.54 and 1.09 head/ha and from Stylosanthes scabra– C. ciliaris pastures at 1.09 head/ha.

Use of legume plant age and biomass, in conjunction with grass biomass (C. rotundifolia) or kg legume/head (S. scabra), to predict soil seed accounted for 87% (C. rotundifolia) and 77% (S. scabra) of the variation in observed values, and gave accurate and unbiased predictions. Prediction of soil seed from total legume biomass was better correlated with observed values for S. scabra (r² = 0.72) than for C. rotundifolia(r² = 0.63), but over-predicted values for S. scabraand under-predicted those for C. rotundifolia.

The results highlight that, with C. ciliaris, the grass biomass needs to be kept below 3000 kg/ha to maintain a C. rotundifolia population. Similarly, the results indicate that biomass of S. scabra may need to be kept below 400 kg/head to avoid the possibility of S. scabra dominance, depending on the companion grass.

Application of the seed production models in assessing the agronomic adaptation of these legumes and in predicting persistence of legumes and other species (e.g. weeds) is discussed.