Relations between summer crops and ground cover legumes in a subtropical environment 1. Effects of a Vigna trilobata ground cover on growth and yield of sorghum and sunflower

Abstract

An annual form of Vigna trilobata (L.) Verdc., a semi-domesticated herbaceous legume, was used to provide additional ground cover under rainfed crops in a subtropical environment. This study reports effects of V. trilobata on growth and yield of sorghum ( Sorghum bicolor (L.) Moench) and sunflower ( Helianthus annuus L.). Crops were sown on a vertisol in south-east Queensland in October and January at densities of 10 and 5 plants m −2 for sorghum and 5 and 2.5 plants m −2 for sunflower, and with or without ground cover legume. Crop rows were 0.8 m apart. V. trilobata seed was broadcast at 150 seeds m −2 when the crops were sown. While a full profile of soil water at sowing, and 330 to 424 mm rainfall during crop growth, led to vigorous growth of both crops and ground cover, crops from both sowings experienced water stress from about anthesis to the middle of grain fill. At the higher crop density, yields of sorghum grain were 453 and 355 g m −2 and yields of sunflower seed were 161 and 227 g m −2 after sowing in October and January, respectively. Yields were decreased to 87%, 70%, 76% and 90% respectively in the presence of up to 330 g m −2 of dry matter of V. trilobata. Sorghum yields were marginally lower at the lower density and decreased relatively more in the presence of ground cover. Neither the yield of sunflower seed, nor the effect of ground cover on it, was affected by density. Competition from ground cover decreased radiation interception by crops through decreasing their leaf area index. It did not appear to affect the efficiency of conversion of intercepted radiation to dry matter. Tiller number in sorghum was decreased by ground cover, except in the low density stand sown in October. Effects of ground cover on the contribution of tiller heads to yield were more severe, leading to a halving of their contribution at the lower density. Lower grain yields in the presence of ground cover were due to lower numbers of seeds m −2 in both crops. Seed size was also smaller in sunflower, but the percentage of oil was not affected. We conclude that under well-watered conditions in south-east Queensland the decreases in yields of summer crops in competition with V. trilobata are likely to be outweighed by the anticipated long-term benefits from using ground cover to decrease soil erosion. The need for information on the processes and consequences of competition under drier conditions is emphasized, so that we can evaluate the broader potential for ground cover legumes in the subhumid subtropics.