Relations between summer crops and ground cover legumes in a subtropical environment 2. Effects of sorghum and sunflower on the growth of Vigna trilobata and Medicago scutellata

Abstract

Vigna trilobata (L.) Verdc. is a low growing legume with potential as a ground cover for controlling soil erosion in cropping lands of the subhumid/semi-arid subtropics of northern Australia. When it was grown beneath sorghum ( Sorghum bicolor (L.) Moench) and sunflower ( Helianthus annuus L.) crops under well-watered conditions on a vertisol in southern Queensland it decreased their yields by about 20% in the first season. This paper reports the effects of competition from crops sown at a normal dryland density or at half the normal density on its growth. V. trilobata sown in pure stand in October produced 531 g m −2 of above-ground dry matter, and sown in January produced 323 g m −2, by the time the grain crops sown at the same time were mature. Yields under mature sorghum sown at the normal density (10 plants m −2) in October and January were 38% and 35% respectively of those from pure stands of ground cover sown at the same time. Under sunflower at the normal density (5 plants m −2), corresponding values were 42% and 22%. The decrease in dry matter yield attributable to competition for water, nutrients and/or radiation from the main crops was proportional to the decrease in intercepted radiation. V. trilobata responded to shade in several ways, but a substantial increase in specific leaf area, together with some additional partitioning of dry matter to leaf, was sufficient to ensure that its leaf area index under crops was similar to that in pure stands. Seed yields of 116 and 84 g m −2 were obtained from pure stands of V. trilobata sown in October and January respectively. However, seed yields under crops sown at the normal density were only 4–18% of those measured in pure stands. Seed size was not affected by treatment. We conclude that, under well-watered conditions in southern Queensland, V. trilobata grown as a ground cover under crops of sorghum and sunflower should be able to produce sufficient biomass to decrease soil erosion, and also produce sufficient seed for regeneration in successive seasons. Its growth and suitability are also briefly compared with those of an alternative ground cover legume, Medicago scutellata (L.) Mill. (snail medic).