Weed Control Studies on Soybean in the Coastal Shrub and Thicket Vegetation Belt of Central Region

Abstract

Protein malnutrition, major dietary problem in developing countries, Ghana included, requires greater attention today than ever before. Soybean, a high protein legume has recently been accepted by Ghanaians as an alternative to groundnuts and cowpea in their diet. On weight for weight basis, the yield of this crop is approximately twice that of meat, cowpea, beans and four times that of cereals. Besides soybean has less pest and disease problems as compared to other well established legumes. However, weed competition is one of the major constraints in the production of the crop. Thus the experiment was to determine:- (a) the critical competition period so as to establish the most economical weeding regime for the crop. (b) the most practical and cost effective weed management practice in soybean fields and (C) residual action of two pre-emergence herbicides, Galex and Stomp, on the field by bioassay techniques. The studies were carried out at the Teaching and Research Farm of the School of Agriculture, University of Cape Coast, during the 1994 Major and the 1995 major seasons. The field experiments were set up as RCBD with four replicates at a spacing of 75cm x 5 cm. Ten weeding regimes were tried in the 1st experiment while the second experiment consisted of 12 treatments which included varying rates of Stomp, Galex, hoeing periods and their combinations. The third experiment employed a bioassay technique using sorghum and millet as test plants to determine residual levels of the two pre-emergence herbicides.

The studies showed 67% and 81% yield reduction as a result of unchecked weed growth in 1994 and 1995 respectively. At 70% attainable yields, the critical periods of competition were found to be 23-35 and 20-32 days after emergence in the 1994 and 1995 seasons respectively. Also, soybean grain yield and its parameters and plant growth decreased significantly as duration of weed competition increased from 10 to 40 days after crop emergence. The herbicides studied gave a relatively higher percentage weed control in the 1994 minor season than in the 1995 major season. This was attributed season to the dominance of Cyperus rotundus L. and Trianthema postulacastrum L. which are known to be efficient plants and therefore better competitors than soybean, in the 1995 major season weed population. The studies showed that both herbicides proved effective against weeds at 2.0 kg/ha except on Cyperus rotundus. In both seasons of study, manual weed control resulted in a financial Advantage and no level of residual, concentration, likely to prove injurious, phytotoxic or detrimental to subsequent crop growth and development was detected.