The effect of weeds on growth and yield of beans (phaseolus vulgaris)
Field studies were conducted in 1979 and 1980 at the Field Station, University of Nairobi, to evaluate dry bean (Phaseolus vulgaris, L) varieties Canadian Wonder, Mwezi Moja and Rose Coco growth and yield as influenced by competition from annual mixed weeds and different weeding .treatments , In one set of experiments, the beans were left to grow in weeds the first four and eight weeks respectively after planting or left to grow with weeds the entire season. In another set the beans were freed of weeds the first four and eight weeks respectively after planting and thereafter kept weed free or were weeded the entire season. The leaf areas, dry weights of whole plants, shoots, roots, stems, leaves as well as plant height and number of branches per plant were determined at intervals during the phase of the vegetative growth. At the end of the growth period, grain yield and yield components were determined. Season-long weed competition reduced bean grain yields by 49.5, 55.5 and 58.0% with ample moisture in 1980 and by 53.0, 58.0 and 67.0% when moisture was limiting in 1979 for Canadian Wonder, Mwezi Moja and Rose Coco respectively. In all instances, the pod number per plant was the most severely affected component of yield and showed similar trend of weed effects to those observed in grain yields. Seeds per pod and weight per seed were not affected by weeds except in cultivar Rose Coco when moisture was limiting in 1979, when in addition, the number of seeds per:'pod was significantly reduced by weed competition. Plant growth as measured by the total plant dry weight and the dry weights of other plant parts named above were significantly reduced by weed competition lasting eight weeks and beyond after planting except stems in Mwezi Moja. Similarly LAI and number of branches per plant were significantly reduced by weed competition beyond the reproductive phase (six to eight weeks after planting). There was no need to keep the cultivar free from weeds beyond the fourth week after planting. However, upto and beyond eight weeks in weeds the bean growth and yield were significantly reduced.