Effect of Integrated Weed Management Practices on Weeds Infestation, Yield Components and Yield of Cowpea [Vigna unguiculata (L.) Walp.] in Eastern Wollo, Northern Ethiopia

Cowpea [Vigna unguiculata (L.) Walp.] is usually infested and its yield is adversely affected by a number of weed species that compete with the crop from germination to harvest, affecting the crop yield adversely. Therefore, an experiment was conducted at Sirinka and Jari, northern Ethiopia during the 2013 main cropping season (July-October). The objectives were to assess the effect of pre-emergence s-metolachlor and pendimethalin on weeds, and growth, yield components and yield of cowpea and to investigate the possibilities of supplementing low doses of herbicides with hand weeding for effective and cost effective weed management. There were 12 treatments comprising: s-metolachlor (1.0, 1.5 and 2.0 kg ha-1); pendimethalin (1.0, 1.3 and 1.6 kg ha-1), s-metolachlor at 1.0 kg ha-1 + hand-weeding at 5 weeks after crop emergence (WAE), pendimethalin at 1.0 kg ha-1 + handweeding at 5 WAE, one handweeding at 2 WAE, two handweeding at 2 and 5 WAE, weed free and weedy checks. The treatments were arranged in randomized complete block design with three replications. 78.6% of the weeds comprised in the experimental sites were the broadleaved. At 20 DAE, application of 2.0 kg ha-1 s-metolachlor at both locations resulted in the lowest broadleaved weeds, sedge and total weed density. Pendimethalin failed to control Commelina benghalensis and Xanthium strumarium. At 55 DAE, low rate of s-metolachlor and pendimethalin when superimposed with one hand weeding were as effective as complete weed free treatment in reducing the broadleaved weeds and sedge density. The minimum weed dry weight was registered with the application 2.0 kg ha-1 of s-metolachlor in both locations; however, at 55 days and harvest, weeds accumulated significantly lower dry weight due to1.0 kg ha-1 s-metolachlor 1.0 kg ha-1 pendimethalin superimposed with hand weeding at both locations. The interaction of location with weed management practices was significant on days to 50% flowering and physiological maturity of the crop, number of pods plant-1, grain and aboveground dry biomass yield and yield loss. The maximum grain yield (4277 kg ha-1) was obtained in complete weed free treatment at Sirinka which was statistically equivalent with complete weed free and two hand weeding treatments at Jari and Sirinka experimental sites respectively. Due to weed infestation throughout the crop growth, the highest yield loss (70.8%) was recorded at Jari while it was 47.5% at Sirinka. The highest gross benefit was obtained with the application of 1.0 kg ha-1 of s-metolachlor superimposed with hand weeding followed by two hand-weeding at 2 and 5 WAE. Therefore, managing the weeds with the application of 1.0 kg ha-1 of s- metolachlor + hand weeding and hoeing 35 DAE proved to be the most profitable practice. However, under the condition of labour constraint and timely availability of the herbicide, pre emergence application of 2.0 kg ha-1 of s-metolachlor should be used to preclude the yield loss and to ensure maximum benefits.