Effect of Pre-emergence Herbicides on Weeds, Nodulation and Yield of Cowpea [Vigna unguiculata (L.) WALP.] at Haik and Mersa in Wollo, Northern Ethiopia

The experiment was conducted at Mersa and Haik, northern Ethiopia during the 2014 main cropping season to assess the effect of pre-emergence s-metolachlor and pendimethalin on nodulation, and yield of cowpea. 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 + hand weeding at 5 WAE, one hand weeding at 2 WAE, two hand weeding at 2 and 5 WAE, weed free and weedy checks. The treatments were arranged in randomized complete block design with three replications. The minimum weed dry weight was registered with the application 2.0 kg ha-1 of s-metolachlor at both locations; however, at 55 DAE and at harvest, weeds accumulated significantly lower dry weight due to s-metolachlor 1.0 kg ha-1 and pendimethalin 1.0 kg ha-1 each pendimethalin superimposed with hand weeding at both locations. The interaction of location with weed management practices was significant on number and dry weight of nodules, 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 Mersa which was statistically in parity with complete weed free and two hand weeding treatments at Haik and Mersa, respectively. Due to weed infestation throughout the crop growth, the highest yield loss (70.8%) was recorded at Haik while it was 47.5% at Mersa.