The effects of weed and insect pest control regimes on the growth and yield of two varieties of cowpea (Vigna unguiculata (L) walp)

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Experiments were conducted to study the effect of four weed control regimes; (no weeding, one-hand weeding, herbicide (Galex) and weed free, and two pest control regimes (no spraying and spraying) on two varieties of cowpea (Asontem and Soronko). The effects of these treatments on the physiology of the crop weed infestation, and some flower pest populations and damage to grains were assessed. Four experiments were conducted between 1995 and 1997. Results revealed that leaf area index (LAI), net assimilation rate (NAR), crop growth rate (CGR), and relative growth rate (RGR) were generally higher in the major seasons than in the minor seasons, with the highest value recorded in the 1997 major season. The no weeding treatment recorded the lowest value for the growth functions. However, the one-hand weeding, herbicide (Galex), and the weed free treatments did not differ in their effects on the growth functions. The study showed that Soronko recorded higher LAI and CGR than Asontem, due to its lager leaf size and hence superior weed competitive ability. The one-hand weeding treatment recorded the highest grain yield mainly because of its greater number of pods per m2. No weeding treatment gave the lowest grain yield and yield components in all the seasons. Highest yield and yield components were achieved in the 1997 major season whiles the lowest was obtained in the 1995 minor growing season. Grain yields were higher in the sprayed than the unsprayed treatments. The cowpea variety, Soronko recorded higher grain yield than Asontem in all the growing seasons, mainly due to the higher number of pods per m2, lower grain damage and better competitive ability with weeds. Sprayed treatments experienced lower insect pest population and damage to grains than the unsprayed treatments. Results of the trial showed negative correlation between grain damage and grain yield. Maruca pod borer (MPB) damage was lowest in the herbicide (Galex) treatment, whiles pod sucking bug (PSB) population and grain damage were lowest in the weed free treatment. Soronko recorded lower flowering pest population and damage than Asontem. The MPB population was lowest in the 1996 major and highest I the 1996 minor seasons, respectively. The PSB population was lowest in the 1997 major and highest in the 1996 minor seasons. The treatment interactions performed better in terms of the number of pods per m2 and grain yield in both varieties. One-hand weeding at 3- weeks after planting with insecticide application gave the best result in the interactions Soronko again was preferred to Asontem in the treatment interactions. The seasons did not significantly affect the treatment interactions on yield and yield components of the two varieties. The weed species population was generally lowest in the 1 997 major but highest in the 1996 major season. The no weeding treatment recorded the highest weed population at the end of the growth periods except for the 1997 major season. Apart from the weed free treatment, the lowest weed population was recorded in the herbicide (Galex) and one-hand weeding treatments for the 1995 minor and 1996 growing seasons respectively. Generally, the weed species population was lower in Soronko than Asontem, whiles the sprayed treatment also had lower weed population than the unsprayed treatment. The weed biomass at final harvest negatively correlated with grain yield. It recorded the highest value in the 1995 minor and the lowest in the 1096 major season. The no weeding treatment recorded the highest weed bioniass. The weed biornass 1 the one-hand weeding and herbicide (Galex) treatments were 1ot signiflcantly (P > 0.05) diffirent although the herbicide (Galex) treatment recorded the lowest weed hiomass .part from the weed free treatment. The cultivation of the crop is recommended in the major season for a higher yield. Similarly, Soronko is preferred to Asontem in both the minor and major seasons. One- hand weeding at 3-4 weeks after planting in combination with insecticide application is the best practice in the cultivation of the two varieties. A pre-emergence herbicide for instance Galex with its dual effect on both broad leaf and grasses could also be taken advantage of use in combination with insecticide protection at flowering for a good cowpea yield. The control of flowering and post-flowering insect pests must be controlled to achieve a higher cowpea yield. The result of the trial demonstrated that weed control at 3 to 4 weeks after planting with the spraying of appropriate insecticide was critical in cowpea production. The LAI, CGR and the number of pods per m2 appear to contribute most to the yield of the two varieties. Agronomic practices should therefore be directed to factors, which promote these functions. It is recommended that if weed competition can be minimised by at least one-hand weeding at the critical stage with good pest management practices, yield in cowpea can be comparable to weed free condition throughout the growth period. Insect pest controls during flowering and post-flowering stages are very essential to prevent damages to reproductive structures. Furthermore, insect pest monitoring strategies should be further developed to enable an average farmer usage thereby avoiding blanket spraying of insecticides.
A thesis submitted to the Board of Postgraduate Studies, Kwame Nkrumah University of Science and Technology, Kumasi, in partial fulfilment of the requirement for the award of the Degree of Master of Philosophy in Agronomy, 1998