The Effect of Organic and Conventional farming systems on weed seed bank, density and dominance in Murang’a and Tharaka Nithi Counties, Kenya

Abstract

Low crop productivity in Africa can be attributed to factors such as poor soil fertility, infestation by insect pests and diseases. Weeds have also been reported to cause drastic yield loses of up to 25% in sub-Saharan Africa. This can be attributed to millions of weed seeds available in every cropping season that competes for nutrients, light, and water with crop thus reducing both yields and quality. Therefore, weeds are crop pests that need proper management if optimal food production is to be achieved. Conventional farming i.e., use of herbicides, mechanical weeding and slash and burn, have been used to control weeds but with detrimental effects to the environment. For this reason, organic weed control measures such as use of cover crop, push and pull technology, crop rotation and use of biological vectors has been actively promoted as sustainable weed control strategies that ensures environmental integrity. However, the efficiency of conventional and organic weed management strategies within high and low input investment is scantly documented. This study seeks to address the following objective; i) to determine weed density in organic and conventional farming system in Murang’a and Tharaka-Nithi Counties, ii) to identify the type of weed species that dominates in organic and conventional farming system in Murang’a and Tharaka-Nithi Counties and, iii) to determine the effects of crop rotational practices on soil weed seedbank in organic and conventional farming system in Murang’a and Tharaka-Nithi counties. The study was conducted in two long term on-station trials set up in Murang’a and Tharaka-Nithi Counties. The trials were laid out in a randomized complete block design with four treatments replicated four times in Tharaka-Nithi and five in Murang’a with test crops i.e., cereals, vegetables, legumes and tuber planted within six season comprising two season per year in a three-year crop rotation. For each trial, weed species emerging from the soil samples, their density and effect of crop rotation on soil weed seedbank will be assessed within two nutrients levels i.e., low and high N and P application rates, under conventional and organic systems. A supplemental irrigation during dry period coupled with 225 kg N ha^-1 and 125 kg P ha^-1 will represent high nutrient input levels applied in commercial systems while 45 kg N ha^-1 and 26 kg P ha^-1 shall be used to mimic the rain-fed low nutrient practices common in smallholders farming systems in the region.  Soil was sampled at a depth of 0-20 cm at the end of every season and transported to KALRO Kabete laboratory. In the laboratory to determine soil weed seedbank density and composition, weed seedling emergence method was used. This involved counting of weed seedling emerging from the soil sample to estimate the size of soil weed seedbank. Analysis of variance was used to determine weed density and dominance in organic and conventional farming systems and the impact of crop rotation on soil weed seedbank. Results from the study will contribute to recommending the efficient weed control methods with respect to farming system and crop rotation.