Effects of improved fallow species and phosphorus application on weed biomass, composition and maize yield in western Kenya

Low soil fertility, in particular phosphorus deficiency, limits maize grain yield to less than 1 t ha-1in many areas of western Kenya. This is confounded by losses from weed competition that is often aggravated under conditions of low soil fertility. An integrated package involving improved fallows with fast growing trees and phosphorus fertilizer is reported to be a promising option to improve soil fertility and crop yields. Little is, however, known about the effects of this package on weed dynamics. This study compared the effect of improved fallow species with other land use systems with or without P application after the fallows on weed biomass and species composition. The study was conducted in 1998/1999 cropping seasons at two farms (Osita and Khwisero) in western Kenya. The experimental design ~as a split-plot replicated four times. The main plots were four land use systems (continuous maize cropping, natural fallow,Crotalaria grahamiana fallow and Tithonia diversifolia fallow). The subplots were plus and minus P applied to maize after the fallows at the rate of 50 kg ha-lThe fallows were established without P application and were clear-cut for cropping after 10 months. At the time of cutting the fallows, crotalaria and tithonia fallows at Osita farm had no weeds present, probably because of the higher standing tree biomass (8.8 t ha-l for tithonia and 13 t ha-l for crotalaria). This resulted in better ground canopy cover than those at Khwisero (4.2 t ha-l for tithonia and 6.7 t ha-l for crotalaria). In contrast to the tree fallows systems that had no weeds, considerable amount of weeds (0.2 to 2.1 t ha') were present in both the continuous maize systems and natural fallow at both sites. Because of the good weed control by the fallows at Osita, weeding frequency in this system was reduced to only two times instead of three for the continuous maize and natural fallow systems. Phosphorus application increased weed biomass in all land use systems by between 1.7 and 2.6 times at both sites. Weed species composition was not affected by P application but fallows significantly cont~olled some weeds such as Richardia brasiliensis until the third weeding at Osita. Overall, the results of the study suggest that while P application may enhance weed biomass and, hence induce a higher weeding frequency, the use of improved fallows with fast-growing trees in rotation with crops may mitigate this problem. This, however, seems to depend on the fallow species used and how well it is established.