Weed Composition in Maize (zea Mays L.) Fields under Smallholder Conservation Farming

Smallholder farmers in southern Africa have reported on increased labour requirements for hoe weeding due to high weed infestations in conservation farming (CF) fields. However, CF proponents claim that weed pressure and labour requirements for weeding decrease within the first three years under the recommended CF practices. An observational study was carried out during the 2008/09 cropping season on 21 maize fields in Wards 12 and 14 of Masvingo District to determine weed composition in fields that had been under CF for different years. Fields were grouped into CF3(under CF for 2 or 3 years) and CF3+ (under CF for 4 or 5 years) with conventional mouldboard plough tillage (CONV tillage) used as the control group. Participatory Rural Appraisal (PRA) techniques were used to obtain farmer perception of the most limiting constraints in CF. Neither crop residue mulching to provide a soil cover of at least 30% soil cover at planting nor cereal/legume rotations had been practiced in the past four seasons by the 17 farmers reported to be practicing CF. The farmers had only adopted the minimum tillage system of planting basins (PB) and the associated improvements in management. Hereafter, the CF fields will be referred to as PB fields. Tillage had no significant effect on weed density and species composition. However, the first post-planting hoe weeding was done at least 15 days earlier (P < 0.05) in PB than in CONV tillage suggesting higher early season weed growth in PB relative to CONV tillage. Three post-planting weedings were carried out in PB compared to only two under CONV tillage. Farmer ranking of the main constraints in PB were low rainfall > input unavailability > labour > pests. It seemed that farmers were committing PB to small acreages equivalent to the inputs supplied by NGOs. Under these low areas, weeds could be managed by available family labour. At least double the maize grain yield was obtained from PB compared to CONV tillage (mean: 1 052 kg ha) probably as a result of improvements in soil fertility and weed management. However, grain yield decreased with increase in weed density at 3 WAP highlighting the importance of early season weed control in maize. As labour requirements for weeding did not decline with time in PB, there is need to investigate the use of herbicides to