Abstract:
Constraints to effective weed management may be the main reason for the small area under minimum
tillage (MT) in smallholder farming in southern Africa. The effect of maize residue mulching and
intensity of hand hoe weeding on the growth of weeds, cowpea (Vigna unguiculata cv. IT 86D-719) and
sorghum (Sorghum bicolor cv. Macia) was investigated in the fifth and sixth years of a conservation
agriculture (CA) field experiment at Matopos Research Station (28830.920E, 20823.320S). The experiment
was a split-plot randomized complete block design with three replications. Tillage was the main plot
factor (conventional tillage – mouldboard plough compared against MT systems – ripper tine and
planting basins) and maize residue mulch rate (0, 4 and 8 t ha 1) the sub-plot factor. Hoe weeding was
done either four times (high weeding intensity) or twice (low weeding intensity) during the cropping
season. Planting and weeding were done at the same time in all treatments. There was markedly greater
early season weed growth in MT systems relative to mouldboard plough (MBP) in both crop species. In
sorghum, MT (planting basins: 40.3 kg ha 1; ripper tine: 34.8 kg ha 1) systems had higher cumulative
weed biomass measured after planting than MBP (29.9 kg ha 1) system. Maize mulching was generally
associated with increased mid- to late-season weed growth in the two crops probably due to improved
soil moisture conservation during periods of low precipitation. Weed suppression by the maize mulch
was observed only in sorghum and limited to early in the cropping season with no effect observed for the
remainder of the sorghum rotation phase. The high weeding intensity treatment had lower weed growth
in both crops and better sorghum yield than low weeding intensity. The MT systems had poor crop
establishment which translated into low yields. Cowpea grain yield obtained from MT systems was less
than 300 kg ha 1 compared to 413 kg ha 1 in MBP. The poor sorghum establishment in MT systems
translated into low grain yield as sorghum grain yield was lowest in planting basins (2602 kg ha 1) and
highest in MBP with 4159 kg ha 1. Results suggest that CA systems require early and frequent hoe
weeding even after four years to reduce weed infestations and improve crop growth. This higher demand
on a smallholder household’s limited labor supply throughout the cropping season will be a key
determinant of the spread and adoption of CA in southern Africa.
