Research in Latin America has shown numerous benefits of rotations with cover crops under conservation agriculture (CA) to manage N and weeds; however, information from Zimbabwe is limited. The objective of the study was to investigate the benefits of rotating maize (Zea mays L.) with different leguminous and nonleguminous cover crops on maize yield, weed management, and decomposition rates of residues. A study was conducted at University of Zimbabwe (UZ), Domboshawa Training Centre (DTC), and two sites at Henderson Research Station (HRS) with contrasting soil types (clay and sand) from 2008 to 2014. Nine different cover crops were rotated with maize and compared with maize monocropping. During the 2012 to 2013 and 2013 to 2014 cropping seasons, a litter‐bag experiment was set within the experiment at UZ and DTC to investigate residue decomposition rates. Decreases in weed numbers of up to 94% were observed in maize–black sunnhemp (Crotalaria juncea L.) treatment at DTC. At UZ, maize after jack‐bean [Canavalia ensiformis (L.) DC.] (7823 kg ha−1) showed up to 61% more yield than continuous maize (4851 kg ha−1). Maize yields after fodder radish (Raphanus sativus L. var. oleiformis Pers.) were, in most cases, not significantly different from those of uniform maize. Decomposition rates were faster in leguminous residues, with velvet‐bean [Mucuna pruriens (L.) DC. var. utilis (Wall. ex Wight) Baker ex Burck] residues being generally the fastest and therefore not suitable for ground cover. Despite the positive effects of rotations, not a single cover crop was identified that provided all the benefits. There is need to characterize niches for specific cover crops within the farming system to assist farmers in their choices and spatial arrangements of cover crops on their farms.
Please cite this article in press as: B. Mhlanga, et al., Effects of relay cover crop planting date on their biomass and maize productivity in a sub-humid region of Zimbabwe under conservation agriculture, NJAS -Wageningen J. Life Sci. (2016), http://dx.
a b s t r a c tRelay cropping of cover crops is a strategy of increasing biomass yields and productivity of maize-based systems. However, there is need to strategically plan the relay cropping to avoid competition between the main crop and the relay cover crops while at the same time obtaining optimum yields from both crops. A study was carried out in a clay soil in a sub-humid region of Zimbabwe to investigate the effect of introducing different relay cover crops at 8, 11 and 15 weeks after planting maize (WAPM) into a standing maize crop on biomass yield of the relay cover crops, their emergence and maize yields in the 2012-13 and 2013-14 seasons. From the results of the study, it was observed that the introduction of relay cover crops late in the season compromises their emergence and hence biomass yields (as low as 0.8 kg ha −1 for blue lupins (Lupinus angustifolius var. angustifolius (L.)). In a season where longer mid-season dry spells were experienced , biomass yields of the relay cover crops were lower than in 2012-13 season. Delays in planting of relay cover crops (i.e. from 8 to 11 and from 11 to 15 WAPM) resulted in yield reductions of around 50%. Relay cover crops introduced at different periods of the season had no significant effects on maize grain and biomass yields. However, there are relay cover crops such as the velvet bean (Mucuna pruriens (L.) DC) and common oats (Avena sativa L.) that showed better emergence even in the sub-optimal conditions (with emergence as high as 90%). Of all the investigated relay cover crops, none could contribute to significant amounts of biomass thus insignificant increases in total plot biomass. There is need to investigate on other earlier planting dates that do not compromise the biomass productivity of such relay cover crops.
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