Speaker
Description
Introduction
Crop diversification is widely promoted as a sustainable intensification strategy due its positive effects on the land use efficiency (LUE) of crop production and its contribution to ecosystem services. Yet, such effects are highly variable, and an increase in LUE can mask decreases in individual crop yields. This can be a problem for farmers, who may value each crop differently. Variability in both LUE and individual crop yield translates into uncertainty for farmers that, alongside land and labour constraints, may discourage adoption of diversified systems.
To reduce this uncertainty, a better understanding is needed of how crop diversification strategies respond to the environment in which they are implemented. In this study, we investigated how yields of maize and legume crops and LUE varied along a rainfall gradient under different diversification strategies on smallholder farms in Zambia. We aimed to determine whether one diversification strategy consistently outperformed others, or whether different strategies should be recommended for regions with different growing season rainfalls.
Materials & Methods
We used mixed modelling to explore three years of data (2021-2023) from an on-farm trial network comprising 29 farms spread across two communities of Zambia’s Eastern Province and two communities in Southern Province. These three years and four communities (12 site-years) provided a gradient of rainfall in the growing season from 448 mm to 1034 mm.
Five cropping systems were tested on each farm, a maize monoculture (the control) and four maize-legume diversification strategies: a rotation, an alternate row intercrop, a two-row strip intercrop, and a four-row strip intercrop. All diversification strategies were managed under conservation agriculture (CA). However, the maize monoculture and the maize-legume alternate-row intercrop were also tested with conventional tillage practices. The diversification strategies followed an additive design, so that the same maize plant population of 44,444 plants/ha was adopted for all cropping systems.
Results
Seasonal rainfall influenced the response of both maize and legume yields to different diversification strategies (P<0.05), but had a stronger effect on legume yields. In drier site-years (<700mm rainfall), legume yields aligned with a gradient in the intensity of shading from maize: highest in alternate row intercrops, intermediate in the strip intercrops, and lowest in the rotation. This suggests that in dry conditions, the shading effect of maize modified the microclimate to the legume’s benefit (perhaps reducing heat and evaporation), outweighing the effect of interspecific competition.
Maize appeared to be more sensitive to competition for water. In drier site-years, it yielded lowest in the four-row strip crop, where maize plants are grown more densely within the strip (50cm between rows) leading to higher intraspecific competition. Yields were highest in the rotation, where maize is not tightly spaced (90cm between rows) and does not compete with legumes, but can benefit from their residual effects. Maize was also affected by soil preparation: in both the monoculture and alternate row intercrops, maize yielded relatively more under CA practices in drier site-years, but in wetter site-years, yields were equal (in the monoculture) or better (in the intercrop) under conventional practices.
In terms of the LUE of each cropping system (considering both crops), the alternate row intercrop (under CA only) and the two-row strip crop had higher land equivalence ratios (LER) than other diversification strategies in drier site-years, while the four-row strip crop had a higher LER in wetter site-years. The rotation had a lower average LER than all intercropping systems due to each crop only being present every other year. This difference further increased in dry site-years due to relatively lower legume yields in the rotation.
Discussion
Eastern Province typically has higher growing season rainfall (approx. 800mm) than Southern Province (approx. 600mm), so the four-row strip intercrop would be recommended for Eastern and the two-row strip intercrop or alternate row intercrop (under CA) for Southern. Across Zambia, maize is typically preferred as the staple crop, but legumes offer a richer source of protein and have a higher market price. Overall, our results suggest that tailoring the choice of crop diversification strategies to the expected growing season rainfall (using long-term averages and seasonal forecasts) could be a win-win for yields of both crops.
| Keywords | Competition, crop diversification; facilitation; intercropping; variability |
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