Leonard Rusinamhodzi

Benefits and challenges of crop rotations in maize-based conservation agriculture (CA) cropping systems of southern Africa

Conservation agriculture (CA) based on minimum soil disturbance, crop residue retention, crop rotations and associations are being promoted in southern Africa to reverse the decline in soil fertility and crop productivity. While agronomic benefits of rotations are known, farm level benefits of rotations in CA systems and how they fit in the smallholder farming systems have not been sufficiently addressed. This paper summarizes agronomic results from 2005 to 2011 of maize in rotation and association with different crops in Malawi, Mozambique, Zambia and Zimbabwe. Rotation with or without legumes improved water infiltration (between 70 and 238%), soil moisture, soil carbon, macro-fauna and crop productivity. However, due to poor market conditions, rotations with legumes were less profitable than maize during the study period. Farmers have fewer difficulties to abandon tillage and there is scope to retain crop residues in situ in areas of limited crop–livestock competition but the adoption of rotations and associations is constrained by socio-economic factors that need to be addressed before all principles of CA can be applied.

Conservation agriculture in Southern Africa: Advances in knowledge

The increasing demand for food from limited available land, in light of declining soil fertility and future threats of climate variability and change have increased the need for more sustainable crop management systems. Conservation agriculture (CA) is based on the three principles of minimum soil disturbance, surface crop residue retention and crop rotations, and is one of the available options. In Southern Africa, CA has been intensively promoted for more than a decade to combat declining soil fertility and to stabilize crop yields. The objective of this review is to summarize recent advances in knowledge about the benefits of CA and highlight constraints to its widespread adoption within Southern Africa. Research results from Southern Africa showed that CA generally increased water infiltration, reduced soil erosion and run-off, thereby increasing available soil moisture and deeper drainage. Physical, chemical and biological soil parameters were also improved under CA in the medium to long term. CA increased crop productivity and also reduced on-farm labor, especially when direct seeding techniques and herbicides were used. As with other cropping systems, CA has constraints at both the field and farm level. Challenges to adoption in Southern Africa include the retention of sufficient crop residues, crop rotations, weed control, pest and diseases, farmer perception and economic limitations, including poorly developed markets. It was concluded that CA is not a ‘one-size-fits-all’ solution and often needs significant adaptation and flexibility when implementing it across farming systems. However, CA may potentially reduce future soil fertility decline, the effects of seasonal dry-spells and may have a large impact on food security and farmers’ livelihoods if the challenges can be overcome.

Crop Rotations and Residue Management in Conservation Agriculture

Yield increases and sustainability of conservation agriculture (CA) systems largely depend on systematic crop rotations and in situ crop harvest residue management coupled with adequate crop nutrition. In this chapter, the beneficial effects of crop residue management and crop rotations on maize (Zea mays L.) grain yield in CA systems under rainfed conditions are explained through a meta-analysis. The effects of crop residue management are most beneficial under rainfed conditions as rainfall distribution is often erratic and seasonal dry spells common. The meta-analysis was based on the weighted mean difference (WMD) effect size using the random effects model. Yield advantages of CA systems over conventional tillage systems were only significant when in rotation, under low rainfall conditions and with large N fertiliser inputs. The WMD for CA with continuous maize ranged from − 1.32 to 1.27 with a mean of − 0.03 t ha−1, and when rotation was included the WMD ranged from − 0.34 to 1.92 with a mean of 0.64 t ha−1. Mulch retention under low rainfall ( 1000 mm per season) reduced the yield advantage with the WMD ranging from − 1.2 to 0.02 with a mean of − 0.59 t ha−1. CA is likely to have the largest impact in low-rainfall environments where increased infiltration of rainfall and reduced evaporative losses are achieved by retaining crop residues. However, it is in these areas that achieving sufficient crop residues is a challenge, particularly in mixed crop–livestock systems where crop residues are needed for livestock feed in the dry season. The results suggest that CA needs to be targeted and adapted to specific biophysical as well as socioeconomic circumstances of farmers for improved impact. The ability of farmers to purchase fertiliser inputs, achieve sufficient biomass production as well as produce alternative feed will allow them to practise CA and possibly achieve large yields.