Weed control under conservation agriculture in dryland smallholder farming systems of southern Africa. A review

Abstract

Human-induced soil degradation has led to declining yields and soil fertility in many parts of the world. Conservation agriculture has been proposed as a strategy to ensure more sustainable land use. While conservation agriculture, based on minimum soil disturbance, crop residue retention, and diversification may improve a range of soil characteristics and can be a potential cropping system for improving farmer resilience to climate change, increased weed pressure is often an impediment to its widespread adoption in southern Africa. Weed control under conservation agriculture in other countries has been linked to increased herbicide use, but concerns about herbicide resistance, access to chemicals, and environmental impacts highlight the need for alternative weed control strategies accessible for smallholders. Farmers in semi-arid regions contend with the additional challenge of low biomass production, which may limit the weedsuppressing benefits of permanent soil cover. This paper reviews the regional applicability of various mechanical (manual weeding, weeding using animal traction, weed seed harvest), thermal (soil solarization, weed steaming, flaming), chemical (herbicides, seed coating), and cultural (crop competition, crop residue retention, intercropping, crop rotation) weed control strategies. For each strategy, benefits and challenges were assessed and contextualized with the circumstances of rainfed smallholder farmers in southern Africa. We found that (1) no single solution can solve all weed control challenges under current conservation agriculture systems; (2) success of weed control strategies is largely contingent upon site-specific conditions, including soil type, dominant weed species, and socioeconomic factors; and (3) practices new to southern Africa, such as weed steaming, merit localized research. Previous reviews have addressed various weed control strategies, but a comprehensive review of strategies available to smallholder farmers in semi-arid southern Africa is lacking. Finding a suitable combination of weed control strategies is critical for encouraging smallholder farmers to adopt and maintain conservation agriculture practices. Keywords Mechanical weed control. Chemical weed control. Herbicides. Cultural weed control. Semi-arid agriculture. No-tillage

Sustainable Weed Management for Conservation Agriculture: Options for Smallholder Farmers

“…In smallholder situations, with limited land area and little mechanization, light surface scraping of the soil with the hand-hoe can be enough to initiate a phytochrome-mediated germination process in certain weed seeds [74]. Emerged seedlings can then be eliminated during a subsequent surface scraping operation [53] and this light operation can be considered compatible with CA. Phytochrome-mediated weed seed germination is also reduced with direct planters designed to cover the minimally disturbed seeding slot with soil and mulch to create 'invisible' no-tillage seeding and so reduce weed germination and emergence [75].…”

“…It can only be considered as potentially applicable in situations of low biomass production which limit soil surface mulch cover with its weed-suppressing attributes. Solarization is achieved by placing plastic sheeting over the soil surface [53] which increases the soil surface temperature to a level sufficient to kill weed seeds. The problem is that high temperatures can also be lethal to bacteria and fungi [53] and so this method of weed seed elimination is not entirely compatible with CA and should only be used for particularly problematic weeds, such as noxious weeds that farmers have the obligation by law to eliminate from their fields.…”

“…Solarization is achieved by placing plastic sheeting over the soil surface [53] which increases the soil surface temperature to a level sufficient to kill weed seeds. The problem is that high temperatures can also be lethal to bacteria and fungi [53] and so this method of weed seed elimination is not entirely compatible with CA and should only be used for particularly problematic weeds, such as noxious weeds that farmers have the obligation by law to eliminate from their fields. Yellow nutsedge (Cyperus esculentus L.) can be controlled by using clear plastic sheet cover for 90 days during a summer fallow [53]; but, of course no crop could be grown during this time.…”

“…The problem is that high temperatures can also be lethal to bacteria and fungi [53] and so this method of weed seed elimination is not entirely compatible with CA and should only be used for particularly problematic weeds, such as noxious weeds that farmers have the obligation by law to eliminate from their fields. Yellow nutsedge (Cyperus esculentus L.) can be controlled by using clear plastic sheet cover for 90 days during a summer fallow [53]; but, of course no crop could be grown during this time. The practice is also of little use against weed seeds that have been deeply buried by soil inversion tillage.…”

“…Soil erosion and soil degradation, inherent in tillage-based systems, increase environmental pollution from agricultural chemical inputs such as fertilizers and pesticides [53]. In fact, soil erosion and degradation jeopardize sustainability of crop production and ecosystem services as well as threaten global food security in the long run [54].…”