Sustainable/Ecological Weed Management

Fairly little is known about smallholder (<1 hectare) weeding practices and their efficacy across sub-Saharan landscape (Dimes et al., 2004). Without understanding a farmer’s current weeding practices, fertilizer applications, soil conditions, and/or reoccurring weed populations, it is difficult to identify effective and realistic weed management strategies (Collinson, 1998). Disconcertingly, many weeding recommendations are based on fixed or predetermined designs conducted on experimental stations. Unfortunately, management (e.g. weeding date) at these stations do not reflect smallholders’ on-farm variability in pest pressure, rainfall, soil erosion or labor availability. Thus the utility and relevance of fixed weeding recommendations to smallholder farmers is limited and potentially erroneous.

Rural farmers are well aware of the repercussions for not weeding thoroughly, but much of the challenge to weed completely or in a timely manner arise from laborer and financial limitations (Kumwenda, 1997).  One of the most critical times for smallholders to weed their intensified grain cropping systems is three to four weeks after sowing. Unfortunately, this period occurs when a smallholders’ food supply and finances for the year have dwindled.  During this time, available household labor will tend to be allocated for off-farm employment (e.g., contract labor) to supplement finances for food before the harvest season (Giller et al., 2011).

In countries such as Malawi, the agrarian population employ highly variable weeding practices. Much of this variation is attributed to their natural, financial and/or social circumstances (Tafirenyika, 2014). The country’s local language, Chichewa, has no fewer than 36 different words that describe weeding actions such as ‘hoeing’ (Orr et al., 2002). This illustrates the complexity of local weeding practices. Several researchers compiled reports detailing descriptions of these manual techniques (see Figure 1) (Orr et al., 2002; Riches et al., 1993; Sileshi et al., 2008).  Researchers agreed these practices were highly advanced and context specific; although, many lacked punctuality, allowing weeds to flower, reproduce, and reemerge (Orr et al., 2009). Based on these findings it would seem frivolous to advise farmers with limited time, income, and labor to weed more timely and often. Rather, it may be more useful to offer alternative techniques that would require less labor and coincide with a farmer’s current crops, weeds, and agroecological/financial conditions.

In short, there are methods to treat weed colonization, such as manual pulling or herbicide application. These control measures, however, require social and financial capital during times when both are most limited. Yet weeds, like all crops, can be controlled or prevented by increasing competition for light, water and nutrients. Altering the soil environment can also limit their growth. Essentially, weeds can be managed by creating a competitive and unsuitable environment. Thus to prevent weeds through ecological management, farmers can utilize a number of modified indigenous techniques such as: intercropping, perennial integration, crop rotation, push-pull technology, increased crop densities, redundancy, mulching or modified fertilizer application.

For example, there are a number of crop rotations that have been successful in eradicating common weeds from smallholder plots such as Striga. In the sub-Sahara, rotation or the use of intercrops (especially with “trap crops”) that include cowpea, yellow gram, pigeon pea, soybean, groundnut, sweet potatoes, cassava, or cotton have shown promise (Rodenburg and Johnson, 2009). In reviewing these crops, pigeon pea has shown potential in suppressing Striga from its ability to utilize iron-bound phosphorous (P) in alfisols. By increasing total P availability, soil fertility is enhanced; thus reducing parasitic-potential (Ae et al., 1993). Furthermore pigeon pea’s dropped biomass can produce an aboveground blanket. This blanket in turn smothers annual weed growth and has been particularly successful with rice rotation (Roder et al., 1997). Snapp et al., (2002) adds that farmers who utilized double-up legume systems (i.e., bush legumes with an understory legume) profited more from total food production as compared those who intercropped. In South Africa cowpea rotation with maize has been utilized by smallholders and yielded positive results (Stringer et al., 2009).  Other noxious weeds, namely blady grass (Imperata cyndrical) have been controlled in West Africa by rotating grains with velvet bean (Buckles et al., 1998; Chikoye et al., 2000; Place et al., 2003). Despite these benefits, farmers must be willing and able to put land into rotation, which will be limited by landholdings sizes and additional socioeconomic constraints.

Climate change will likely compound the battle smallholders face against weeds. As temperatures rise and droughts protract, C4 plants will gain a growing advantage over C3 plants like rice (Rodenburg et al., 2011). With little resources (e.g., biomass, irrigation) to increase soil organic material or moisture, smallholder lands may degrade more rapidly, making them a breeding ground for weeds. Solutions like no-till soil management have been developed for the anticipated events. Yet, when such resolutions require expensive inputs like herbicide, the tradeoff can be too drastic for smallholders (Robertson and Swinton, 2005).

Learning lab member Timothy Silberg is investigating integrated approaches for controlling weeds under intensified grain systems that are adapted to Malawian smallholder weeding practices, crops, and socioeconomic and agroecological conditions.

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