Book Description

Abstract: Concerns about public health and environmental quality due to the use of pesticides in conventional agriculture have driven increased demand for organic products. Although growers have obtained higher prices and demand with organic products, many farmers are reluctant to transition to organic agriculture. Farmers view the challenge of weed management and risk of lower output as barriers to converting to organic production. The mandated three years before organic certification can be used to suppress weeds and improve soil fertility for enhanced yields in the first year of organic production. Smother cropping is an alternative strategy of weed management that uses living plants in monoculture or mixture to control weeds with the potential to improve soil fertility. Potential smother crops and smother crop mixtures, their effectiveness without chemical or mechanical management, mechanisms of suppression, and impacts on productivity under organic management are not fully understood. In this study, we investigated the use of smother cropping and associated transition strategies for weed suppression and productivity through 1) evaluation of smother crop species and mechanisms of weed suppression through a literature review; 2) determining the potential of using tef [Eragrostis tef (Zucc.) Trotter] and warm-season annual crop mixtures; 4) assessing smother crop planting dates; and 5) comparing mechanical and cropping-based organic transition strategies. The results of this research indicate that crop growth and ancillary management practices are most important in determining the effectiveness of smother crops. Exploitation of ecological niches in designing smother crop systems and targeting specific weeds can improve weed suppression. Tef can be used to suppress annual weeds under organic management, but is a weak competitor against Canada thistle [Cirsium arvense (L.) Scop]. In designing smother crop mixtures, the choice of grass species in mixture can affect biomass production. The effect of grass species in crop mixture dynamics may be related to height, morphology, spread, and aggressivity. Multi-species mixtures can increase ground cover by smother crops and reduce the cover of weeds, but are not more effective than monocultures in suppressing weed biomass. Canada thistle is a particularly problematic weed for organic growers and planting smother crop mixtures when root carbohydrate reserves are at a seasonal nadir can improve suppression. Crop mixtures of warm-season, highly competitive crops were most effective at suppressing Canada thistle while a mixture of cool-temperature adapted species suppressed annual weed biomass. Smother cropping and the use of high-diversity prairie species as organic transition strategies were most suppressive of weed density and biomass after three years of transition. Compost application improved vegetable yields in the first organic year, while plant available nutrients had the greatest influence on potato yield and organic matter strongly affected tomato yields in comparison to other soil variables. Transition strategies before conversion to organic agriculture can influence productivity and weed populations. Smother cropping is a viable strategy for organic transition, but the choice of crops and management must be carefully considered in order to realize optimal benefits.