Processes Involved in the Weed Suppressiveness of Hairy Vetch and Implications for Weed Management in Vegetable Production

View the project final report

• 2006 annual report

Commodities

• Agronomic: corn, hay
• Vegetables: carrots, cucurbits, onions, peppers, tomatoes

Practices

• Animal Production: feed/forage
• Crop Production: application rate management, cover crops, crop rotation
• Natural Resources/Environment: biodiversity
• Pest Management: allelopathy, weed ecology
• Soil Management: soil quality/health

Weed population dynamics studies confirmed the weed suppressive effects of hairy vetch in cropping systems. Laboratory studies demonstrated that the effects were species dependent and strongly suggested allelopathy as a major mechanism involved. Under field conditions the inhibitory effect of hairy vetch residue was greatest during the first two weeks, suggesting that growers should avoid planting cash crop during that window of time. In addition to weed suppression, use of legume cover crops like hairy vetch can help reduce nitrogen fertilizer inputs in subsequent crops. This could help improve the long-term sustainability of the farm and reduce nutrient leaching.

Introduction:

Hairy vetch (Vicia villosa Roth) is a winter annual native to Europe and Asia. It is a winter hardy species that can accrue between 4.82-7.68 tons/ha in biomass, and it can contribute approximately 44.8 kg/ha of nitrogen (Ngouajio, personal communication) to the soil due to fixation (UC SAREP 2002). These characteristics along with its allelopathic potential against weeds make hairy vetch an ideal cover crop for regions with a temperate climate.

In 1989, White et al. found that hairy vetch aqueous extracts reduced corn and cotton germinations by up to 44 and 42%, respectively, depending on concentration. Corresponding radicle length reductions were 39 and 62%. In the same study, they found that the germination and radicle growth of pitted morning glory, wild mustard, and Italian ryegrass were all inhibited to some degree in the presence of hairy vetch extract. Screening tests run by Fujii in 2001 showed that water extracts of hairy vetch reduced radicle elongation in lettuce by 88% and hypocotyl growth by 11% compared to a non-treated control. However, germination was not affected. Using a methanol extract of hairy vetch radicle length was reduced by 82%, while hypocotyl growth was decreased by 48%. In this case, germination was decreased by 10% (he used 67g/L to extract for water and 250g/L for methanol).

In 1993, Hoffman et al. conducted a two year field study comparing various killing methods for hairy vetch prior to crop planting. The found that living and chopped hairy vetch reduced weed emergence and thus density, where as the rolled and glyphosate killed treatments did not. All treatments however, were found to reduce corn yield compared to the bare ground, weed-free control. Hairy vetch that is left living has been shown to suppress weeds longer than desiccated hairy vetch; however, weed densities in both were less than a bare ground control (Teasdale and Daughtry 1993). This finding suggests that something other than light transmission or temperature buffering is contributing to the reduction in weed density. More recently, in a two year study Ngouajio and Mennan (2005) reported reduced marketable cucumber yields in the presence of hairy vetch residues compared to a bare ground control during the second year. The yields in the hairy vetch plots during both years were significantly lower than those in the rye plots and sorghum sudan grass plots. This same study hairy vetch was shown to reduce weed density and biomass by 99 and 91%, respectively, compared to the bare ground control.

In cover crops that accrue a lot of biomass, competition appears to play a large role in reducing weed populations. However, the amount of biomass accumulated by hairy vetch does not account for the degree of weed reduction observed, perhaps further evidence of allelopathy (Fujii 2001). Drought conditions have been shown to exacerbate the growth inhibition caused by hairy vetch (Hoffman et al. 1993; Ngouajio and Mennan 2005). Therefore, it follows that the responsible allelochemicals are likely water soluble, resulting in higher concentrations under conditions, such as drought, that reduce leaching.

The allelopathic effects of the cover crops previously discussed are all at varying stages of research and development. Some such as rye and sorghum, have already had their allelochemicals isolated and identified and are looking at how to put that knowledge to use. Others such as hairy vetch are at the beginning stages of collecting evidence to support claims of allelopathy. More information is necessary prior to attempting to identify and understand the allelochemicals and their interactions in hairy vetch (Dakshini et al. 1999).

Understanding the allelopathic effects of hairy vetch on vegetables and weeds could help select the appropriate cover crop and crop rotation. The subsequent works examine the use of hairy vetch improve weed management and the sustainability of vegetable production systems. The objectives of these studies are as follows:

Objective 1. Study of the response of weed populations to hairy vetch residue.

Objective 2. Determination of the role of allelopathy in the weed suppressiveness of hairy vetch.

Objective 3. Measurement of the effects of timing of cucumber planting after hairy vetch kill on yield.