- Vegetables: tomatoes
- Pest Management: allelopathy, biological control, weed ecology
A Virginia study evaluating weed suppression of a biculture cover cropping system compared to a monoculture system determined that there may not be an advantage to the biculture system. Data from this study suggests that a cover cropping system that utilizes oilseed radish significantly decreases weed means. However, as evidenced with this study, the oilseed radish may pose a weed problem later in the growing season because of its inability to sufficiently winterkill in Virginia’s mild winters.
Weed management in vegetable production systems can be difficult because of the limited number of herbicides available and the degree of control necessary to maintain adequate yields (Creamer & Bennett, 1997). According to a 1986 study by Pimental & Levitan, farmers spend more than $3.6 billion annually on herbicides; however weeds still reduce yields by about 10%. An alternative option to herbicide application is the use of cover crops. Farmers incorporate cover crops into their production practices for several reasons, which include nutrient acquisition, erosion control, and weed suppression (Hill et al, 2006). Cover crops can be used in weed suppression by leaving a layer of residue on the soil that serves as organic mulch (Teasdale, 1993). When these residues remain on the soil surface, they can modify seed germination by limiting light availability, decreasing soil temperatures, and altering moisture content (Creamer et al 1996). Other types of interference can occur, which includes allelopathy. Rice (1974) defined allelopathy as harmful effects on one plant species through the release of toxic chemicals into the environment by another.
Allelopathic weed suppression by several plant families has been well documented. Creamer et al (1996) determined that rye, crimson clover (Trifolium incarnatum L.), hairy vetch (Vicia villosa Roth.), and barley (Hordeum vulgare L.) all suppressed the emergence of eastern black nightshade in a field study. They also found similar results when a mixture of these cover crops was used.
Even though cover crops offer a wide range of benefits, including weed suppression, it is important to note that negative effects from alleopathic cover crops has occurred. Putnam (1986) and Teasdale (1996) both reported results where cover crop residue has reduced establishment, growth, and yield of cash crops. In a 2005 study conducted by Ngouajio and Mennan, marketable yield of cucumber was significantly lower when planted behind hairy vetch compared to other cover crops.
While cover crops are an important and necessary component of sustainable agriculture, it is important to understand how these cover crops not only affect weed species, but cash crop species, as well. Cover crop bicultures, which consist of two cover crops planted together with the hopes of complimenting each other’s characteristics, is yet another aspect of sustainable production systems. It is necessary to understand how one cover crop will affect the other when planted in a biculture. If allelopathic effects occur on the cash crop, then there is potential for negative effects on other cover crops.
Fabacaceae (legume) and Brassicaceae are two families of cover crops that are often recommended in a sustainable production system because of their unique properties and benefits on the soil. The use of allelopathic legume cover crops is of great interest because of their ability to fix nitrogen (Hill et al 2006). Several studies have shown the allelopathic potential of legume cover crops (Njoujio & Mennan 2005; Teasdale 1996; Hutchinson & McGiffen 2000). One such group of legume cover crops that have shown strong allelopathic capabilities is the vetches (Njouajio & Mennan 2005, White et al 1989; Hill et al 2006). Vetches, which include hairy vetch, purple vetch, and lana vetch, perform well over a wide range of soils, can fix over 100 pounds of nitrogen per acre and release about half of it to the following cash crop (Schonbeck & Morse 2006). They also make soil phosphorus more available and provide habitats for beneficial insects.
Cover crops in the brassica family, which includes daikon, oilseed, and fodder radishes, are often chosen as cover crops because they are deep rooted crops that can help open subsoil hardpan (Schonbeck & Morse 2006). This characteristic is especially important in areas where traditional tillage has left a layer of hard soil just under the disturbed soil area. Other advantages include conservation of soluble nitrogen and rapid canopy closure to help prevent weed seed germination (Schonbeck & Morse 2006). These cover crops are also known to have strongly allelopathic root exudates, which can leave behind a weed-free seedbed after winterkill.
Planting a brassica and legume cover crop as a biculture could be very beneficial. In areas where traditional agricultural practices, such as mold-board plowing, have left a hardpan under the soil, the brassica cover crop could help break-up this layer. Incorporating a legume cover crop, that will help fix nitrogen, could prove to be very beneficial, especially in areas where the soil contains very low organic matter. Weed suppression could also be increased by incorporating the two families, instead of planting a monoculture cover crop system. However, it is important to understand how these crops will not only affect one another, but the cash crops that would follow behind this system. If increased weed suppression occurred from the biculture system, it stands to reason that cash crops could be negatively affected, as well. It is necessary to conduct field studies examining this system before it is recommended to growers.
The objectives of this project were to determine: 1. compatibility and weed suppression of a brassica and legume cover crop in a biculture production system and 2. effects on a cash crop when brassica and legume cover crops are planted as a mono- and biculture.