Cover crops improve soil health and can be beneficial for weed management by suppressing weeds at different stages. During the cover crop growth period, cover crops can reduce weed growth and seed production through direct competition or allelopathic effects (Chou, 1999; Liebman and Davis, 2000). Live cover crops such as red clover may also provide a beneficial habitat for weed seed predators such as crickets or carabid beetles, which can increase weed seed mortality (Davis and Liebman, 2003). After incorporation into soil, cover crop residues may suppress weeds by inhibiting weed emergence and growth through allelopathy (Kumar et al., 2008b; Weston, 1996), immobilizing nitrogen (Dyck and Liebman, 1994; Kumar et al., 2008a; Samson, 1991), mulch effects (Teasdale, 1998; Teasdale and Mohler, 1993, 2000), or through interactions with pathogens (Conklin et al., 2002). In many northern states of the United States, short-duration cover crops such as buckwheat, yellow and brown mustard, and oats can be planted in spring before short-duration, late-planted vegetable crops for weed management in vegetable cropping system.
Brassica cover crops can suppress weeds (Al-Khatib et al., 1997; Boydston and Al-Khatib, 1994; Haramoto and Gallandt, 2004; Krishnan et al., 1998), nematodes (Mojtahedi et al., 1993), insects (Blau et al., 1978; Williams et al., 1993), and diseases (Angus et al., 1994; Sarwar et al., 1998). Brassica plants contain glucosinolates that are further hydrolyzed by the enzyme myrosinase to form isothiocyanates (ITCs), compounds toxic to a variety of soilborne plant pests, including weeds (Brown and Morra, 1997; Morra and Kirkegaard, 2002). Reports have described the role of ITC in inhibition of plant growth and seed germination (Petersen et al., 2001; Teasdale and Taylorson, 1986). Greenhouse and field studies have reported the role of brassica cover crop residues in reducing emergence and growth of weeds (Al-Khatib et al., 1997; Boydston and Hang, 1995; Haramoto and Gallandt, 2004; Krishnan et al., 1998). Among mustard cover crops, brown and yellow mustard are readily available, inexpensive, and thought to have good weed-suppressive potential. For example, Shuler et al. (2005) found that brown mustard residue was effective at reducing weeds in a subsequent snap bean crop. Yellow mustard (var. Idagold) has high glucosinolate content and is therefore thought to have good weed-suppression potential (Haramoto and Gallandt, 2005b).
Buckwheat (Fagopyrum esculentum Moench) is commonly used in organic vegetable production and has potential as a short-duration cover crop in both organic and conventional systems. Its fast growth and short life cycle make it an ideal choice for niches where the soil would otherwise be left bare during short periods in the late spring or summer. Studies have demonstrated the strong weed-suppressive abilities of buckwheat during crop growth (Creamer and Baldwin, 2000; Golisz et al., 2002; Iqbal et al., 2003; Tominaga and Uezu, 1995). Buckwheat residues may also contribute to weed management by inhibiting emergence and growth of weeds (Haramoto and Gallandt, 2005b; Xuan and Tsuzuki, 2004). Freshly incorporated buckwheat residues reduced and delayed the emergence of redroot pigweed (Amaranthus retroflexus L.) and lambsquarters (Chenopodium album L.) (Haramoto and Gallandt, 2005b). Pellets made of buckwheat shoots reduced the dry weight (60%) and density (75% to 80%) of weeds in paddy rice (Oryza sativa L.) (Xuan and Tsuzuki, 2004). Like with mustard cover crops, chemicals isolated from buckwheat have inhibitory effects on weeds (Golisz et al., 2007; Iqbal et al., 2002, 2003; Xuan and Tsuzuki, 2004). In field and growth chamber studies, soil amended with buckwheat residues reduced emergence of various weeds, including Powell amaranth (Amaranthus powellii S. Wats) (Kumar, 2008). For some weed species (e.g., Capsella bursa-pastoris), this suppression was the result of immobilization of nitrogen, whereas for Powell amaranth, allelochemicals concentrated in shoot tissue appear to have played an important role (Kumar et al., 2008a, 2008b).
Hairy galinsoga [Galinsoga ciliata (Raf.) Blake], a summer annual broadleaf weed with worldwide distribution, can reduce yields from 10% to 50% in a wide range of vegetable crops (Warwick and Sweet, 1983). Chemical weed control options for this species are limited within many vegetable crops because many registered herbicides have limited efficacy against this weed. Moreover, hairy galinsoga can produce up to1.3 million seeds/m2 in unmanaged fields (Brainard, unpublished data). Hairy galinsoga can also produce viable seeds very rapidly, often within 35 to 40 d of plant emergence (Brainard, 2002; Warwick and Sweet, 1983). Most studies evaluating weed-suppressive ability of cover crops during cover crop growth period estimate only reduction in biomass and do not estimate number of weed seed produced. It is very important to estimate number of seeds produced by hairy galinsoga under cover crop canopies at cover crop termination in the context of a short-season green manure crop followed by a short-season vegetable crop. This is because fresh hairy galinsoga seeds are nondormant, and seed produced during the cover crop growth period will create problems in the succeeding vegetable crops. Competition from a rye cover crop reduced hairy galinsoga seed production and delayed the timing of seed production (Brainard, 2002). Because hairy galinsoga seeds are relatively short-lived (3 to 4 years), practices that prevent seed production can be useful in rapidly reducing hairy galinsoga populations (Warwick and Sweet, 1983). The potential for cover crops to inhibit seed production and emergence of this weed in subsequent vegetable crops has not been examined.
Although cover crop residues can suppress weed emergence and biomass production, crop establishment and yield may be affected. However, large-seeded species (e.g., snap beans, corn) are more tolerant to physical and chemical stresses caused by cover crop residues than are many small-seeded weed species (Westoby et al., 1996). Adverse effects of residues on seed germination are also avoided when transplants are used. However, effects of cover crop residues on transplant growth have not been extensively tested.
Because nitrogen is an important stimulant of weed seeds of many weed species and a critical factor in the growth of both weeds and crops, the C:N ratio and decomposition rates of cover crops can have important effects on weed–crop competition.
The central objectives of this study were to examine the: 1) effects of spring-sown buckwheat, brown mustard (Brassica juncea L.), yellow mustard (Sinapis alba L.), and oat (Avena sativa L.) cover crops on hairy galinsoga growth and seed production during cover crop growth; and 2) residue effects of these cover crops on establishment of hairy galinsoga and four short-duration vegetable crops planted immediately after cover crop incorporation. We hypothesized that cover crops would reduce seed production of hairy galinsoga during cover crop growth and that cover crop residues would suppress hairy galinsoga more than large-seeded [snap bean (Phaseolus vulgaris L.) and pea (Pisum sativum L.)] and transplanted [lettuce (Lactuca sativa L.) and swiss chard [Beta vulgaris var. cicla (L.) K. Koch] vegetable crops.
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