The intensive vegetable production in the low desert valleys of southern California has traditionally relied on high inputs of pesticides and synthetic fertilizers. However, recent concerns about increased regulatory pressure to reduce nutrient input from agricultural fields to the nearby Salton Sea and decreased investment return have encouraged growers to adopt more sustainable production techniques such as crop rotation and the use of cover crops (Hartwig and Ammon, 2002).
Cover cropping may provide nitrogen for cash crops, add organic matter to soil, and improve the physical properties of soil (Aguiar et al., 2001; Creamer and Baldwin, 2000; Hartwig and Ammon, 2002). At the same time, planting cover crops could help suppress pest populations and reduce soil erosion, nutrient leaching, and contamination of surface and groundwater (Hartwig and Ammon, 2002; Hutchinson and McGiffen, 2000). However, cover crops could also reduce the subsequent crop yield (Al-Khatib et al., 1997; Scott and Knudsen, 1999). Although the practice of cover cropping is highly recommended in sustainable production, cover crops and crop management practices have to be carefully selected and investigated to maximize these benefits.
The common vegetable crop rotation in the low desert provides a niche in the cropping cycle that is ideal for a warm season cover crop. Lettuce is planted in the fall (September to October) followed by spring muskmelon in February to March. After muskmelon harvest in June and July, there is not enough time to grow a third cash crop before the next lettuce season in the fall. In addition, excessive summer temperatures preclude the production of many vegetables in the area (Hutchinson and McGiffen, 2000). Production fields are traditionally fallowed during the hot summer months, creating a gap in the production season of 2 to 3 months to integrate cover crops into the vegetable production system.
Summer cover crops that fit into current desert production practices have to be heat-tolerant and capable of producing adequate biomass in a relatively short cycle (Aguiar et al., 2001). Cowpea and sudangrass are adapted to tropical conditions and tolerate high temperatures (Creamer and Baldwin, 2000). Cowpea is a leguminous cover crop used in many parts of the southern and western United States (Ehlers et al., 2002; Ehlers and Hall, 1997; Hall and Frate, 1996). Sudangrass is also a common warm season forage crop that forms a dense canopy that effectively shades out weeds. Both cowpea and sudangrass have been shown to suppress weeds and other pests when rotating with cash crops and to recycle excess nitrogen in the soil (Danso et al., 1991; Hutchinson and McGiffen, 2000; Roberts et al., 2005; Wang et al., 2006, 2008).
The objective of this work was to evaluate cowpea and sudangrass as potential cover crops for improvement of growth and yield in a lettuce–muskmelon rotation system in the low desert of southeastern California under three management systems, which included conventional, integrated crop management (ICM), and organic.
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