Intercropping, the agricultural practice of cultivating two or more crops in the same space at the same time, can make more efficient use of arable land and increase per-hectare profitability for growers (Kahn, 2010; Lithourgidis et al., 2011). Potential benefits of intercropping include increased water, nutrient, and light efficiency and reduced pest pressure (Machado, 2009; Risch, 1983; Sullivan, 2003). Intercropping also may provide insurance against crop failure (Horwith, 1985; Lithourgidis et al., 2011). If one crop of the mixture fails due to adverse environmental conditions, the other crop(s) might withstand the stresses and remain harvestable and profitable (Clawson, 1985). Intercropping has shown to be more productive when the component crops differ greatly in growth duration (Fukai and Trenbath, 1993).
Increasing land productivity and grower profits are likely the greatest benefits of intercropping. Success and efficiency of intercropping are commonly measured using the land-equivalent ratio (LER) (Francis and Decoteau, 1993), developed by Mead and Willey (1980). The LER is calculated as the yield of a crop in an intercrop system relative to the yield of that crop in a monocrop system (Kahn, 2010). The resulting number is a ratio and specifies the amount of land needed to grow intercropped vs. the amount of land needed to grow both crops individually and produce an equivalent yield. Intercropping favors the growth and yield of the species when the LER is greater than 1.0, but when LER is lower than 1.0, intercropping negatively effects the growth and yield of the plants grown in mixtures (Willey and Rao, 1980). However, according to Garnier et al. (1997), evaluating an intercropped vs. a monoculture system based on LER can be deceiving, and they proposed an alternative index. Garnier’s index compares the yield of the intercropped or polyculture with the most-productive monoculture. Garnier proposes that for a polyculture to be better than a monoculture, it must yield better than the best monoculture. Trenbath (1974) had the same conclusion.
Although intercropping may improve land productivity and may improve grower profits, the combination of two or more crops may necessitate significant alterations in normal crop-production practices compared with monoculture of each crop (Willey et al., 1983). Many researchers have reported difficulties in managing combined crops, with negative consequences for one or both crops (Boehner et al., 1991; Lesoing and Francis, 1999; Santalla et al., 2001). Successful multiple cropping requires superior management skills (Bowen and Kratky, 1986). Because agronomic recommendations for many intercrop combinations do not exist (Machado, 2009), it is necessary to delineate management practices for an intercrop combination.
Intercropping watermelon and upland cotton is an effective approach to share land resources and production inputs as well as improve grower profitability and is being rapidly adopted by Georgia growers (Eure, 2014). A major impediment to the success of watermelon–cotton intercropping systems in Georgia is management of glyphosate-resistant Palmer amaranth (Eure et al., 2012, 2013; Tankersley et al., 2011). Appropriate registrations of agrichemicals to be used in both crops can be an obstacle for intercropping as well (Kahn, 2010).
In South Carolina, ≈7000 to 9000 acres of watermelon are planted annually (U.S. Department of Agriculture, National Agricultural Statistics Service, 2017). The greater part of this planted watermelon acreage employs a plasticulture production system incorporating drip irrigation. Of the roughly 158,296 reported acres of cotton planted in South Carolina during 2007, about 9% of the crop received irrigation that year (Payero and Khalilian, 2017), primarily from overhead irrigation systems. This is well below the reported 36% of upland cotton receiving irrigation in the United States in 2007 (U.S. Department of Agriculture, National Agricultural Statistics Service, 2012).
If intercropping of watermelon and cotton is to be successful in South Carolina, it is necessary to define the required modifications in standard practices. Thus, research was conducted to identify those alterations and to determine the feasibility and profitability of intercropping watermelon and cotton.
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