Weed management in agricultural systems is the single greatest input of labor and chemicals (Dayan et al., 2011; Wang et al., 2009). Weeds compete with the crops for moisture, nutrients, and light. Therefore, weeds, more so than any other pest, have the greatest adverse impact on crop yields (Dayan et al., 2011). Yield losses from 40% to 80% have been cited because of weed populations left unmanaged and outcompeting cash crops for resources (Akobundu, 1987; Karlen et al., 2002). The cost of weed control, along with the associated yield losses due to weed competition, has been estimated at more than $15 billion annually in developed nations (Buhler, 2003). Because of their highly effective mode of action and relative low cost, synthetic herbicides have been a reliable tool for weed control in conventional systems (Dayan et al., 2011; den Hollander et al., 2007). However, the increasing demand for organic food and concerns over the potential detrimental effects these herbicides have on human health and on the environment have driven demand for nonchemical alternatives for managing and controlling weed populations (Dayan et al., 2009; den Hollander et al., 2007; Webber et al., 2014).
In organic systems, herbicide options for weed control are limited (Peruzzi et al., 2007). This creates a challenge for organic farmers and is the largest obstacle for producers considering transitioning from conventional production systems (Barberi, 2002). Weeds are an even bigger problem in low-input systems and, in particular, with slow-growing vegetable crops such as pepper that are inferior competitors (den Hollander et al., 2007; Isik et al., 2009). This is especially problematic when it comes to managing perennial weeds (Wedryk et al., 2012). Organic farmers tend to rely on physical, cultural, and mechanical techniques such as mulching, smother cover crops, cultivation, and direct manual weeding to suppress weeds (Bilalis et al., 2010; Isik et al., 2009; Mulvaney et al., 2011).
Intercropping is a potentially effective cultural control strategy for weeds in organic production systems. Intercropping is the practice of growing two or more crops within the same area such that there is biological and agronomic interaction (Mohler and Stoner, 2009; Vandermeer, 1989). Often used as a way to introduce biodiversity into agroecosystems (Unlu et al., 2010), intercropping can also be used in conjunction with other practices such as soil solarization and cover cropping as a management strategy for reducing pest pressure and suppressing the spread of disease, particularly in organic and low-input farming systems.
The ability of the cash crop to outcompete weeds is enhanced in intercropping systems through an increase in resource capture by the desired species which reduces the availability of resources such as light, water, and nutrients to weeds (Barberi, 2002; Baumann et al., 2002; Saudy, 2015), as well as through allelopathic interactions (Iqbal et al., 2007). However, the extent of competitive interactions depend on factors such as crop geometry, canopy architecture, planting density, planting time, and crop growth rate (Isik et al., 2009; Keating and Carberry, 1993). Plants with different forms can be intercropped to create a more complex multilayer system that mimics natural ecosystems (Denevan, 1995). The ability of a multilayer intercropping system to suppress weed growth is typically owed to a reduction in light transmittance due to an increase in canopy density (Baumann et al., 2002). Aboveground plant biomass has been found to be positively correlated with the interception of solar radiation by the canopy (Kiniry et al., 2005). For example, Bilalis et al. (2010) found that a maize (Zea mays)–legume intercrop led to higher canopy cover and more efficient use of solar energy, thereby increasing weed suppression. Therefore, the ability of complex multilayered mixed cropping systems to maximize canopy density and increase radiation interception makes them a potentially useful tool for weed management in organic and low-input farming systems (Saudy, 2015).
We previously reported that an increase in plant functional diversity led to overyielding of harvestable fruit; i.e., increased overall production per unit land area (Franco et al., 2015). Although it is likely that several factors may have contributed to overyielding in mixed cropping systems, it is possible that weed suppression may have played a key role in our observations. In this article, we evaluate the ability of watermelon to reduce weed biomass and explore the relationships between weed suppression, yields, aboveground plant biomass, and LAI. The primary objective of this study was to test the ability of watermelon and diverse intercropping systems to suppress weeds in a low-input organic production system. We hypothesized that cropping systems consisting of watermelon and more architecturally complex multilayer canopies would reduce weed pressure as compared with monocultures; i.e., less complex systems. We also hypothesized that this reduction in weed biomass would be the result of increased leaf area and aboveground plant biomass intercepting solar radiation. The species used in this study were peanut, watermelon, okra, cowpea, and pepper.
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