Florida is the largest citrus-producing state in the United States, and with 302,929 ha bearing fruit it, accounts for 74% of U.S citrus production (Florida Agricultural Statistics Service, 2006). Total crop value of Florida citrus during 2004–2005 was US$742 million. However, conventional citrus growers are facing increased international competition, relatively low citrus prices, new virulent diseases, competition with residential development for land and water resources, and more stringent environmental regulations (Athearn, 2004).
Because excessive use of potentially harmful agrochemicals may affect biodiversity, environmental quality, food safety, and farmers' health, there is increased interest in more sustainable production systems including organic farming (Reganold et al., 2001). With reduced synthetic pesticide and fertilizer use, organic production can preserve both groundwater resources and fragile ecosystems (Mader et al., 2002). Conversion to organic production also allows growers to benefit from special marketing niches and grower-friendly price mechanisms (Athearn, 2004).
Weeds compete with crops for water, nutrients, and light. Serving as potential hosts for insect pests and diseases, weeds can also interfere with soil tillage, irrigation, and harvest operations (Liebman and Davis, 2000), and thereby increase labor requirements and production costs. Adequate weed control is one of the main challenges during the conversion to organic systems. Changes in weed population dynamics during this process requires implementation of alternative weed management strategies (Bond and Grundy, 2001).
Weed management has been identified as the primary concern of organic farmers (Sooby, 2003; Walz, 1999). For organic citrus groves, weed control accounted for more than 30% of annual production costs and the majority of the labor costs (Muraro et al., 2003). Included in these costs are disking, mowing, and hand labor to remove vines growing into tree canopies or weeds near tree trunks of young trees. The majority of Florida citrus growers interviewed in 2001 expressed a strong interest in the use of cover crops (CC) to prevent soil degradation and to suppress weed growth (Scholberg, unpublished).
Cover crops may suppress weeds by either reducing resource availability (Ngouajio and Mennan, 2005) or by inhibiting weed growth via allelopathy (Reberg-Horton et al., 2005). Access to light, nutrients, water, and soil as affected by CC may affect weed persistence (Ngouajio and Mennan, 2005) and the composition of weed flora in citrus groves (Wright et al., 2003). Cover crop residues may also alter soil microbial ecology or increase microbial diversity, resulting in enhanced weed seed predation by soil microorganisms and decreased weed seed vigor (Gallagher et al., 1999; Ngouajio and McGiffen, 2002), or may affect weed population dynamics (Jordan et al., 2000). Cover crops may also increase soil C and N content, two principal components regulating soil biological activity (Wagger et al., 1998), and thus increase the presence of beneficial organisms that suppress biological competitors such as weeds (Kremer and Li, 2003), parasitic nematodes (Wang et al., 2006), and soil pathogens via allelochemicals (Bailey and Lazarovits, 2003).
Successful weed suppression using summer CC has been reported for annual crops such as rice (Oryza sativa L.) preceded by pigeon pea (Cajanus cajan L.) as a CC (Roder et al., 1998), and corn (Zea mays L.) by using velvet bean (Mucuna atropurpureum L.) (Caamal-Maldonado et al., 2001). Using annual winter CC such as winter rye (Secale cereale L.) (Fennimore and Jackson, 2003) or crimson clover (Trifolium incarnatum L.) and subterranean clover (Trifolium subterraneum L.) (Barberi and Mazzoncini, 2001) may provide adequate weed control for the following corn crop.
Perennial peanut (PP) is a warm-season perennial legume with a wide area of adaptation ranging from 31ºN to 35ºS latitude (Prine et al., 1981). Reported uses of PP include soil conservation, pasture, hay, and a replacement for turf (French et al., 2001). Because of low water and nutrient requirements and the absence of serious pests (Baltensperger et al., 1986, French et al., 2001), PP may provide an environmentally sound and ecologically important component of sustainable citrus production in Florida (Mullahey et al., 1994). When mowed two to three times a year, PP can also provide 60 to 112 kg·ha−1·year−1 N to citrus trees or alternatively serve as a high-value forage. Some citrus growers may thus opt to use PP to enhance the sustainability and profitability of their citrus production systems as well.
Successful weed suppression via use of a perennial CC has been reported by Perez-Nieto et al. (2005) in coffee by using Arachis pintoi L. (nonrhizomal PP). Although the use of annual CC and PP in conventional citrus and vegetables systems in South Florida has been studied extensively (Mullahey et al., 1994; Roe et al., 1994; Rouse and Mullahey, 1997), before this study, no information was available regarding the effectiveness of annual and perennial CC in suppressing weeds in organic citrus production systems.
Most studies typically report the effectiveness of CC in suppressing weeds in qualitative terms such as providing “adequate” weed control. But there is a lack of a universally applicable and more accurate index to quantify the effectiveness of cover crops in suppressing weeds across production environments. We therefore designed a new index that can be used as a management tool to evaluate the effectiveness of CC in suppressing weeds by calculating the ratio of biomass of both CC and weeds. We devised the term cover crop weed index (CCWI) for this ratio.
The overall objectives of this study were 1) to identify suitable CC species for weed suppression in organic citrus groves, 2) to develop a quantitative index for assessing the effectiveness of CC in suppressing weeds in an organic citrus system, 3) to contrast the performance and weed growth dynamics of PP-based systems with that of annual CC systems via the use of this index, 4) to evaluate changes in weed growth as affected by annual and perennial CC treatments, and 5) identify optimal CC mixtures suited for organic citrus production. The following hypotheses were tested in organic citrus systems: 1) annual CC will suppress weeds more effectively compared with PP, and 2) summer CC will accumulate more biomass and consequently will suppress weeds more effectively than winter CC.
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