Market forces and environmental considerations have encouraged some California grape growers to adopt organic management practices. In 2008, there were an estimated 23,000 acres of organic vineyards statewide (Klonsky, 2010). Weed management during the transition from conventional to organic farming presents agronomic (Barberi 2002; Bond and Grundy, 2001; Ngouajio and McGiffen, 2002; Smith et al., 2009) and economic (Park and Lohr, 2005) challenges to grape growers. This transition period is marked by an increase in weed populations when the transition is managed by growers who are accustomed to using synthetic herbicides to control weeds (Martini et al., 2004). Few organically acceptable herbicides have been shown to be efficacious, reliable, and economical (Capps and Lanini, 2008). In recent years, some organic herbicides have been registered; however, their efficiency and cost-effectiveness in comparison with other weed control methods has not been directly compared.
Mechanical cultivation has long been known to be an effective weed management practice, but it can generate dust, a regulatory issue [particulate matter (PM10)] in the San Joaquin Valley (SJV) of California, and depending on the terrain, wind, and rainfall or irrigation pattern, may also accelerate soil erosion (Gago et al., 2007). Handhoeing is another potentially effective weed management practice, though it is prohibitively laborious and expensive. Other examples of alternative weed management practices include the use of steam (Kolberg and Wiles, 2002; Shrestha et al., 2004, 2012), flame (Ascard 1998; Ullao et al., 2010), microwaves (Sartorato et al., 2006) reduced soil disturbance cultivators, cover crops, and mulches (Mennan and Ngouajio, 2012). Each of these tools has specific limitations in vineyards and other cropping systems. For example, Shrestha et al. (2012) reported that steam, propane flaming, and an organic herbicide (d-limonene) provided 3 to 6 weeks of weed suppression in nonbearing almond (Prunus dulcis) orchards. Thus, repeated applications of these treatments were necessary, which may not be cost-effective. McCue and Schupp (1991) observed limited success with rotary tillage for in-row weed management in apple (Malus ×domestica) orchards. Baumgartner et al. (2007) suggested that more than two passes with a Clemens cultivator may be required to reduce weed biomass to levels comparable with that obtained by glyphosate, a synthetic herbicide.
In-row cover crops have been successfully used in orchards and vineyards in several areas. Mennan and Ngouajio (2012) found that brassica (Brassica sp.) cover crops provided season-long weed control, and a mulch of hazelnut (Corylus avellana) husks provided upto six months of weed suppression in hazelnut orchards. Similarly, Steinmaus et al. (2008) showed that mulched cover crops provided similar or greater weed control than tillage or synthetic herbicides in northern California vineyards. Elmore et al. (1997) also found benefits of cover crop mulches in weed suppression in vineyards in central California, although Fredrikson et al. (2011) suggested that cover crop residue of 2.5 to 15 kg·m−2 was required to provide effective in-row weed control in vineyards in western Oregon. However, cover crops in the SJV are mostly limited to winter annuals because of restrictions imposed by the climate and cultural practices (Hirschfelt, 2000).
The efficacy and cost-effectiveness of many of the alternative weed management practices discussed earlier can also vary from one region to another and among various grape commodities. For example, perennial cover crops that are suitable for wine grape vineyards are not suitable for traditional raisin vineyards since they would be destroyed by the soil preparations needed to facilitate grape drying (Hirschfelt, 2000). Hence, organic weed control practices need to be evaluated in the SJV to determine whether they are suited to local conditions and if they are cost-effective. Therefore, the objective of this study was to compare the efficacy and cost-effectiveness of various weed control methods and their impacts on crop yield and quality in organic raisin and wine grape vineyards in the SJV.
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