Organic production systems rely heavily on cultural and mechanical weed control, with minimal dependence on herbicides that are approved for use in certified organic crops. Herbicides allowed for use in certified organic production systems are derived from natural sources. Some are derived from essential oils originating from plants (Dayan et al., 2009; Gaskell et al., 2000; Tworkoski, 2002). These herbicides are sprayable, applied postemergence, nonsystemic, and tend to be nonselective, although tolerance varies among crops.
Herbicides derived from essential oils have been investigated for weed control benefits in certified organic crop production. The majority of the reported studies on herbicides derived from natural products were on warm season crops and weeds. Other researchers have evaluated rates of several commercial products, application timing, and combinations with cultivation in many cropping systems and regions (Boyd and Brennan, 2006; Boyd et al., 2006; Curran et al., 2004; Johnson et al., 2012). Common conclusions drawn from these studies were that none of the essential oil herbicides provided any residual weed control, small weed size was critical for maximum efficacy, environmental conditions affected herbicide performance, and rates necessary for maximum efficacy made the treatment costly. Additionally, sprayer configuration was reported to be an important consideration to ensure maximum efficacy, with high sprayer output volume necessary to ensure adequate spray coverage of targeted weeds (Boyd and Brennan, 2006; Boyd et al., 2006; Curran et al., 2004; Evans and Bellinder, 2009; Shrestha et al., 2012; Young, 2004). Previous research showed that essential oil herbicides were ineffective when applied with sprayers calibrated for low output; 6 gal/acre (Brainard et al., 2013) and 8 gal/acre (Ferguson, 2004).
Vidalia® sweet onion is a dry-bulb onion grown in Georgia as a cool season crop. In 2011, Vidalia® sweet onion production accounted for the largest portion (20%) of the Georgia vegetable crop farm gate value compared with other vegetable crops (Wolfe and Shepherd, 2012). Vidalia® sweet onion plantings are restricted by Federal Marketing Order 955 to region in southeastern Georgia (U.S. Code of Federal Regulations, 1990). The acreage is stable, with the 2011 Vidalia sweet onion acreage estimated at 14,210 acres. Within this region, the economic impact of Vidalia® sweet onion production is substantial and valued at $159 million.
There have been efforts in recent years to expand the niche market of certified organic Vidalia® sweet onion. Crop production budgets exist for organic Vidalia® sweet onion production (R.L. Torrance, unpublished data). The two most costly inputs into organic Vidalia® sweet onion production are cost of transplants ($1800/acre) and hand weeding ($1500/acre). Any effective weed control system that reduces or eliminates the need for hand weeding will provide a significant savings directly to growers. Recent research efforts focused on an integrated system of summer solarization, cultivation with a tine weeder, and clove oil (Johnson et al., 2012), with the stated objective to lessen the need for hand weeding. Of the weed control tactics studied, cultivation with a tine weeder was the most consistent and effective approach. However, it was noted that timing of the initial cultivation was crucial and delays in the initial cultivation would greatly reduce overall weed control success. That research also indicated that clove oil provided inconsistent weed control benefits and its use was not correlated with yield responses. However, clove oil might provide a specialized niche in organic Vidalia® sweet onion production by controlling emerged seedling weeds, if consistency is improved. While the production region for Vidalia® sweet onion is in southeastern Georgia where winters are mild with rare incidence of snowfall or frozen soil, applications will be under winter conditions for the region. These conditions could reduce efficacy of contact herbicides like clove oil. Therefore, studies were initiated in 2010 to determine if herbicide adjuvants improve clove oil efficacy without the need for higher sprayer output volume in organic Vidalia® sweet onion production.
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