Since the 1970s, over 90% of U.S. broccoli has been produced in California and Arizona (U.S. Department of Agriculture, 2012). Production and consumption of broccoli has increased in recent years, and increased demand has spurred interest in increasing production in the eastern United States to supply the large eastern population centers. Considerable research is currently directed toward developing cultivars and cultural practices suitable for production regions of the eastern United States (Cornell University, 2011). Effective integrated weed management is essential to the success of eastern broccoli production. Weed management in eastern production may require measures additional to those used in California and Arizona because of differences in weed species common in the regions and higher rainfall in the east that increases weed growth and stimulates frequent weed seed germination.
Clomazone is registered for weed management in direct seeded and transplanted cabbage production in the United States, except in California where the herbicide is banned (FMC Corp., 2005). However, it is not registered for the other cole crops [broccoli, brussels sprouts (Brassica oleracea Gemmifera group), cauliflower (B. oleracea Botrytis group), collards (B. oleracea Acephala group), kale (B. oleracea Acephala group), and others] that are in the same species. Cultivars within a cole crop type are frequently called a crop group. Clomazone is an important component for weed management in cabbage and several other vegetable crops because it provides residual control of several important annual grass and broadleaf weeds. The recommended use rates for clomazone for all crops range from 0.15 to 1.5 lb/acre and are based on soil type and crop tolerance. The higher rates of clomazone recommended for highly tolerant crops control more weed species and provide longer lasting control than the low rates recommended for less tolerant crops. The recommended use rates for transplanted cabbage are 0.25 and 0.5 lb/acre for coarse and fine soils, respectively; clomazone can also be used on direct seeded cabbage at up to 0.5 lb/acre.
Hopen et al. (1993) evaluated the response of 36 genetically diverse cabbage cultivars to clomazone applied before transplanting. Most of the cultivars were tolerant of clomazone, chlorosis was minor and short lived, and yields were not reduced. Three cabbage cultivars that exhibited higher levels of chlorosis, including the important hybrid cultivar, Bravo, also had yields reduced by 1.0 lb/acre clomazone in one of the two years of the study. Natural variation in clomazone tolerance among cultivars or genotypes within crop species has also been reported for bean [Phaseolus vulgaris (Sikkema et al., 2006)], corn [Zea mays (Keifer., 1989)], cucumber [Cucumis sativus (Al-Khatib et al., 1995; Staub et al., 1991)], pumpkin [Cucurbita maxima, C. moschata, C. pepo (Harrison and Keinath, 2003)], rice [Oryza sativa (Mudge et al., 2005; Scherder et al., 2004; Zhang et al., 2004)], sweetpotato [Ipomoea batatas (Harrison and Jackson, 2011)], and watermelon [Citrullus lanatus (Harrison et al., 2011)]. The objective of this study was to evaluate the clomazone tolerance of broccoli cultivars in comparison with cabbage cultivars using greenhouse and field experiments to assess the potential for safely using clomazone for weed management in broccoli.
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