Crop tolerance (CT) to weed interference is the ability of the crop to endure or avoid competitive stress from weeds without substantial reduction in growth or yield. Historically, CT has been a fundamental component of weed management, although the role of CT waned with the introduction of selective herbicides in the mid-20th century. In the last two decades, there has been renewed interest in more fully exploiting CT to reduce risk of weed control failure (Callaway, 1992; Lindquist and Mortensen, 1998). For instance, recent surveys documented 57% of conventionally managed sweet corn fields suffered yield loss as a result of weeds despite extensive reliance on herbicides for weed control (Williams et al., 2008c). An increasing prevalence of herbicide-resistant weeds and a growing organic market in several crops, including sweet corn, have fueled interest in CT playing a larger role in weed management. In agronomic crops such as wheat, rice, and soybean, efforts have been made to breed for cultivars more competitive with weeds (Jannink et al., 2001; Lemerle et al., 2006; Zhao et al., 2006).
Some corn hybrids vary in CT. In dent corn, which has been the subject of considerably more research on CT than sweet corn, several authors have identified canopy architecture traits important to CT, including plant height, canopy density (Lindquist and Mortensen, 1998), leaf uprightness (Sankula et al., 2004), canopy closure rate, and maximum leaf area index (Lindquist et al., 1998). Weed interference can influence certain growth characteristics such as height (Cavero et al., 2000; Maddonni et al., 2002) and leaf azimuth orientation (Maddonni et al., 2002). In addition, the mere presence of weeds, and not direct competition for resources, can influence crop growth. Rajcan et al. (2004) found that the presence of low-growing sod influenced light quality in such a way to alter dent corn growth early in the season. Even at low weed densities, differences were observed in sweet corn hybrid response to weed competition (Williams et al., 2006, 2008b).
The residual weed community (i.e., plants surviving management and persisting to harvest) of most sweet corn fields is often dominated by a small number of weed species. Wild-proso millet (Panicum miliaceum L.) is among the most abundant residual weed species observed in sweet corn (Williams et al., 2008c) and is a serious problem in a number of other horticultural and agronomic crops (Harvey and Porter, 1990; Wilson, 1993). Wild-proso millet has a long period of germination and seedling emergence, tolerates most herbicides used in sweet corn, and is a prolific seed producer.
Research on sweet corn-weed competition has included a relatively small number of hybrids (e.g., two to four hybrids) and has produced limited information on crop growth responses to weed interference. Moreover, the seed industry would like to identify cultivars with high CT suitable for organic production systems. The goal of this study was to examine considerably more hybrids than previously reported to gain a more complete understanding of sweet corn tolerance to weed interference. Using wild-proso millet as a model weed, specific objectives were to 1) quantify tolerance in crop growth and yield to weed interference; 2) determine the extent to which crop growth and yield tolerance variables are associated; and 3) identify hybrids differing in tolerance to weed interference.
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