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Knowledge of weed community structure in vegetable crops of the north central region (NCR) is poor. To characterize weed species composition present at harvest (hereafter called residual weeds) in processing sweet corn, 175 fields were surveyed in Illinois, Minnesota, and Wisconsin from 2005 to 2007. Weed density was enumerated by species in thirty 1-m2 quadrats placed randomly along a 300- to 500-m loop through the field, and additional species observed outside quadrats were also recorded. Based on weed community composition, population density, and mean plant size, overall weed interference level was rated. A total of 56 residual weed species were observed and no single species dominated the community of NCR processing sweet corn. Several of the most abundant species, such as common lambsquarters and velvetleaf, have been problems for many years, while other species, like wild-proso millet, have become problematic in only the last 20 yr. Compared to a survey of weeds in sweet corn more than 40 yr ago, greater use of herbicides is associated with reductions in weed density by approximately an order of magnitude; however, 57% of fields appeared to suffer yield loss due to weeds. Sweet corn harvest in the NCR ranges from July into early October. Earlier harvests were characterized by some of the highest weed densities, while late-emerging weeds such as eastern black nightshade occurred in fields harvested after August. Fall panicum, giant foxtail, wild-proso millet, common lambsquarters, and velvetleaf were the most abundant species across the NCR, yet each state had some unique dominant weeds.
Atrazine has been the most widely used herbicide in North American
processing sweet corn for decades; however, increased restrictions in recent
years have reduced or eliminated atrazine use in certain production areas.
The objective of this study was to identify the best stakeholder-derived
weed management alternatives to atrazine in processing sweet corn. In field
trials throughout the major production areas of processing sweet corn,
including three states over 4 yr, 12 atrazine-free weed management
treatments were compared to three standard atrazine-containing treatments
and a weed-free check. Treatments varied with respect to herbicide mode of
action, herbicide application timing, and interrow cultivation. All
treatments included a PRE application of dimethenamid. No single weed
species occurred across all sites; however, weeds observed in two or more
sites included common lambsquarters, giant ragweed, morningglory species,
velvetleaf, and wild-proso millet. Standard treatments containing both
atrazine and mesotrione POST provided the most efficacious weed control
among treatments and resulted in crop yields comparable to the weed-free
check, thus demonstrating the value of atrazine in sweet corn production
systems. Timely interrow cultivation in atrazine-free treatments did not
consistently improve weed control. Only two atrazine-free treatments
consistently resulted in weed control and crop yield comparable to standard
treatments with atrazine POST: treatments with tembotrione POST either with
or without interrow cultivation. Additional atrazine-free treatments with
topramezone applied POST worked well in Oregon where small-seeded weed
species were prevalent. This work demonstrates that certain atrazine-free
weed management systems, based on input from the sweet corn growers and
processors who would adopt this technology, are comparable in performance to
standard atrazine-containing weed management systems.
Weed management systems used by sweet corn growers, including the role of atrazine, are poorly characterized. Management records of 175 fields throughout the major sweet corn production areas of the Midwest were surveyed from 2005 to 2007. Seventy-four percent of sweet corn fields in the Midwest were grown in rotation with soybean or corn. Interrow cultivation was used on 48% of fields, and atrazine use was higher in those fields without interrow cultivation. A majority of fields (54%) received both PRE and POST herbicide applications. Mesotrione was applied below the registered use rate in two-thirds of the fields in which it was used POST. Atrazine rates in sweet corn were highest when the preceding crops were other vegetables, compared to preceding crops of soybean or corn. Selective herbicides are used extensively in U.S. sweet corn production, accounting for 94% of total weed management expenditures which average $123/ha. Growers treated 66% of fields with one or more applications of atrazine at an average total use rate of 1.35 kg ai/ha. The estimated annual net cost to replace atrazine in U.S. sweet corn production with the broad spectrum broadleaf herbicide, mesotrione, is $9.2 million.
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