Field trials were conducted in 2004 and 2005 to identify sulfonylurea (SU) herbicides that would provide improved weed control, minimal soil residual, and crop safety to SU-resistant chicory. SU-resistant chicory had previously been selected in vitro for resistance to chlorsulfuron. Our research evaluated three commercial, nonresistant and three breeding lines of SU-resistant chicory. Each of the cultivars was treated POST at the two true-leaf growth stage with either foramsulfuron, rimsulfuron, rimsulfuron plus thifensulfuron, tribenuron, thifensulfuron, thifensulfuron plus tribenuron, triflusulfuron, flumetsulam, or imazamox at normal use rates. Established plant densities and root yields of SU-resistant chicory breeding lines were greater than or equal to the densities and root yields of commercial cultivars. The plant density of commercial chicory cultivars was reduced by rimsulfuron, rimsulfuron plus thifensulfuron, tribenuron, and thifensulfuron plus tribenuron, but SU-herbicides did not reduce the density of SU-resistant breeding lines. SU-resistant chicory differed in cross-resistance to SU-herbicides, with tribenuron causing the most crop injury and thifensulfuron, the least. Weed control varied between the SU-herbicides. The greatest reduction in weed biomass occurred with tribenuron, thifensulfuron plus tribenuron, and rimsulfuron plus thifensulfuron; the least reduction occurred with triflusulfuron, foramsulfuron, and rimsulfuron. Chicory root yields comparable to the hand-weeded treatment were achieved with rimsulfuron plus thifensulfuron and thifensulfuron plus tribenuron treatments. The SU herbicides that met the initial project objectives of crop tolerance and improved weed control were combinations of rimsulfuron plus thifensulfuron and thifensulfuron plus tribenuron.

Corn and soybean growers across Indiana were surveyed to assess their perceptions about the importance of preplant and POST weed control timing, focusing mainly on soybean production. Despite studies demonstrating the importance of planting into a clean field, almost a third of Indiana growers do not think it is important to plant into a weed-free seedbed and 74% do not use residual herbicides in glyphosate-resistant soybean production systems. Growers who farmed less than 200 ha were more likely to overestimate the ability of soybean to tolerate weed interference than growers who farmed more hectares. Growers who manage smaller farms were also more likely to use a one-pass weed control program than larger growers. This suggests that yield losses to weed interference may be greater for smaller farms than for larger farms. Weed size and density were the most common criteria used by growers to decide when to apply herbicides. This suggests that field scouting plays an important role in the decision-making process of growers. However, a substantial proportion of growers apply POST herbicides to large common lambsquarters and giant ragweed in an attempt to minimize the number of trips across the field for weed control. Delayed control of these species likely contributes to reduced crop yields, higher application rates, and to the survival of treated plants. Opportunities to improve control and increase yields through more optimal herbicide use appear possible for Indiana corn and soybean growers.

Weed management is a perennial challenge for growers, and continual innovation is essential to maintain the effectiveness of management technologies. The first generation of herbicide-resistant crops revolutionized weed control. However, weeds are adapting to crop systems that rely on a single mode of herbicide action. Crops with resistance to multiple modes of herbicide action could help maintain weed management. GAT/HRA is a new multiple herbicide–resistance technology for corn, soybean, and other crops. GAT/HRA combines metabolic glyphosate inactivation with an acetolactate synthase (ALS) enzyme that is insensitive to ALS-inhibiting herbicides. The mechanism to inactivate glyphosate is the glyphosate N-acetyltransferase enzyme, which transforms glyphosate into a nonphytotoxic metabolite. The gat gene is derived from a naturally occurring soil bacterium and optimized by repetitive gene shuffling and screening. The resistance mechanism to ALS-inhibiting herbicides is a double-mutant, highly resistant ALS (HRA) that is insensitive to all five classes of ALS herbicides. GAT/HRA crops will maintain natural tolerance to selective herbicides and thus provide more weed management options for growers to help deter weed spectrum shifts and delay the evolution of herbicide-resistant weeds.

The vast majority of crop hectares in the United States are treated with chemical herbicides annually. The adoption of herbicides for weed control was rapid in the 1950s and 1960s. Herbicides replaced the use of millions of workers to pull and hoe weeds by hand and greatly reduced the use of tillage for weed control. Costs of production were reduced and crop yields increased because herbicides were cheaper and more effective than hand weeding and cultivation. Organic crop growers cite weed control as their greatest difficulty in crop production because they are not permitted the use of chemical herbicides. They substitute hand weeding and cultivation for herbicides at a greatly increased cost and with reduced effectiveness. Aggregate studies that estimate the value of herbicides assume that growers would substitute a certain amount of hand weeding and tillage if chemicals were not used, which would not be sufficient to prevent yield losses totaling about 20% of U.S. crop production.

There are nearly 75 species in the genus Amaranthus, part of the Amaranthaceae family, worldwide. In this large genus, there is a distinct group of 10 species that are dioecious (separate male and female plants). In contrast to the monoecious Amaranthus spp. which are represented by species endemic to every continent, the dioecious Amaranthus spp. are all native to North America. The Amaranthus spp. have a long documented history of being fellow travelers with humans. In recent years, three dioecious Amaranthus spp.: Palmer amaranth (Amaranthus palmeri S. Wats.), common waterhemp (Amaranthus rudis Sauer), and tall waterhemp [Amaranthus tuberculatus (Moq) Sauer] have become major weeds to row crops in North America. Palmer amaranth has become a very troublesome weed to cotton (Gossypium hirsutum L.), corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] production in large parts of the southern United States, and the waterhemp complex is now a major weed pest in corn and soybean production in the midwestern United States (Horak and Loughin 2000; King 1966; Mabberly 1997; Robertson 1981; Sauer 1950, 1955, 1972; Wax 1995).