A 2-yr field study was conducted from 2002 to 2003 on a Dundee silt loam soil at the Southern Weed Science Research Unit Farm, Stoneville, MS (33°26′N latitude), to examine the effects of hairy vetch cover crop (hairy vetch killed at corn planting [HV-K], hairy vetch killed in a 38-cm-wide band centered over the crop row at corn planting [HV-B], hairy vetch left alive [HV-L], and no hairy vetch [NHV]) and glyphosate postemergence (broadcast, banded, and no herbicide) application on weed control and yield in glyphosate-resistant corn. Two applications of glyphosate at 0.84 kg ae/ha were applied 3 and 5 wk after planting (WAP) corn. Hairy vetch dry biomass was higher in HV-L (4,420 kg/ha) and HV-B (4,180 kg/ha) than in HV-K (1,960 kg/ha) plots at 7 WAP. Hairy vetch reduced densities of pitted morningglory, prickly sida, and yellow nutsedge in HV-B and HV-L compared with NHV plots, but hairy vetch had no effect on densities of barnyardgrass, johnsongrass, and large crabgrass at 7 WAP regardless of desiccation. Total weed dry biomass at 7 WAP was lower in HV-B and HV-L than in HV-K and NHV plots. Corn yield was higher in HV-K (10,280 kg/ha) than in HV-B (9,440 kg/ha) and HV-L (9,100 kg/ha), and yields were similar between HV-K and NHV (9,960 kg/ha). Glyphosate applied broadcast resulted in the highest corn yield (11,300 kg/ha) compared with a banded application (10,160 kg/ha). These findings indicate that hairy vetch cover crop has the potential for reducing the density of certain weed species in glyphosate-resistant corn production systems; however, optimum weed control and higher yield were obtained when glyphosate was used.

Soft red winter wheat response to the herbicides AE F130060 00 plus AE F115008 00 applied alone or with the safener AE F107892 at the three-leaf, two-tiller, and six-tiller growth stages was determined in a field experiment in North Carolina. AE F130060 00 at 25 g ai/ha plus AE F115008 00 at 5 g ai/ha, twice the anticipated use rate, applied with safener injured wheat 9% but did not affect grain yield, grain test weight, number of spikes, number of kernels per spike, or kernel weight. Results were similar with safener at herbicide–safener ratios of 1:1 and 1:3 regardless of the wheat growth stage at application. Without the safener, AE F130060 00 plus AE F115008 00 applied at the three-leaf, two-tiller, and six-tiller growth stages injured wheat an average of 27% and reduced grain yields 5, 5, and 11%, respectively. Yield losses were attributed to reduced numbers of kernels per spike. AE F130060 00 at 12.5 g/ha plus AE F115008 00 at 2.5 g/ha plus AE F107892 at 15 g ai/ha did not affect grain yield or yield components.

Research was conducted in winter of 2000 and spring of 2001 to determine the extent of simazine-resistant annual bluegrass in Mississippi. Samples of annual bluegrass seed or mature plants were collected from 71 locations across the state and grown in the greenhouse. Four weeks after treatment with 22.4 kg ai/ha simazine (10× rate), samples from 31 of 71 locations (30 golf courses and one nongolf course) evaluated in the greenhouse had simazine-resistant annual bluegrass plants. Thus, 43% of the golf courses tested in the greenhouse had triazine-resistant annual bluegrass present. When natural field populations were treated with an equivalent simazine rate, simazine-resistant annual bluegrass plants were observed in 90% of the sites found to be resistant in the greenhouse screen.

‘Russet Burbank’ potato tolerance to dimethenamid-p applied preemergence at 0.7, 1.4, or 2.9 kg ai/ha was assessed in field studies conducted at Aberdeen, ID and Ontario, OR. Although crop injury was evident approximately 2 wk after treatment, most injury had diminished by row closure 2- to 3-wk later. Initial injury did not translate to yield loss and U.S. No. 1 and total tuber yields in dimethenamid-p–treated plots were similar to yields in the untreated, weed-free control.

A 3-yr study was conducted in Wheatland County, Alberta to determine if agronomic practices of growers influenced the occurrence of herbicide resistance in wild oat. Wild oat seeds were collected in 33 fields in 1997 and in 31 fields in each of 1998 and 1999 (one field per grower). Seedlings were screened for resistance to two acetyl-CoA carboxylase (ACCase) inhibitors, imazamethabenz, an acetolactate synthase (ALS) inhibitor, and triallate, a thiocarbamate herbicide. A questionnaire on herbicide resistance awareness and management practices was completed by each grower. Both ACCase and ALS inhibitor resistance in wild oat were linked to a lack of crop rotation diversity. In addition, ALS inhibitor–resistant wild oat was associated with conservation-tillage systems and recent use of herbicides with that mode of action. Results of this study suggest that timely tillage and inclusion of fall-seeded and perennial forage crops in rotations will effectively slow the selection of resistance in this grass species.

A general life cycle model was modified to demonstrate how agronomic practices and weed biology factors affect the rate of appearance of herbicide-resistant downy brome, jointed goatgrass, and wild oat in Pacific Northwest wheat cropping systems. The model suggests herbicide rotation strategies for cropping systems that include imidazolinone-resistant wheat as a weed management tool. Simulation of continuous annual imidazolinone-resistant winter wheat and imazamox herbicide use resulted in the resistant soil seed banks of downy brome, jointed goatgrass, and wild oat surpassing their susceptible soil seed banks in 5, 7, and 10 yr, respectively. Reducing the initial seed bank density of downy brome before beginning a rotation that includes imidazolinone-resistant winter wheat reduces the likelihood of selecting for herbicide-resistant biotypes. The best simulated management option for reducing the total jointed goatgrass soil seed bank in low-precipitation areas is an imidazolinone-resistant winter wheat–fallow rotation. Rotations that include winter and spring crops and rotations that include non–group 2 herbicides minimize herbicide resistance selection pressure and reduce the wild oat soil seed bank.

“Here we go round the prickly pear Prickly pear prickly pear Here we go round the prickly pear At five o'clock in the morning.”

The Hollow Men, T. S. Eliot (1925).