Spring barley often is grown in rotation with winter wheat, and sometimes barley can overwinter in the subsequent winter wheat crop reducing grain yield and quality. Studies were established during 1996 and 1997 in winter wheat fields in southeastern Washington and near Moscow, ID, respectively, to evaluate control of ‘Steptoe’ volunteer barley with MON 37500, diclofop, and fenoxaprop/2,4-D/MCPA. Herbicides were applied to volunteer barley at two growth stages: two leaves to four tillers and more than four tillers with stems beginning to elongate. MON 37500 at 0.018, 0.026, and 0.035 kg ai/ha visibly controlled volunteer barley 83% or more at both application times. Diclofop at 1.12 kg ai/ha did not control volunteer barley, whereas fenoxaprop/2,4-D/MCPA at 0.66 kg ai/ha controlled volunteer barley 64 to 97% in 1996, but only 0 to 23% in 1997. In 1996 and 1997, volunteer barley density was reduced 80 to 99% in MON 37500-treated plots compared to the untreated control plots. Wheat grain grade was #1 for all MON 37500 treatments compared to grade #4 in 1996 and #3 in 1997 in the untreated plots. Grain price was reduced by dockage (barley kernels) for MON 37500-treated wheat $0 to $3.12/metric ton (MT), whereas price was reduced $23 to $26/MT for grain from untreated plots. In greenhouse studies, visible injury and height and biomass reduction varied among the 36 barley varieties treated with MON 37500.

Field experiments were conducted near Chickasha, OK, in 1999 and 2000 and near Perkins, OK, in 2000 to evaluate the effects of Palmer amaranth on harvest efficiency of grain sorghum and full-season competition with that crop. Weed densities were 0 (the weed-free check), 1, 2, 4, 6, 9, 12, and 18 plants/15 m of row. In the harvest efficiency experiments, each additional weed per 15 m of row increased grain moisture before cleaning by 0.7 and 0.2% at Chickasha in 1999 and at Perkins, respectively. After cleaning, it increased moisture by 0.2% in both the experiments for each weed. Foreign material increased 67, 2, and 3 kg/ha at Chickasha in 1999, Chickasha in 2000, and Perkins, respectively. At Chickasha in 2000, sorghum seed loss through the combine increased 11 kg/ha for each additional weed per 15 m of row. Grain grades improved at Perkins at higher weed densities. In the competition experiments, grain yield decreased by 1.8 to 3.5% for each increase of 1 weed/15 m of row. More weeds resulted in higher weed dry weight. Each kilogram of Palmer amaranth dry weight per plot reduced grain yield by 5.3 to 9.1%. In 2000, sorghum seeds per panicle were reduced by 27 to 50 for each weed. Grain grades generally decreased as weed density increased at Chickasha in 2000 but not at Perkins.

Field experiments were conducted in Oklahoma to quantify the wheat grain yield losses and price discounts resulting from season-long interference with cheat, feral rye, Italian ryegrass, jointed goatgrass, and wild oat. Plots were seeded to individual weeds at one of seven seeding rates, and wheat was planted in all plots at a uniform rate. Maximum weed densities were 89 (cheat), 80 (feral rye), 158 (Italian ryegrass), 170 (jointed goatgrass), and 120 plants/m2 (wild oat). Wheat grain yield losses caused by interference from the maximum density of each weed species were 19 (cheat), 55 (feral rye), 20 (Italian ryegrass), 21 (jointed goatgrass), and 28% (wild oat). Wheat grain total price discounts caused by interference from the maximum density of each weed species were 22 (cheat), 368 (feral rye), 26 (Italian ryegrass), 36 (jointed goatgrass), and 64 cents/hectoliter (wild oat). Of the five weed species included in this research, interference from feral rye had the greatest effect on wheat grain yield and price.