Leaf washing was used to determine relative rates of uptake of 2,4-D and dicamba dimethylamine salts by wheat following field application of a postemergence tank mixture (3:1) of 476 g ae ha-1 at the 4- to 5-leaf growth stage. Foliar herbicide deposits and total acid residues in the crop were determined the day of spraying as well as 2 and 7 d after spraying. Leaf washing of foliage samples collected on the day of spraying indicated that the crop/weed canopy intercepted 51% of the application, and that dicamba was taken up more quickly than 2,4-D. Within a few hours of spraying, approximately 80% of the deposited dicamba and 65% of 2,4-D were taken up by the crop. A 35-mm rain during the first 48 h after application reduced plant surface deposits of both herbicides by more than 98%. During this 2-d period, dicamba residues within the crop were metabolized more quickly than those of 2,4-D. There was little difference in effectiveness among water, 10% methanol, 25% methanol, or 90% methanol in removing plant surface deposits of 2,4-D and dicamba from wheat.

In continuous wheat or barley or in a canola/barley rotation, wild oat control every year over 4 yr maintained wild oat seedling populations at 3 plants/m2 or less. Failure to control wild oats annually increased wild oat populations (>200 plants/m2 by the fourth year) in continuous wheat dramatically, while in the other two cropping systems, populations increased to only 40 plants/m2 or less by the fourth year. In the continuous wheat and in the canola/barley rotation, wild oat control every year generally provided the best economic returns when prices and costs were averaged over 4 yr; in continuous barley, the average return was better when wild oats was controlled only in the second or third years rather than every year.

The effect of duration of wild oat competition on spring wheat yield and growth was determined in time-of-removal experiments conducted over a three year study period in Saskatchewan, Canada. Failure to remove wild oat reduced wheat yield 28 and 39% at wild oat populations of 64 and 188 plants per m2, respectively. Wheat yield was not reduced by wild oat densities of 64 or 118 plants per m2 until the six- and seven-leaf stage of wild oat, respectively. Removing wild oat at 64 plants per m2 before the seven-leaf stage and 118 plants per m2 before the five-leaf stage did not increase wheat culm or fresh weight production.