The response of susceptible (S) and resistant (R) green foxtail biotypes to increasing dosages of trifluralin, applied PPI in rapeseed and preemergence incorporated (PEI) in wheat, was investigated in field experiments in 1989 and 1990. Differences in response between the biotypes to PPI- and PEI-trifluralin were 7- and 12-fold, respectively, based on density and shoot biomass determinations 4 wk after emergence. Nine- and 14-times higher dosages of PPI- and PEI-trifluralin, respectively, were required to reduce R-seed production by 50% than to reduce S-seed production by the same amount. At the recommended trifluralin dosage in rapeseed (1.4 kg ha−1), the density of S-plants 4 wk after emergence was reduced by 84% compared with untreated plots, whereas the density of R-plants was reduced by only 4%. The effective kill (seed yield reduction) was 99% and 42%, respectively. At the recommended dosage in wheat (0.9 kg ha−1), the density of S-plants 4 wk after emergence was reduced by over 99% compared with less than 36% for R-plants. The effective kill was 97% and 14%, respectively. Based on determination of effective kill, the selection pressure of trifluralin on green foxtail is greater when the chemical is applied PPI in rapeseed than when applied PEI in wheat, even though initial density reductions are less in the former than the latter.

The response of trifluralin-susceptible (S) and -resistant (R) green foxtail biotypes to herbicides belonging to several chemical groups was compared to determine the cross-resistance pattern of the R-biotype. Dose-response experiments conducted in the growth chamber indicated that R-green foxtail was resistant to other dinitroanilines and a chemically unrelated mitotic disrupter herbicide, but not to nine other herbicides belonging to seven chemical families. The response of S- and R-green foxtail to increasing dosages of ethalfluralin, applied PPI in rapeseed, was investigated in a field experiment in 1989 and 1990. The R-biotype was 7 times more resistant to ethalfluralin than the S-biotype based on density determinations 4 wk after emergence. Seven times higher dosage was required to reduce R-seed production by 50% than to reduce S-seed production by the same amount. The initial reductions in density of R- and S-plants at the recommended dosage of ethalfluralin in rapeseed (1.4 kg ha−1) was 35% and 95%, respectively. The effective kill (seed yield reduction) of R- and S-biotypes was 55% and 99%, respectively. The results indicate that ethalfluralin will not effectively control R-green foxtail. However, several other herbicides with different mechanisms of action can be used to effectively control R-foxtail, thereby reducing any adverse effects of their interference on crop production.