The effect of weather factors on the performance of herbicides to control Alopecurus myosuroides in winter wheat

SummaryThe effect of weather factors before and after spraying on the activity of the herbicides isoproturon and clodinafop-propargyl was assessed in field experiments from 1997 to 2000. The herbicides were sprayed at four rates on eight to 15 occasions between October and April and efficacy was assessed as ED50 for the surviving panicles of Alopecurus myosuroides Huds. The ED50 values (N = normal dose) varied between 0.18 N and 1.16 N for isoproturon and 0.11 N and 1.51 N for clodinafop-propargyl. The effect of weather factors was investigated using multiple regression techniques for 1 to 14 days before and after spraying. These regressions were significant (P <0.001) for each of the individual years. Between 86% and 97% of the total variation in response to both herbicides was accounted for, but the factors were not consistent from year to year. However, when the 3 years were combined for each herbicide the regression was also significant (P <0.001) for isoproturon (78% variation accounted for) and for clodinafop-propargyl (72% variation accounted for). The current blanket advice, that the best control results from applications to small plants, was not always correct.