The cool and dry conditions that Iowa has experienced over the last week or so may contribute to poor activity of soil-applied herbicides. Weeds may germinate and emerge through the herbicide treatment unless there is sufficient soil moisture to place the herbicide in solution in the soil water where it is available to the germinating weed seedling.
Monitor fields closely to determine if weed germination events have occurred. If weeds are just beginning to germinate, and if a timely rain occurs, the herbicide may have sufficient activity to provide control. This is true for a number of preemergence herbicides such as the triazines and the ALS inhibitor herbicides. However, the size of weed, initial soil moisture, herbicide application rate, and weed species are extremely important in determining whether or not the herbicide will provide delayed control. Timely field observations are critical to ensure success of a weed management program.
||Woolly cupgrass escaped in soil-applied herbicide
||Timely mechanical weed control is critical for optimum weed management
If weeds have just begun to germinate, but not emerged, or if the weeds have germinated very shallow, mechanical control tactics such as rotary hoeing or harrowing are likely the best option. If weed size is too large for a rotary hoe to be effective, and if the crop has developed sufficiently, cultivation should be considered. Postemergence herbicide treatments may cause excessive injury to crops under stress. Delay any postemergence application until the crop recovers. However, growers also must consider both the size of the weeds and the crop. If weeds are too large, as specified on the label, the decision to spray or not must be made regardless. If crops are relatively small, and the herbicide treatment is too early, the crop may not be large enough to compete with later developing weeds
This article originally appeared on page 81 of the IC-478(11) -- June 2, 1997 issue.