Scouting After Herbicide Applications
Good Day! Hope you are well!
Rainfall for last week ranged from zero inches to 0.2 inch with an average of 0.05 inch. The high temperature for the past week ranged from 66 to 85 degrees Fahrenheit with an average of 78 degrees Fahrenheit. That is two degrees below the normal for this time of year. The high temperature forecast for the coming week will be somewhat above normal.
No change in the number of cases of Highly Pathogenic Avian Influenza (HPAI) this past week.
The crops are progressing very nicely in the county. Overall the crops look pretty good.
Postemergence herbicide applications have been made in most corn and many soybean fields. Scouting before making a postemergence herbicide application is critical to successfully control weeds in a field. However, scouting that same field after the herbicide application is just as important. Scouting is necessary to determine whether all weeds were controlled, to determine the cause if not controlled, and to make plans for any future applications as quick as necessary.
If contact herbicides were applied, scout fields 5 to 10 days after application. If translocating herbicides were applied, scout fields 7 to 14 days after application. Scouting on the early side of each range is preferable to learn the most about the effectiveness of the herbicides, however weeds may not be dead at that time and require a second scouting trip to determine death. If the weather is cold and cloudy it usually takes longer to determine if the weeds will die. Timely scouting after application is important for all crops, but most important for soybeans.
The number one reason to scout timely after a postemergence herbicide application is to respond quickly in case a second application is necessary. Waiting to scout too long after the initial application, especially when needing a contact herbicide, will allow the weeds to become too large making it difficult for the second herbicide application to be effective.
If more than a single weed species is present after a herbicide application and the injury symptoms are somewhat uniform, then an incorrect rate, poor application method, or poor weather conditions likely caused the herbicide to be less effective. In this situation, ideally choose a herbicide having a different site of action that will effectively manage the weeds present, although applying the same herbicide may be effective in the second application.
If marestail, waterhemp, kochia, or common ragweed are present after the postemergence application of glyphosate (Group 9), ALS-inhibitors (Group 2), and/or PPO-inhibitors (Group 14), suspect herbicide resistance. If wild oat is present after the use of an ACCase inhibitor (Group 1) herbicide, then suspect resistance. If plants of these species are dead while other plants appear normal and all other plants respond somewhere between dead and near normal, then you can almost guarantee resistant biotypes are present in the field. It is possible to find waterhemp and common ragweed that are resistant to Group 2, 9, and 14 herbicide sites of action. When plants are surviving due to resistance, herbicide(s) having an alternative site of action will need to be applied from 14 to 21 days after the initial application. Contact herbicides should be applied at the 14-day interval while translocating herbicides can be applied closer to the 21-day interval.
Timely scouting after a postemergence herbicide application is critical to responding quickly with an effective second herbicide application. Stopping weeds from producing seeds that have survived one or more herbicide applications is very important to reducing herbicide-resistant weeds, even if that means hand-weeding. Hand-weeding is the most effective way to prevent herbicide-resistant weeds.
Have a great week!
