Rain Delays and Weed Control (Prostko)
Recent rains over the last few days at the UGA Ponder Farm (1.75″+) have kept me out of the field for today so I thought it would be a good time to squeeze in a quick blog. The frequent rain showers we have had over the last month or so have caused many problems with growers who have not been able to get in the field to make pesticide applications. Since its now July 20 and getting late for just about everything, here are a few things to think about in regards to weed control in various crops:
1) The application cut-off date for all dicamba applications in tolerant soybean was June 30 and is July 30 for tolerant cotton.
2) If at all possible, growers need to avoid making applications of Cobra or Ultra Blazer to peanuts during the 60-80 DAP time frame. There have been yield losses (5-10%) associated with the application of these herbicides during this period (usually R5-R6 or beginning to full seed stage). Cobra can legally be applied up until ~100-105 DAP since it has a 45 day PHI. The PHI for Ultra Blazer is 75 days.
3) Yes, it is my opinion that it always a better idea to apply a POST grass herbicide at least 3 days before a peanut fungicide application rather than tank-mixing them together. However, growers in a tight bind who just have no other choice but to co-apply (fungicide + POST grass herbicide) should consider increasing the rate of the grass herbicide (5-10%) to help overcome potential antagonism. They must also realize that they still might not get optimum control especially since grasses are likely larger than desired. Growers will be wasting money on the grass herbicide if applied to large plants. Generally, flowering grasses are way too big to control with these herbicides. The POST grass herbicides (Select, Fusilade, Poast) have a 40 day PHI. Check out page 234 of the 2021 UGA Pest Control Handbook for more information about grass control in peanut:
4) 2,4-DB has a 60 day PHI so legally it can be applied up until ~80-90 DAP (for most peanut cultivars). There are at least 8 different formulations of 2,4-DB out there and these labels can vary slightly. Late applications of 2,4-DB can be tank-mixed with most fungicides. Don’t expect any home-runs here since 2,4-DB is not very effective on most weeds that are growing above the peanut canopy (except common cocklebur).
5) Since field corn harvest is rapidly approaching, I have gotten a few questions about the pre-harvest control of annual morningglories. My usual recommendation is to apply a combination of Roundup + Aim within 3 days of harvest. This treatment will not miraculously remove the running morningglory plants, but it will desiccate them enough to facilitate harvesting (i.e. makes the vines more brittle and less likely to wrap). See below for more specifics (from 2021 UGA Pest Control Handbook, page 73).