Many growers know the benefits of fall herbicide treatments, and like how fields look the following spring. We know that it’s not always possible to complete harvest and then still find the time, weather, or field conditions to get herbicides applied. This is just a reminder that we have a lot of time yet to apply herbicide this fall. In OSU weed science plots, we have typically applied most of our fall herbicides in early to mid November, but have occasionally applied into December and maintained effectiveness on winter annuals and dandelion. When we get a period of very cold weather in later November, there is typically a decline in dandelion and thistle (change from green to purplish) so that control decreases, but we seem to still control the winter annuals. So we should still have up to 6 weeks yet to apply herbicides, although we may be up against increasingly wet field conditions.
Following our previous article on fall herbicides, where we discussed how to proceed without glyphosate, we were told that 2,4-D prices have increased considerably, and supply may be short. There are apparently some ongoing adaptations of fall herbicide programs to minimize use of 2,4-D and glyphosate both, and we are getting questions about using dicamba as the base for treatments instead of 2,4-D. Among all of the herbicides we have used in fall, dicamba seems to be affected by cold weather the most and require considerable help from another herbicide to obtain comprehensive control. Some considerations based on our research:
- Do not apply dicamba alone – it won’t be effective enough and misses some key weeds (same can be said for 2,4-D which misses chickweed)
- When using dicamba as a base to mix with lower rates of 2,4-D or glyphosate, use a dicamba rate of at least 0.5 lb ai/A. In the mixtures mentioned below, use a rate between 0.25 and 0.5 lb ai/A, depending upon how effective the mix partner is.
- Mixtures of dicamba and metribuzin can be “good enough”. Our most typical mix has been 0.25 lb ai dicamba plus 0.38 lbs ai metribuzin. We have not tested mixtures of dicamba with simazine.
- We have tested mixtures of dicamba with some ALS inhibitors (Harmony Xtra and Express) in the fall in wheat, and they have worked well. We have not tested these combinations in the fall in a fallow situation but our assumption is that they will provide adequate control there as well. This would include combinations of dicamba with products containing tribenuron and/or rimsulfuron, or chlorimuron.
- In our research, we do not use adjuvants with dicamba/2,4-D. If they are mixed with glyphosate, we add AMS. Treatments containing metribuzin, simazine, and the ALS inhibitors are usually applied with crop oil concentrate.
Different topic – the shortage or anticipated shortage of various products has caused some growers to buy and take possession of herbicide this fall, including filled shuttles, for use next spring (maybe this is a common practice for some growers – we don’t follow how herbicide moves around). Be sure to know the storage requirements for situations like this – minimum temperature, etc. Also be aware that changes in the product – separation or settling out of certain components – can occur over time, and there may be recommended procedures to prevent this or restore the integrity of product at the end of long storage. Check with manufacturer and distributor representatives for the appropriate information.