By Larry Steckel, Extension Weed Specialist
There continues to be quite a few folks wanting to know how fomesafen applied last year could affect grain sorghum planted this spring. The concern is understandable as grain sorghum is very prone to injury from fomesafen carryover.
The answer to this question is not completely straight forward for a number of reasons. First there are numerous herbicides that contain fomesafen and they can range from a 0.56 lb ai/gallon material to a 2.883 lb ai/gallon material. Therefore, one has to think in active fomesafen/A and not just ounces of a given trade name. Please keep in mind that the maximum season use rate of fomesafen is 0.375 lbs ai/A. Below is a list of a number of herbicides that contain fomesafen and their corresponding maximum ounce rate that provides 0.375 lb ai/A of fomesafen.
| Product | Fomesafen lb ai/gal | max oz/a =0.375 lbs ai/ A |
| Reflex | 2 | 24 |
| Flexstar | 1.88 | 25.5 |
| Prefix | 0.95 | 45.6 |
| Flexstar GT | 0.66 | 72.7 |
| Flexstar GT 3.5 | 0.56 | 85.7 |
| Dawn | 2 | 24 |
| Rhythm | 1.88 | 25.5 |
| Intimidator | 0.67 | 71.6 |
| Marvel | 2.883 | 17.1 |
| Cheetah Max | 1 | 48 |
Another reason that it is not a straightforward answer is that the likelihood that fomesafen is going to be a problem in a given grain sorghum field this year will vary due to a number of factors. The timing of application(s) last spring/summer, the total amount of active ingredient of fomesafen applied last summer, soil type and weather all have a great impact on carryover issue this spring.
Here are a couple of items to consider when assessing this issue.
- Maximum fomesafen season use rate is 0.375 lbs ai/A
- There is a 10 month recropping to grain sorghum requirement. This would dictate that a fomesafen applied July 1, 2014, could harm grain sorghum if it is planted before May 1st 2015. Grain sorghum could not be planted before June 1, 2015 if fomesafen was applied on August 1st 2014.
- DO NOT plant grain sorghum in 2015 if the maximum amount of fomesafen (0.375 lb ai/A) was exceeded incrop in 2014 as grain sorghum injury is likely. This would be an off-label application.
A June planting date for grain sorghum from both an agronomic stand point as well as insect management standpoint would suggest that fields with probable fomesafen carryover should be planted to soybean this spring.
If one has concerns that a field may have some fomesafen carryover then a soil test can be taken. Some have elected to do this already. If you go this route it is important to note if you are taking a 6” deep sample or a 3” deep sample, etc.….. This has huge impact on how the results are interpreted. Also please note that those results could vary across a field so they will need to be taken from a cross section of a field.
Some folks have said that since they sprayed a dense canopy of soybean and weeds last fall there will be less likelihood for carryover. That is true to a very limited degree. However research has shown that enough fomesafen will make it’s way to the soil to be a problem this year.
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