The 2021 field corn production season has not been great. Cold, cloudy, wet weather has many farmers scratching their heads about the way some of their fields look. There are many possible causes of these problems (i.e. fertility, disease, insects, nematodes, weather, herbicide carryover, etc.). My colleagues and I have tried to address these issues as needed. With that in mind, I would like to give you my thoughts on the potential for Strongarm (diclosulam), applied to peanuts in 2020, to have carried over to field corn in 2021.
1) In general, I am of the opinion that the potential for herbicide carryover (assuming 1X rates were applied) for many herbicides in Georgia is low. Why? We have warm temperatures, lighter/low OM soils, abundant rainfall, and soils that never freeze (maybe in extreme North Georgia). These are conditions that are very favorable for the microbial degradation of herbicides (the major breakdown method for most herbicides).
2) The labeled rotation restriction for field corn following an application of Strongarm is 18 months. Consequently, any grower/consultant/county agent who actually reads the label might automatically assume that this could be the cause of many field corn problems. But, there is more to this story.
3) What does Strongarm injury to field corn look like? See the pictures below. These are from research trials that I conducted in 2013/2014 where I applied various rates of Strongarm to irrigated field corn immediately after planting (i.e. PRE). Remember that the labeled rate of Strongarm is 0.45 oz/A.
4) The NOEL (no effect level) for Strongarm’s influence on field corn yield (based upon 3 years of irrigated field trials in south Georgia) is 0.029 oz/A (1/16X rate).
5) The greatest/longest reported half-life for Strongarm is 43 days(range of 13-43 days). Thus, based upon our data, it would take 172 days or 5.7 months for the 0.45 oz/a rate to breakdown to this 0.029 oz/A rate. So, field corn planted after that time period would be unlikely to be affected by Strongarm carryover (in irrigated fields and normal weather patterns).
6) A few other random thoughts:
a) Sweet corn is way more sensitive to Strongarm than field corn.
b) PRE applications of Strongarm would be more likely to carryover than POST applications (i.e. more herbicide reaches the soil).
c) Much of the Strongarm used in Georgia peanuts is applied at reduced rates. Many growers who use Strongarm PRE only apply 0.225 oz/A. Many growers who apply Strongarm POST only use 0.30 oz/A.
d) The potential for Strongarm carryover would be greater in dryland fields vs. irrigated fields.
7) I will never say never but it is my opinion that Strongarm applied at 0.45 oz/A or less to irrigated peanuts is unlikely to cause carryover problems in irrigated field corn planted more than 6 months (7+ months even safer) after application. But, it is never a good idea to use Strongarm in dryland peanut fields that will be rotated to dryland field corn and never a good idea to use Strongarm in any peanut field (dryland/irrigated) rotated to sweet corn.
8) The label is the law so growers who choose to plant field corn earlier than 18 months after an application of Strongarm do so at their own risk. It is not very likely that Corteva would ever change the current Strongarm field corn rotation restrictions based upon previous UGA test results.
9) Soil/tissue tests for pH/fertility problems and a nematode test are always very helpful when diagnosing field problems since these types of problems can look very similar to herbicide injury.
10) The refereed journal article for this research is as follows: Prostko, E.P. and T.M. Webster. 2015. Field corn response to diclosulam. Crop, Forage, & Turfgrass Management 1:1-5.