Replanting: Consider Previously Used Herbicides (York)

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The cold, wet weather we have experienced this spring has not been conducive for good cotton stands and early growth. And, we are beginning to see seedling diseases become an issue. A number of growers have or likely will face a decision to replant either to cotton or to soybeans. In either case, the herbicide(s) applied to the first planting need to be considered.

If replanting to cotton:  In most cases, one can replant cotton with no concern over herbicides used on the original planting. The exception is Warrant (or Warrant Ultra).

Acetochlor, the active ingredient in Warrant, is absorbed by the hypocotyl (portion of stem below cotyledons) as the cotton plant emerges. If the cotton emerges timely, before much acetochlor is released from the capsules, crop tolerance is good. We do see injury on cotton that lays in the ground for extended periods before germination. We think that is because the capsules have released more acetochlor than the emerging cotton can handle.

For the same reason, one must be careful when replanting behind Warrant. While you are deciding whether or not to replant, the capsules have released a good deal of acetochlor. Then, when you replant into that acetochlor, the new seedlings can absorb a damaging amount of herbicide as they emerge.

Based upon research in Georgia by Stanley Culpepper, the following are recommendations when replanting cotton on land originally treated with Warrant or Warrant Ultra:

  1. Cotton can be replanted 2 weeks after the original application if the land is strip-tilled, with ripper shank attached, before replanting. The strip-till unit, with ripper shank, will pull up enough untreated soil to dilute the acetochlor.
  2. If no tillage is performed, wait 3 weeks before replanting.

If replanting to soybeans:  Soybeans are tolerant of flumioxazin (Valor, others), trifluralin (Treflan, others), pendimethalin (Prowl, others), fomesafen (Reflex, others), and acetochlor (Warrant). Hence, any of these products applied to cotton will not be an issue if replanting to soybeans. Pyrithiobac (Staple, others) is hard on soybeans; if pyrithiobac was applied preemergence on cotton, replanting to soybeans is discouraged.

That leaves diuron (Direx, others) and fluometuron (Cotoran) to think about. Labels for diuron and fluometuron will not let you replant to soybeans. However, we conducted research at three locations in 2013 and 2014 to determine the potential to replant to soybeans after application of diuron or fluometuron to cotton. We broadcast either 1.5 pt of Direx or 1 qt of Cotoran at cotton planting. Then, we planted soybeans 3, 6, or 9 weeks after Direx and Cotoran application. Soybeans were planted no-till or the land was disked prior to planting soybeans.

We often think that deep disking will dilute the herbicide and lessen injury to replanted soybeans. Interestingly, we observed the opposite. We had less injury when the soybeans were planted no-till.

Soybeans were injured only 1 to 15% (6% averaged over locations) when planted 3 weeks after Direx application, and yield was not reduced. There was no injury when soybeans were planted 6 weeks after Direx application.

Cotoran was more injurious to soybeans than Direx. Soybeans planted 3 weeks after Cotoran application were injured 6 to 33% (17% averaged over locations) and yield was reduced at 2 of 3 locations. There was little to no injury and no yield impact when soybeans were planted 6 weeks after Cotoran application.

Soybeans, as we all know, respond to planting date. On the average, later planted soybeans yield less. That showed up very clearly in our research. Even though soybeans planted 3 weeks after Cotoran were injured and yield reduced, the yield when planted 3 weeks after Cotoran was still greater than the yield of no-Cotoran soybeans planted with a 6- week delay.

This research can be viewed at http://www.cotton.org/journal/2015-19/3/upload/JCS19-613.pdf.

When replanting to soybean following cotton treated with diuron or fluometuron, one should avoid metribuzin-containing soybean herbicides.