RALEIGH — No one likes to replant any crop, but sometimes it is inevitable. For various reasons, we lose our stand. When we lose a cotton stand, we must make a decision to either replant to cotton or to plant another crop. Much of that decision depends on the calendar. When it becomes too late to replant cotton, soybean is the crop we typically turn to.
Recently, I posted an article on the shortage of Cotoran this year. More specifically, I addressed alternatives to Cotoran for control of glyphosate-resistant ragweed. Direx, Reflex, or especially a combination of Direx plus Reflex is our best option in that situation. Apparently some of you read that, and now I am getting questions about the injury potential of Direx versus Cotoran and the risks of replanting soybean behind Direx-treated cotton.
No one likes to see herbicide injury on their cotton. However, as I responded to one grower this week, you can’t have your cake and eat it too. In other words, if you believe preemergence herbicides are beneficial in terms of weed control (and in this day of resistance, I hope you feel that way), then you have to accept some risk of herbicide injury. Specifically related to Cotoran and Direx, one can get injury from either herbicide. It can happen when rainfall moves a significant amount of the herbicide into the root zone of small cotton. Thankfully, we don’t see significant injury from either herbicide in most cases. But, to the question at hand, which of the two is more likely to cause injury? In my experience, Direx is at least as safe as Cotoran.
So, can we plant soybean behind a failed stand of herbicide-treated cotton? For most cotton herbicides, the answer is easy. Do not attempt to plant soybean behind Staple. On the other hand, there would be no concerns planting behind Dual, Liberty, Outlook, Prowl, Reflex, Roundup, Treflan, Valor, or Warrant. That leaves Cotoran and Direx. Can you replant to soybean if you have Cotoran- or Direx-treated cotton and lose your stand? Labels for both of these herbicides basically say you cannot replant to soybean in the same year. But, if you have no choice, what is the risk and would it help to till the land before replanting in an attempt to dilute the herbicide?
During 2013 and 2014, one of my graduate students, Lewis Braswell, did some interesting and useful work to determine the risk of replanting to soybean behind a failed stand of Cotoran- or Direx-treated cotton. Cotoran at 1 quart per acre or Direx at 1.5 pints per acre was broadcast in late April. Then, soybean was replanted at 3, 6, or 9 weeks after Cotoran or Direx application. An additional variable was tillage (deep disking) or no tillage prior to replanting to soybean. The experiment was conducted at three locations over the two-year period.
Interestingly, soybean was more tolerant of Direx than Cotoran. At one location, we saw little injury from either herbicide, even when soybean was planted three weeks after cotton herbicide application (see Table 1 below). At the other two locations, Cotoran caused more injury than Direx when replanting occurred at three weeks. The typical injury symptoms (yellowing, burning of lower foliage) were observed along with stand reduction. However, Jim Dunphy has been telling us for years that soybean can compensate for thin stands, and that was evident in this experiment. When replanting occurred at six or nine weeks, there was little injury from either herbicide.
The yields were interesting. At Rocky Mount in 2014, good yields were obtained with all treatments (see Table 2 below). Compared to plots with no cotton herbicide, neither Cotoran nor Direx reduced yield with any replant delay. At the other two locations, neither Cotoran nor Direx impacted soybean yield if replanting was delayed six or nine weeks. With the three-week delay, Cotoran reduced soybean yield 13 to 16% but Direx did not statistically reduce yield.
Soybean planting date had no effect on yield at Rocky Mount in 2014 (Table 2). The experiment was on good land and rainfall during most of the season was excellent. Regardless of planting date, we met Dunphy’s goal of the crop being three feet tall and the middles lapped by early bloom.
We had a strong effect of planting date at both locations in 2013 (Table 2). Compared to the no cotton herbicide plots with a three-week replant delay, delaying replanting until six or nine weeks reduced soybean yield 23 to 24% and 43 to 46%, respectively. Also note that while replanting three weeks after Cotoran application reduced yield relative to the yield of the no cotton herbicide check, the yield when replanting three weeks after Cotoran was still greater than the yield when replanting after a six-week delay because of the strong influence of planting date on yield potential.
If you have to replant soybean behind Cotoran or Direx, will it help to aggressively till the land before replanting? Based upon this experiment, the answer is no. We did not see a tillage by herbicide or tillage by replant delay interaction. However, averaged over herbicides and replant delays, yield of soybean planted with no disking was 12 and 16% greater at Lewiston and Rocky Mount in 2013 but not different at Rocky Mount in 2014 (data not shown).
So, here is the take-home message. This research suggests fear of having to replant to soybean is not justification to avoid use of Direx.
— Dr. Alan York
Named Professor Emeritus
Crop and Soil Sciences – NC State University
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