‘Houston, we have a problem’

• By Larry Steckel •

A year ago this month, I posted a blog titled “Dicamba in Tennessee: Year 3.” At that time, the Tennessee Department of Agriculture had fielded 45 official dicamba drift complaints. There were some issues on poor grass control, but overall weed control was good. So what is the status of Xtend weed management in 2019?

It was fairly calm for most of the spring and early summer on the dicamba drift front, but there has been a flurry of calls to Extension and the TDA as of late. The change seen last year where the drift injury reported moved from non-Xtend soybeans to other broadleaf plants continues into this year.

Most of the reports have been on tree nurseries, trees in state parks and, of course, gardens and landscaping from home owners. Of the roughly 1.5 million acres of soybeans in our state, no more than 30,000 acres is planted to anything other than Xtend soybeans. Reports and field visits would suggest that a good percentage of them have dicamba injury symptoms ,which is similar to last year.

In 2019, the weed control in the Xtend system was not near as good as in the first few years this technology was used. Most notably, the grass species, especially junglerice, goosegrass and Johnsongrass, have become very troublesome and expensive for growers to control.

Several cotton growers this year stated their weed control costs have doubled to tripled trying to control grass in Xtend cotton. Moreover, prickly sida has become more of an issue and is in my mind a weed shift as glyphosate has never provided good control of that weed. And it is clear that a 0.5 lb rate of dicamba is not improving that level of control.

Palmer control slips

Also in 2019, a number of retailers and farmers told me that the Palmer amaranth control experienced with Engenia or XtendiMax + glyphosate had slipped compared to when they first used that mixture in Xtend crops. To test this observation, we screened dicamba on Palmer collected from these fields to Palmer amaranth that was sourced back in 2014 prior to the introduction of Xtend crops.

In order to get Palmer amaranth from this year, we took soil from fields where dicamba had been disappointing in its control and grew out seedlings. We screened both populations with an Engenia rate of a 0.25 lb, 0.5 lb (12.8 oz/A), 1 lb and 2 lbs.

Picture 1.

palmer pigweed dicamba screening
Picture 1

The first observation of this one test would suggest that their concerns were not without merit. At 10 days after application, the 0.25 lb rate of Engenia had provided complete control of the 2014 Palmer amaranth but only partial control of the 2019 population (Picture 1).

This would be consistent with field visits where many of the escaped Palmer amaranth from Engenia + Roundup PM were often times found on field edges where sprayers would slow down, resulting in less pressure from the pump to adjust for the speed and likely less coverage.

The first couple years folks used dicamba, they were getting control even on the field edges but not in 2019.

Picture 2.

dicamba pigweed screening
Picture 2

There was also a noticeable difference 10 days after application of the 0.5 lb (labeled rate). The 2014 Palmer amaranth was completely controlled while there are still some surviving from the 2019 population (Picture 2).

This too would be consistent with field observations from this spring where a sequential application of a labeled dicamba product, or a follow-up application of Liberty, was often needed to finally get control. Complete control was seen with the higher Engenia rates.

The status of the Xtend weed management system here in 2019 could be better. First, off-target dicamba issues have not appreciably improved from 2018, and many feel the dicamba symptomology showing up on broadleaf plants across the landscape continues to be an issue.

From a weed management perspective, we need a new game plan on grass control as what we are doing now is not working on junglerice, goosegrass or Johnsongrass. We also need a new plan on prickly sida. Finally, many have reported that more follow-up applications were necessary to control Palmer.

History was made 50 years ago this month with the Apollo 11 moon landing. I think we can sum up Tennessee’s Xtend weed management experience in 2019 not so much with Apollo 11 but a quote attributed to Apollo 13: “Houston we have a problem.”

Dr. Larry Steckel is University of Tennessee Extension weed specialist. He can be reached at lsteckel@tennessee.edu

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