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PPO/glyphosate resistance in Palmer not an overnight event

PPO/glyphosate resistance in Palmer not an overnight event
How widespread is PPO/glyphosate-resistant pigweed? How long did it take to develop?

It is becoming clear we aren’t at the beginning of things with respect to detecting PPO/glyphosate-resistant Palmer amaranth in the Mid-South. It became established two to three years ago.

From greenhouse screens for PPO-resistant Palmer amaranth in Arkansas and Tennessee, we continue to find new fields. Other neighboring states are finding it as well. Folks at the University of Kentucky have confirmed PPO-resistant Palmer amaranth in at least one county in western Kentucky.

A recent Jason Bond Twitter post indicated it is present in Mississippi. Moreover, Bond believes the PPO-resistant Palmer is likely as widespread in Mississippi as it is in Arkansas and Tennessee.

The PPO-resistant Palmer amaranth did not appear overnight across four states. It had to take a couple years to achieve this.

In the fields with PPO-resistant Palmer amaranth we have run experiments trying to assess the percent infestation of PPO-resistance in the population. What we found in a couple of fields is about 25 percent of the Palmer amaranth is PPO-resistant. From a genetic standpoint that suggests the resistance in those fields was established two years ago.

We can safely say PPO resistance will be more widespread this spring and in a higher proportion of the population in fields where it was confirmed in 2015. In short, PPO resistance will be more the rule than the exception across much of the Mid-South this year.

How are we are going to manage this new problem?

To begin with, we need to have some indication if the PPO-resistant Palmer amaranth can still be controlled with a pre application of a PPO herbicide. The early indication from some greenhouse work suggests that the length of residual control of PPO-resistant Palmer amaranth is reduced about 50 percent compared to PPO-susceptible Palmer. This means we need to start utilizing more metribuzin and Group 15 herbicides like Dual Magnum, Zidua and Outlook in the pre-applied window.

Liberty is the last herbicide standing that will control Palmer amaranth post in our soybean fields in 2016. As I visit with growers and consultants at winter meetings it is clear that there is a big rush to acquire LibertyLink soybeans to combat this new problem.

For folks new to utilizing Liberty to manage Palmer amaranth, there are some tactics to use to improve the consistency of control. Coverage is critical, so apply at least 15 gallons per acre and utilize nozzles that will give you good coverage. I prefer some of the dual fan nozzles like the Wilger Y or the Greenleaf TADF.

The time of day Liberty is applied makes a big difference on pigweed control. For best results, apply Liberty in the window of two hours after sunrise to about an hour before sunset.

In soybeans consider mixing a PPO herbicide like Flexstar, Prefix or Ultra Blazer in with the Liberty. In our research that tankmix provides more consistent Palmer control than either one applied alone. Also, from a resistance management standpoint, it makes a lot of sense because the Liberty can control the PPO-resistant Palmer and the PPO herbicide can control those rare individuals that may be Liberty-tolerant.

We also need to utilize cultural weed control practices. Across Tennessee many fields have a cover crop. Following a very warm December, the covers look to be in good shape and should help provide control of horseweed and good suppression of Palmer amaranth.

In some of our 2015 research we examined the effect of cover crops on Palmer amaranth in soybeans. We found Palmer in a non-cover treatment took eight days to emerge and reach 4 inches tall while in a cover crop of cereal rye and vetch blend it took 30 days to reach the same stage. Cover crops able to buy that much time can be all the difference in getting Palmer sprayed timely post.

Another cultural practice — rotating soybean fields where PPO-resistant Palmer amaranth was a problem in corn or grain sorghum — is another good way to go.

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