Tennessee: How Extensive Is Herbicide Resistant Pigweed?

We will learn a lot over the next couple of weeks on the extent of the glyphosate and PPO resistant Palmer amaranth.  In an effort to get an early estimate of where we are at, we randomly collected Palmer amaranth from fields in Shelby, Hardeman, Tipton, Madison, Weakley, Obion, Gibson, Carroll, Lake and Dyer counties. 

We sent these samples to Dr. Bryan Young at Purdue University to determine if they had the PPO resistant gene.  Their test can also tell if the resistant trait is heterozygous or homozygous resistant. 

That is, did the resistance gene come from both parents or just one.  We just received some of the results.

If we take into account the survey we conducted last year with the results to date from this year, we are finding that about 20% of the fields we sampled showed some PPO resistance.

In almost all cases we are finding 10 -20% of the total number of plants sampled from each field have tested positive for PPO resistance.  That is good that most fields are at that low a level or is it?

The key is how large is the Palmer amaranth soil seed bank.  If it’s low then 10 to 20% of the population being PPO resistant is manageable with a chopping crew. 

If the Palmer pigweed soil seed bank is more robust then even 10% of the population escaping a PPO herbicide is only manageable with a disk and replanting to Liberty Link soybean.   I have had reports of both occurrences happening already this year.

We also went in and sampled Palmer amaranth in a field that had escaped a Prefix and glyphosate application when it was 2” tall. What we found in a random sample of those Palmer was that 97% of them contained the resistant gene. I heard a serious chopping crew was sent to that field.

Another interesting result of the survey to date is that all the Palmer amaranth that has tested positive for the PPO resistance gene has been heterozygous resistant.  That is, the Palmer only contains one of the possible two genes for PPO resistance.

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In the Midwest where PPO-resistant waterhemp is common, homozygous resistant plants is more the rule than the exception.  That we are only detecting heterozygous resistant (just one gene) suggests we are on the early end of the problem.

Unfortunately, the resistance will most likely get worse both in spread and in a higher proportion of Palmer amaranth containing 2 genes for PPO-resistance as the gene becomes more endemic in our Palmer population.

For now, with most of our Palmer amaranth containing just the one PPO-resistant gene, we have seen that a full rate of fomesafen applied in the middle of the day on <2” pigweed can provide up to 60% control.   This clearly is not good enough but will be better control than one can expect if the Palmer amaranth contained two PPO-resistant genes.  

This research was sponsored by the Tennessee Soybean Promotion Board and we appreciate their support as we couldn’t do this and other applied research without it.