Not Scared Of Palmer Pigweed? Better Recalibrate Your Thinking, Says This Consultant

Editor’s Note: The author, Matt Hagny with Pinnacle Crop Tech in Salina, Kansas, has been an independent crop consultant specializing in no-till systems since 1994.

Some farmers outside of the South still seem a bit too relaxed in their approach to Palmer pigweed control. That could be an expensive oversight. This threat isn’t going to be easily overcome — like we did with marestail, ragweed, and many other weed-resistance problems.

Those are weak sisters compared to Palmer pigweed. Its characteristics give Palmer a competitive edge well beyond that of most weedy species.

For one thing, Palmers are obligate outcrossers — meaning, the male and female flowering parts are on completely separate plants. These are termed dioecious. So, they cannot self-pollinate, and that’s a rarity in the plant kingdom. This results in far higher genetic diversity. (Waterhemp and other pigweeds native to North America are also dioecious.)

Any new and advantageous traits – such as herbicide resistance – can quickly sweep across entire regions. 

Other advantages include:

  • Palmer’s pollen can travel dozens of miles.
  • Palmer’s tiny seeds can hitch rides on almost anything, including the digestive tracts of migratory birds, like ducks and geese.
  • Female plants produce enormous amounts of seed – up to several hundred thousand on a single plant.
  • It can grow up to 8 feet tall with stalks several inches in diameter at their base, which can make soybean harvest problematic or even impossible.
  • The weed loves hot, dry conditions. Keep in mind that Palmer is a native of the desert Southwest. Early on, some people speculated that Palmer would not take hold in the Midwest because the weather is too cool. That was wishful thinking, based on reports now from states bordering the Great Lakes. Remember, its outcrossing nature and high genetic diversity make it extremely adaptive.

Limited Post-Emerge Options Now

With all those advantages, Palmer is a formidable opponent and it’s now resistant to all but two post-emerge options — paraquat and Liberty and its generic equivalents.

If this species develops resistance to those chemistries, we’re in really deep water. No other weed is resistant to so many things and develops and spreads resistance as fast, with the possible exception of waterhemp in the Corn Belt. But Palmer stands out among summer weeds, with its highly aggressive growth capabilities and its prolific seed production.

Now do you understand why I think 100% control of Palmer pigweed should be the goal? I have clients who are well on their way to that — sometimes getting by with no post-emerge herbicides in their beans. Those growers actually get by with just a little rogueing.

Let’s Get Paranoid About This Weed

If you think I’m too alarmist, consider that land values and rents in the Deep South are strongly affected now by Palmer pressure. With only two modes of action (MOA) remaining consistently effective for post-emerge control, Palmer will likely get worse before it gets better. Granted, some new MOAs and other technologies are in the pipeline, but do not expect any silver bullets in the next 5 to 10 years.

Many of you have read by now that a Palmer pigweed population at a Kansas site is resistant to both 2,4-D and dicamba applied post-emerge. But this merely confirms what I and others have been saying since last summer or even prior to that. We have seen many instances where Palmers weren’t dying after 2 timely applications of full rates of dicamba in Xtend beans.

They didn’t just survive – those Palmer plants went on to set seed. In meetings and through my newsletters, I have a long and very public record of warning about the risks of selecting for resistant populations by overusing certain MOAs. I’m not a prophet, just a guy who understands weed and pest adaptation to control measures. What we’re seeing now was inevitable.

Further note that the Kansas State University report about dual resistance also mentions that this population is resistant to the shoot-inhibitor metolachlor, as well as post-emerge PPOs — something I suspected was happening several years ago.

All this is on top of widespread Palmer resistance to ALS chemistries, atrazine (pre and post), glyphosate, and post-emerge HPPDs (Callisto, Laudis, etc.) in Kansas.

Don’t Assume You’ll Knock It Out In Corn

Are you planning on controlling Palmer in your 2019 crops with post-emerge dicamba products (including Status in corn)? If so, you might be in for a rude awakening. Palmer has been surviving to some degree all along, but the escapes mostly stayed below the crop canopy and went unnoticed. The female plants still set seed.

Even scarier, every generation is more resistant and more vigorous, so we’ll soon find dicamba-resistant Palmers towering above the crop, having shrugged off the onslaught of dicamba. That could happen this year in isolated spots. If not now, then soon.

Approaches Going Into 2019

Here’s the best advice I can give…

Do everything possible to avoid relying on post-emerge dicamba (or 2,4-D, or HPPDs, or glyphosate) for Palmer control this year. If your corn hybrids are LibertyLink, use Liberty plus atrazine – and whatever else you want or need that doesn’t antagonize Liberty post-emerge.

Go with an HPPD (e.g., Callisto) pre-emerge plus dicamba preplant or pre-emerge. Soil-applied dicamba is still effective on all Palmer biotypes, but it’s only good for a short time, maybe one or two rains. Higher rates somewhat help extend this control. Results, as they say, can vary.

For soybeans, applying the max rate of sulfentrazone for your soil is the absolute best money spent, and it’s one of the gentlest herbicides on soybeans so long as it gets rained in before you seed the beans. Go with 0.37 lbs/acre a.i. for fine-textured soils above 2% OM. For sandier soils or high pH, cut back on the rate.

For cleaning up escapes, Liberty in LL/Enlist beans works if applied timely and under appropriate conditions. For Xtend beans, you can apply dicamba preplant or pre-emerge. In the eastern half of Kansas, fomesafen is labelled, and can help considerably if used before Palmer emergence. Depending on crop rotations, you may want to use less than the full rate. (That’s a bigger subject that you may want to explore on your own.)

Beyond those recommendations, you’re down to shoot-inhibitors (Zidua, S-metolachlor, Warrant, Outlook) and those give less bang for the buck than the other options. However, they may be necessary for moderate to high Palmer pressure, or if you have no post-emerge control options.

Rogue diligently. Whatever herbicide program you use, it’s important to deal with escapes by rogueing. Your ability to grow summer crops and your land value depend on it. I do a lot of rogueing while scouting fields, so this isn’t an armchair recommendation.  

Do it yourself or hire a crew. Pulling is more sure-fire than hoeing. With hoeing, you must chop below the lowest node, or they will regrow.  If rogueing occurs early enough, the plants can be left in the field. Later in the season, as Palmers begin to set seed, you will need to bag the plants and carry them out of the field for disposal.

Inhibiting Palmer Germination – Other Methods

If you have a good, thick cover crop of cereal rye or another winter cereal like triticale or barley, this can work as well as or maybe even better than some herbicides in no-till beans. Please note that it is hardly ever sufficient on its own, especially not if the cover crop is thin in places.

And certainly not if your planter or drill does any significant soil disturbance when planting the soybean crop. But a good cover crop provides a very worthwhile level of suppression.

Another help is gaining a fast canopy. For soybeans, this means narrower rows, 10-inch spacing instead of 20-inch, for example. However, I’m reluctant to run the extra openers unless there’s a thick mat of rye cover crop ahead of the beans. Again, you don’t want to disturb too much soil and trigger more weed germination.

For beans planted directly into corn or milo stalks with no rye cover crop, I’m also reluctant to trample any more stalks or compact the soil more by running extra openers, especially south of I-80 on upland ground. We already have too many issues with erosion. And soil moving around, even on a micro scale, breaks the barrier of your soil-applied chemicals, resulting in outbreaks of Palmer and whatever other seeds want to germinate.

Parting Comments

So, good luck in the Palmer battle this season. For more details on herbicide choices, see the February 2019 Crop Health Workshop booklet webinar that I co-hosted with Leland Baxa. Leland has ample experience with herbicides and resistance management. The material helps sort through all the choices.

Palmer pigweed demands attention, whether the information comes from me or someone else. I’m reminded of a widely quoted comment by Billy McLawhorn, a veteran North Carolina crop consultant, who said:

“Of all the problems I’ve read about in farm magazines, Palmer pigweed was the only one that was just as bad as they said it would be when it finally got here.”

Let’s take that to heart, shall we?