Palmer Pigweed Prevention – It’s Possible – 5 Things To Remember

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Phillip McKibben
believes that the cheapest way to control Roundup-resistant Palmer pigweed is to never let it get established. Over the last 5 years, this north Mississippi crop consultant has gone to great lengths to do just that.

Along the way, he has learned a good deal about how to spot problems and keep fields Palmer-free. His insights are worth knowing, particularly if you’re in an area where resistant pigweed hasn’t taken hold yet.

McKibben’s approach with his farmers might be described as educational but it also has a strong hint of revival meetings and lessons on the hard reality of letting even one pigweed slip by.

Early on, after urging one client to pull Palmer pigweeds that had been developing, McKibben found a monstrous female Palmer pigweed that had grown wide and tall on one of the farm’s ditchbanks. He sawed it off at the base and loaded the plant into the back of his pickup. That single weed virtually filled up the short bed of his crewcab truck (see photo above) and lapped over the sides. The plant’s trunk, McKibben said, was as big as his arm.

“I took it to the farmer’s shop and had him and his two sons come out in the lot to see it,” McKibben told the farmer: “I’ve been recommending that you pull these scattered Palmer pigweeds, so I wanted to show you what this particular pigweed species looks like if left alone.”

His intention, McKibben said, was “to put the fear of God in their hearts.” He succeeded. The family threw themselves into cleaning up ditchbanks and any Palmers they found in their fields.

On a couple of other occasions, McKibben has driven clients to fields outside their area so they could see what “20,000 pigweeds per acre looked like in a field that had already been sprayed 4 times with Roundup. That got their attention.”

McKibben may occasionally pull up a 6-foot Palmer pigweed and show it to the farmer just to emphasize the need to move quickly. “It’s one thing to tell the farmer that I’m finding a few Palmers,” McKibben says, “but it gets the point across if I’ve got that plant with me and it’s just starting to wilt.”

Through the process of trying to keep fields Palmer-free, McKibben has become a keen observer of how the weed tends to develop, at least in his area.

Much of what McKibben has learned boils down to 5 key points:

#1 – Don’t blame the birds for Palmer pigweed infestations.

While it’s often said that birds can carry the seed or that winds can move pollen from resistant plants into areas with no resistance, McKibben says any potential problems in his farmers’ fields seem to start with mechanical movement.

Birds could possibly have been a factor, he adds, “in one particular location” across all the acreage he works. But even certain random plants that might be written off as bird deposits could otherwise be explained by other human activity.

“I’ve found a single resistant plant, on occasion, in a field of wide-row soybeans and it was growing precisely in that one-inch seed band, not in the middles,” he says. “What are the odds that a bird would drop the seed right there? That leads me to believe that the seed came with the soybean seeds.

“I’m not necessarily being critical of how the seed were cleaned. Considering how small pigweed seeds are, at some point one of them might get through when soybeans are cleaned. It may be one seed per 500 acres or one per 50 acres, but if it produces a female plant then it still might turn out enough seed to start an initial population.”

#2 – Humans move most resistant Palmer pigweed seed.

McKibben, who mainly works in the state’s hill country, first saw Palmer pigweed emerging in fields where several of his farmers produced sweet potatoes. Foundation seed potatoes – brought in from an eastern state – harbored resistant pigweed seed, and that was all it took to get an infestation going.

Not surprisingly, he’s dealt with pigweed “planted” by combines that had been used for custom harvesting elsewhere or had been bought “barely used” from sellers in areas with heavy pigweed pressure.

Not every case can be directly traced, although the evidence points to some type of mechanical movement. After highway crews spray Roundup-type herbicides along a right-of-way, McKibben sometimes spots a lone, still-healthy Palmer pigweed among the dead vegetation.

Or, he’ll find a single Palmer on the edge of a field where it adjoins a turnrow or county road. If a combine passed through the area after harvesting acreage with Palmer infestations, it might have dropped a seed or two along the way, he speculates. When he first spots Palmer pigweeds in a field, those plants typically are within 2 feet of the field edge.

#3. You can make headway in preventing or turning back Palmer pigweed.

Overall, out of several hundred fields his firm works, McKibben says that about 60% are in “great shape,” meaning they started Palmer pigweed-free and remain so, even when other farmers around his clients are heavily fighting the weed.

“I feel good about another 20%,” he says. In those fields Palmer has been contained where populations developed.

That’s a total of 80%. With most of the remaining 20%, “we’ve reduced the seed bank compared to what we had in years past. In just a few cases we’ve gone backwards where weather delayed residual herbicide applications.”

Critical situations, he adds, tend to arise when farmers take on new acreage that already has been infested with resistant Palmer. The challenge is to minimize spreading seed and eliminate pigweed emergence going forward.

#4. Palmer pigweed doesn’t happen overnight.

“When people find spots of maybe 100 escaped pigweed after an application, they often assume that they’re looking at the first or second year of an infestation. But the problem started earlier, I suspect. For whatever reason, a single seed may have gotten into the field and nobody noticed or thought much about that lone plant. But that resulting plant went to seed. Even without a male plant nearby to provide pollen, a female plant may produce a few seeds.”

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If that generation goes unnoticed, he adds, the populations will build a bit more. Those escapes that someone finally notices may be the result of three or even four years of buildup.

A single plant one year might mean 10 more plants the next year and 100 the year after that. If 1,000 plants emerge in that fourth year and treatments take out 90% of them, 100 plants are left to keep pressure rolling into the future.

When his farmers see a lone pigweed plant now, they “wade into the crop and pull it up,” says McKibben. “You can’t afford to let it go. Otherwise, you’re just asking for an infestation.”

#5 – Diligence and scouting pay big time.

Of his farmers who remain Palmer-free, “they really aren’t doing anything much differently than before Palmer pigweed became established in this area except that they exercise extreme diligence and close timing.”

To some degree they’re also using more residual herbicides, he says. But the key factor with herbicides is proper timing.

Scouting also allows McKibben and his farmers to tailor herbicide programs to the populations at hand.

“I hear people talking about ‘declaring war on Palmer’ or going with a ‘take no prisoners’ approach to pigweed management,” McKibben says. “More than likely, that means they’re going with an aggressive program on every acre, even if they don’t actually have Palmer in every field.”

In certain fields he can find red root or smooth pigweed but not Palmer. Those other pigweeds are simpler and cheaper to control, McKibben points out.

“If all you have are smooth or red root pigweed, you can get by for as little as $20 an acre compared to an $80-an-acre program where you’re spending extra to control Palmer that’s not even there.”

The cost of controlling Palmer comes into sharper focus with McKibben’s farmers who also produce sweet potatoes, a crop they typically rotate with soybeans. Where farmers rent land for 5-year stretches to grow that high-value crop, pigweed must be aggressively attacked early on.

One farmer sent pulling crews into a field 6 times one year and 4 times the next year to mitigate Palmer pressure.

“In one case, a farmer spent $18,000 (on manual weeding) to deal with Palmer on a 60-acre field, over and above what he also paid for herbicides,” McKibben recalls. “That’s a situation nobody wants to repeat.”