Since he farms on weekends, Jake Fisher, superintendent of the Delta Research Station in Portageville, Mo., has good insight into issues growers face. He recently asked what we were doing about pigweed. Of course, he knew the answer (we're working on it), and we knew what he meant (keep working on it!).
We continue work on Palmer amaranth control in corn, rice and soybeans. But I especially want to mention work in cotton targeting pigweed problems. These pigweed studies are being funded by Cotton Incorporated's state support program.
Ten years ago, pigweed was considered easy to control. A good recommendation was “spray something.” Today, we consider the weed difficult, and the most intensive treatments seem to fail.
It's also strange to consider what's happened since Roundup Ready has become popular. Ten years ago, morningglory was our biggest problem. Then we switched to glyphosate, which is considered to be weaker on morningglory and stronger on pigweed. But it's morningglory concerns that have faded while pigweed is capturing headlines.
Here's one more angle on pigweed: in northern Missouri, growers complain of the waterhemp species of pigweed while we complain of the Palmer amaranth species. These two pigweeds have some differences, but the big similarity is they are both causing headaches. Waterhemp seems to be troublesome north of US Highway 60 and Palmer dominates south of the highway.
Reid Smeda (a weed scientist in Columbia, Mo.) planted waterhemp at Portageville, and it went away after three years.
In soybeans, Roundup Ready has been the answer. If you have pigweed problems, Roundup Ready is probably the way to go. Just keep your eyes open — some of the waterhemp pigweed has been acting like it's glyphosate-resistant. The chemists can't agree on what's actually happening regarding the resistance claims. But whatever you call it, the weed sometimes survives treatment.
Many say that Palmer amaranth is a good candidate to develop glyphosate resistance, and it is. But while we've all been watching pigweed, mare's tail turned out to be the dark-horse candidate that won the resistance race.
Roundup Ready cotton has also been a God-send for pigweed control. However, I'm not so sure that it isn't causing some of the pigweed problems. Roundup Ready cotton fields often have good pigweed control, with an occasional big escape every 200 feet or so. These escapes die if you spray glyphosate on them, so they aren't resistant. What we are probably seeing is that plants right in the drill are physically missed by the post-directed spray.
Also, the use of residual herbicides has declined greatly, allowing Palmer amaranth to continue to germinate. Palmer will grow up to a foot per week, and glyphosate has no residual activity, so an occasional plant does seem to be slipping by.
In cotton, we've had some good luck with early postemergence Dual/glyphosate mixes. With post-directed treatments, it's more important when you spray than what you spray. Residual herbicides should be used in post-directed, hooded and layby treatments. But if you spray on time, most will work.
We might want to consider making more applications of residual herbicides and fewer directed applications of straight glyphosate. Also, we still see benefits from pre-emergence herbicides even though they aren't popular.
Palmer amaranth is also a problem in corn. In most cases, the cause is that most corn herbicides are designed for cooler Iowa weather. Our warmer, wetter, longer summers wear out the best herbicides. This gives rise to all kinds of late-season weeds that germinate when corn dries out and ceases to shade the soil.
Also, a lot of our pigweed have ALS resistance, and ALS chemistry is popular in corn.
Recently, Ford Baldwin mentioned Palmer as being troublesome in rice. The big cause there is that Command happens to be weak on pigweeds, and Command has displaced propanil (a good pigweed herbicide). For the record, Facet, Ricestar, Regiment, Londax, Permit and Clincher also do not control pigweed.
We've tended to ignore broadleaf weeds until flood-up time, but maybe we need to be jumping on them sooner in a Command program. The flood will control pigweed, but if they are thick, they will severely inhibit rice growth before flooding.
We will continue to work on pigweed control programs and share our results. However, for the foreseeable future, and regardless of the crop, we should recognize the following basic truths:
- Many herbicides provide excellent short term pigweed control.
- Success requires a well-planned program using more than one herbicide.
- In many cases, residual herbicides are key.
- Good weather and good luck helps.
- Ninety-nine percent control of a million weeds leaves a lot to be desired.
Andy Kendig is Extension Weed Specialist at the Missouri University Delta Center in Portageville, Mo.