Weed Webinar Reviews Summer Research
Published: Friday, February 5, 2021
During his preamble to Monday night's Controlling Pasture Weeds Virtual Field Day presentation, Purdue University Extension forage specialist Keith Johnson drew from the Parable of the Sower to underscore what every farmer who has ever tangled with Canada thistle knows.
"Weeds have cursed planet Earth for quite a while, haven't they?" Johnson posited to the statewide webinar audience after noting how the plants choked off by thorns bore no grain.
Unfortunately, no amount of cursing will make weeds go away, or make the job of getting rid of them any easier.
The problem, and what can be done about it was put into perspective by the results of a pasture weed control study conducted by Purdue College of Agriculture specialists and a representative from Corteva AgriScience last growing season.
Canada thistle was the predominant weed and main focus at the dry winter testing site at the Purdue University Scholer Beef Farm, but curly dock, broadleaf plantain, lambsquarters, ragweed and lady thumb were also present, and competing with white and red clover, according to Marcelo Zimmer, Purdue weed science Extension specialist.
The plot underwent a series of practices and treatments, with five different herbicides, to determine the best way to deal with dastardly weeds that are the source of so much misery on a variety of levels.
The plot was under constant observation by twice-weekly scouting, drone imagery and even round-the-clock trail cameras.
Purdue weed science professor Bill Johnson explained how unwanted plants can directly affect the Angus/Simental-based herd of 165, pasture-grazed, spring-calving cows at the Scholer Beef Farm.
"Some of these weeds compete with desirable forage species, some of these weeds can be somewhat toxic to animals, some of the weeds can have burrs or things like that that get into the hides and the fur and cause problems there," Bill Johnson said. "Weeds can interfere with fencing, can interfere with lots and driving lanes, and water movement and things like that.
"So there's a lot of good reasons to manage these pastures to maximize productivity of the animals and productivity of the forage species that are being produced."
Like the blob in the old horror movie, researchers know they can't stop weeds; they can only hope to contain them. It's a never-ending battle.
A common practice is for farmers, Keith Johnson said, to "clip" their pastures, but timing is critical and that method can be more of a cosmetic fix than a remedy, especially when it comes to perennials like night shade.
"If the effect of clipping is to deplete food reserves to cut back the competitiveness of some of the perennial weeds so your forage species can have an advantage, clipping is gonna be effective in driving down populations," Bill Johnson said. "If it doesn't (help forage plants gain an advantage), then the impact will be a lot less.
"Particularly for annual broadleaf weeds, once the seed heads are above the clipping height, if we can get those weeds clipped before viable seed is produced, clipping can be very effective."
There are chemical controls that can all but wipe out the weeds found in pastures, but not without also damaging the forage plants.
"I think a key thing to remember with the plant species we grow in the pasture is that we don't have herbicide-resistance traits in them like we do our agronomic crops (like corn and soybeans)," Bill Johnson said. "Herbicides are a good tool, but they're not gonna be the big hammer.
"The big thing I try to get across to people as far as effective management of weeds in pastures is to manage your forage stand to be as competitive as possible."
That requires planting forage plants most suitable for location, geography and soil type while managing soil fertility, pH, grazing, harvesting and the placement of feed bunks.
"Anytime you can minimize overgrazed or highly trafficked areas that kind of beat all species back, then that's gonna help with suppressing weeds by the forages as well," Bill Johnson said.
Summer annual weeds are easier to control early in the season before they start producing seed heads because they're more sensitive and less likely to produce more seeds.
It's generally more effective to spray herbaceous perennials such as horse nettle, Canada thistle or ironweed, in late summer or early fall when they're starting to send carbohydrates down to the roots, Bill Johnson said.
Biennial weeds, on the other hand, are a whole other animal.
"Typically, your biennial weeds are going to be more sensitive in the vegetative stage than they are the reproductive stage, but if you go into a pasture you're going to have both the first-year growth and the second-year growth," he added. "So that one's a little bit trickier question to answer."
Furthermore, products with active ingredients for treating pasture weeds aren't nearly as plentiful as those designed for corn or soybeans.
"In terms of a low-cost treatment that's very effective on summer annual broadleaf weeds, a 2,4-D mixture, whether it's a commercially available product or one you mix yourself, is going to accomplish a lot of your weed-control goals," said Bill Johnson, reiterating that it needs to be applied before seed heads develop. "Two, 4-D and dicamba, as kind of late-spring treatment, can also be very good on thistle if (it's) actively growing and it's in the bud stage.
"Triclopyr is a very good herbicide for woody perennials, like your milkweed, dog beans, tree seedlings, multi-floral rose and things like that. It's not quite as good some of the summer annuals. Clopyralid has been around for several decades (and) is very effective on thistle, and aminopyralid as well. Metsulfuron has activity on both woody perennials and herbaceous summer annuals but it can be a little bit more limited on one or the other so it's really important to know what species you're going after."
What's important to keep in mind, Bill Johnson said, is that the legumes in the pasture, such as clovers, are also going to be adversely affected.
"With almost every one of those active ingredients, you cannot re-seed immediately (and) some of them have as long as a two-year rotational restriction before clovers can be re-seeded," he continued. "Some of these herbicides can be sprayed when the cows are on the pasture and some require you to take the cows off of the pasture for a specific period of time."
Keith Johnson added that there can be some residual effects through urination and defecation on legumes in a paddock where livestock that ingested herbicides are moved to. It's important, therefore, to read the product label for information on when animals can safely be moved.
"Wherever (ingested herbicides) are excreted, they have the ability to damage the legumes in that particular paddock," Bill Johnson agreed. "They also have the ability to get into water sources as well, so that's a key thing to keep in mind."
Purdue Extension digital agriculture coordinator John Scott explained how drones were used to not only identify weeds with photographic and video capture, but to monitor their growth and demise over the entire course of the growing season instead of just one point in time.
"We flew this probably once every couple of weeks and collected these images we turned into maps," Scott said. "You could really potentially start to unlock some pretty cool stuff that's going on in this pasture."
Said Keith Johnson, "It was really easy to pick out those control plots because we really did get some pretty good control of weeds where we did have these different five herbicides applied."
Purdue's drone can also spray herbicides and spread seed.
"I've used that to spread some untreated cereal rye on pastures at my own home where the pastures are pretty uneven," Scott said. "We've got lots of differences in topography and in some places it's steep enough, I know from experience, and you can't get a tractor across.
"Using this technology, I can get across where I can't go otherwise, which is pretty phenomenal. On top of that, if you had the right licensures and certifications, you can also spray with this thing in those same hard-to-reach areas. You could spray a chemical to control weed pressure.
"I think we're just scratching the surface. I think there's a lot more we can do with this technology. Quite honestly, if we can dream it we can do it with a lot of this stuff."
The Purdue drone, Keith Johnson pointed out, can carry about 22 pounds of clover seed which can cover four acres of pasture.
"I can see that this is a great opportunity to use that type of tool for that type of purpose," he said.
Return to Top of Page