Weed management is a never-ending war for farmers and ranchers everywhere. This year’s University of Missouri Extension Southwest Center Field Day near Mount Vernon was focused on that very topic.
“If you plan to spray, there should be some things you consider beforehand,” said Kevin Bradley, University of Missouri state weed scientist.
Bradley pointed out mowing is an option to consider in some situations, but producers should determine if mowing or an herbicide application is more economical. Mowing considerations include the cost of labor and fuel and can cost $15 to $25 or more per acre. The cost of spraying an herbicide depends on the product and the rate it is applied. Another consideration is the type of weed being targeted.
“In the end, it’s very hard to mow a perennial weed to death,” Bradley said.
Bradley recommended spot spraying from an economic standpoint if possible and especially if targeting blackberries or multiflora rose.
The weed scientist also spoke about metsulfuron, an herbicide sold generically for low costs and used in springtime herbicide applications for tall fescue seedhead suppression. One study showed 14 to 61 percent seedhead reduction when the fescue was sprayed at 6 inches tall in the vegetative stage. At 12 inches at the boot stage, suppression was 53 to 88 percent.
He did warn “significant yield loss” can occur but careful application timing can minimize the loss. However, this is a once a year concern.
Another critical consideration in weed management is identification. Bradley stressed he cannot make recommendations unless he knows what the weed is. He also said knowing the growth habits helps determine the best timing of the application and herbicide selection.
Bradley also broke weeds down into categories. The first — poisonous weeds — included Perilla mint, poison hemlock and nodding spurge. He explained Perilla mint grows in shady locations, has a minty smell and is seen in pastures around the state. It becomes a problem when animals have no other forage choices or the weed is sprayed and animals are left with access to the plant. If producers have a Perilla mint problem, Bradley recommended an application of 2,4-D.
Nodding spurge is another common weed usually emerging in July and August. It has a milky sap and can come up to 8-10 inches.
“Usually anything with a milky sap is somewhat poisonous,” Bradley said. Nodding spurge is still poisonous if baled. Metsulfuron, he added, provides excellent control of the weed.
Another consideration is poison hemlock. The weed scientist recommended spraying when it’s still in the rosette stage and reminded producers that it is one of the first weeds to green up in the spring. He recommended applications of GrazonNext, Grazon P+D or Remedy.
Bradley said wild indigo is another poisonous plant that he is getting more and more calls about each year. For producers with a wild indigo problem, he recommended using picloram-containing products such as Tordon.
Bradley’s second category was spiny weeds including horsenettle, musk and bull thistle, and blackberries. Horsenettle is a “very, very common” weed throughout the state and can bloom several times a year. For control, he recommended pre-bloom applications of GrazonNext, Grazon P+D or Weedmast, depending on the price and the desired legume replant interval.
For thistle problems, producers should apply at the rosette stage, as bolted plants are harder to control.
“The timing is really more important than the herbicide,” Bradley said.
“Blackberries are some of our hardest to kills weeds in pastures,” he continued. For upright blackberries, do not mow the season of treatment and consider spot spraying for better, more economical results.
“Early fall is an excellent time for getting the herbicide to translocate to the roots,” Bradley said, adding he recommends an application of metsulfuron two to three weeks before first frost.
His third category of weeds included those cattle are unlikely to eat such as sericea lespedeza, ironweed and spotted knapweed.
Although sericea lespedeza is not listed as a noxious weed in Missouri, Bradley ranked it as the top of his list due to its prolific seed production and allelopathic properties. His No. 1 recommendation was 1.5 pints of PastureGardHL per acre.
For ironweed, he recommended a triclopyr application.
Spotted knapweed is the most recent addition to the noxious weed list in Missouri.
“If you have any patch of it, I recommend getting rid of it,” Bradley said, adding it also has allelopathic properties.
“It’s probably never going to go away,” he added, explaining he has seen infestations across the state.
Gatlin Bunton, MU graduate student in weed science, gave an update on research he has worked on involving pasture weeds.
“Horsenettle was found in every pasture we visited,” Bunton said, adding of the 66 pastures surveyed, horsenettle was found at rates of 2,943 weeds per acre. Common ragweed was the most visual weed and was found in 97 percent of pastures at a rate of over 6,600 weeds per acre.
He added 79 percent of the pastures had low or very low rates of phosphorus and 23 percent were low in potassium. Soil pH was also low at an average of 5.9. The recommended soil pH level for pastures is 6 to 6.5. For every one unit increase in soil pH, weeds were reduced by 4,100 per acre. For each 0.1 ppm unit increase in phosphorus and potassium, he saw 162 and 12 fewer weeds per acre, respectively. When forage groundcover was at least 70 percent, weed density was reduced by at least 44 percent.
Bunton also studied the crude protein and digestibility of the weeds he found in pastures.
“Dandelions are a pretty high quality plant as far as forage value goes,” Bunton said, adding it has pretty good protein levels and exceptional digestibility. Common ragweed and large crabgrass had good levels as well until they got too mature.
Eric Oseland, another MU graduate student, told producers, “Palmer amaranth is the No. 1 weed to watch in the U.S.”
Palmer amaranth grows faster than its waterhemp cousin and has female and male plants which contributes to its genetic diversity. He recommended spraying Palmer amaranth at a height of 4 inches.
The No. 1 way to tell the different between waterhemp and Palmer amaranth, Oseland said, is Palmer amaranth has very long petioles. Palmer amaranth will also have a long, dense and bristly panicle.
Oseland explained waterfowl have helped transport seeds from the weed across the state. One study found an average of 18 pigweed seeds per duck. Contaminated equipment, feed and seed have also helped the spread of the weed.
“What we’re finding is, if you look for it you’re probably going to find it,” Oseland said, adding pigweed species were found in 98 percent of birdseed mixes they look at in one study with about 8,000 seeds per pound per mix.
“Palmer that gets spread to this area is probably going to be Roundup resistant,” Oseland said. For control in pastures, he recommended using almost any non-selective broadleaf herbicides.
Oseland also spoke about a horseweed population survey in Missouri.
“We’ve essentially lost the ability in Missouri to control horseweed with Roundup,” he said. In the survey, they did not find a population that was not resistant to Roundup and had a similar issue with Firstrate. Dicamba and 2,4-D applications are still proving to be effective, however.
“Dicamba continues to be an issue,” Oseland said, explaining as of Sept. 3 the Missouri Department of Agriculture had received 216 alleged dicamba complaints. The problem has been reported statewide.
Oseland said the No. 1 factor in dicamba issues is a lack of appreciation of soybean’s sensitivity to the herbicide — just 0.005 percent of the labeled rate of dicamba causes significant visual crop response in soybeans.
“It’s way more sensitive than any other crop-herbicide combination,” he said.