Glyphosate-resistant weeds have crept into neighboring states but really had not been an issue here. Now, according to Richard Proost, an agronomist with the University of Wisconsin-Extension Nutrient & Pest Management Program, Wisconsin has two confirmed species of glyphosate-resistant weeds.
When Proost addressed the topic at a seminar in Richfield, he said, "Anything we do long enough will cause resistance and persistence of new weeds. The problem is there have been no new modes of action discovered in the last twenty years. We have new products on the market but no new modes of action."
Proost said there are confirmed glyphosate-resistant weeds in almost every state now. In Wisconsin, there are 14 resistant weeds that cause concern (two of which are glyphosate resistant): giant ragweed (glyphosate resistant in 2011 and also resistant to ALS classes), horseweed (glyphosate resistant in 2013), common lambsquarter, eastern black nightshade, giant fox tail, green fox tail, Kochia, large crabgrass, smooth pigweed, velvetleaf and tall waterhemp.
"We will likely see the list get larger for glyphosate resistant," he said. "We have to rotate the use of our herbicides to avoid this."
He also described ways to determine if problems with surviving weeds are caused by resistance or other issues.
"Resistance is not late flushes of weeds that came up after spraying, tolerant species, unfavorable weather, poor herbicide applications and mistakes," he says. "Weed survival can also be caused by using the wrong herbicide rate, the wrong adjuvant or mixing in the wrong order, or even forgetting to actually add the herbicide to the tank."
He also described ways to identify weeds that have developed resistance.
"Be suspicious when a weed is labeled for control but is healthy after spraying, while other weeds around it died," he said.
"Be suspicious if the weed is the only species left in the field, or the weed was not controlled in the same patch in the past and the patch is getting bigger," he added. "If the weed was not controlled by different herbicides with the same mode of action, it could be developing resistance. This will happen if the same mode of action has been used over and over again.
"I understand your decision to use a herbicide again because it did a good job on a particular weed, but if you continue to use it, that weed will develop resistance."
Regarding concerns about Palmer Amaranth, he said the problem is it is a prolific seed producer with as many as 100,000 to 500,000 seeds per plant. It is a small seed that can easily ride along on harvesting equipment and is also spread by wildlife, wind and manure.
It is also a concern because the seed will emerge any time between April and August, so spraying might catch early weeds, but weeds still appear later in the season.
He showed photos of the plant in soybean fields, illustrating that a field that had a small patch one year developed a larger patch the second year and completely took over the entire field the third year.
"Another reason this is a threat is that it will grow up to three inches a day," Proost said. "It is tolerant to post herbicides unless it is very small, but since it grows so fast, it is difficult to catch it early. It is also resistant to ALS inhibitors."
It has both male and female plants so it can rapidly transfer the resistant trait to other weeds in the field.
Palmer Amaranth resistance has been confirmed in eleven states, including Wisconsin.
"The Wisconsin farm picked up seed that came along with the cotton seed they purchased from the south," he said. "The seeds were fed and got spread in the fields through manure. Cotton seed was also the source for Michigan and Indiana.
"If you buy cotton seed for your cows, ask the dealer where it comes from. If it comes from the south, it could be carrying the seed."
Proost also produced photos to help farmers identify the plant in their fields. The plant lacks hair on the stem and leaf surface, so it is different than pig weed. It has a singular leaf tip notch, a rosette patterned growth and diamond-shape leaves.
"If you believe you have it, contact your county agent," he said. "Vince Davis (UW-weed specialist) will come out to check it.
"It is important to get the weed out before it produces seed. When you pull plants, burn them, and do not drag them out across the field because the seed will drop."
Proost also suggested that moldboard plowing a field, even in a no-till situation, will help because the seed will not stay viable a long time, and burying it would be good.
"Scout your fields so you can catch it early," he said. "Watch the fence lines, too, and if you see it, get rid of it right away."