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Tough spring for weed control

As rains continue to delay tillage, herbicide application and planting, weeds are becoming more of a concern. In some ways, the warm temperatures and soggy soils this week will only serve to amplify the problem, said Justin Petrosino, an agronomist for Stewart Seeds in Bowling Green.

“Thankfully growers have 2011 to look back on as a reference. Cooler temperatures have luckily left us with marestail that, before this week, hasn’t begun to bolt. With a week in the 70s and 80s this may start to happen,” Petrosino said. “In the fields this past week I’ve seen both common and giant ragweeds beginning to emerge along with lambsquarters. A few winter annuals and weeds like dandelions are getting pretty large and flowering making them tougher to control. Thankfully most of the spring emergers only have a few leaves out so they are still relatively easy to control. It is still early for waterhemp to emerge so fields in west central Ohio will still be able to hold waterhemp back with a residual if it is applied in the next week or two. If burndowns and tillage happen this week we will hopefully be tackling weeds that are much shorter then label requirements for control.”

Though weeds will get a boost from the heat this week, hopefully soils will dry enough to get back into the fields.

“With the temperatures forecasted this week we should get some very good drying days in and will hopefully be able to get burndowns on later in the week. Some growers have even told me they are ditching the row gator for a smaller ATV sprayer to hit their problem fields first, and then concentrate on fields that received tillage or a fall herbicide program second,” Petrosino said. “Thankfully at the moment we are in a better place than we were in 2011 since we’ve had a cool spring, but like any year the goal for spring weed management is still the same. The first goal is to kill all emerged weeds before soybeans and corn go into the ground.”

With the soggy spring, there may be some adjustments needed to accomplish this goal.

“This may mean leaving 2,4-D in the burndown on soybeans and waiting the 7 to 14 days depending on the rates and formulations used. The yield loss from waiting a week to plant soybeans in May is insignificant compared to the potential yield loss from marestail and the seed production we will see if marestail gets out of hand,” Petrosino said. “If a grower must drop 2,4-D they must replace it with something effective against marestail. Depending on the product they select they may need to switch up the residual product they are using. If they switch 2,4-D to a product containing saflufenacil they may need to move to a residual other than a PPO inhibitor like products containing sulfentrazone and flumioxazin. This may mean relying on metribuzin, which can lower control on triazine resistant lambsquarters and waterhemp. This may require a change in post-emergence products. The story here is a single change in the burndown could mean changes to both the residual and the post emergence herbicides, along with changes in application requirements. If a grower is going to make a change they need to do their homework to make sure the new program is going to control the weeds they are struggling with. They need a strong burndown that will control marestail, ragweeds, and other tough to control weeds along with a residual herbicide with activity against those tough to control weeds. The worst thing a grower can do is move to a single pass single mode of action weed control program. This will lead to a messy field come harvest time and a lot of weed seeds in the seedbank.”

Ohio State University Extension herbicide specialist Mark Loux points out that situations like this spring can be addressed in the future with a strong fall program.

“It’s obvious from the calls and emails we have received, along with observations of our research plots, that there is a substantial difference in weediness between the fields treated with herbicides last fall versus the lack of a fall treatment,” Loux said. “Among other benefits, the fall treatment does definitely allow a clean start in the spring that persists for a while and ‘buys time’ in a delayed planting situation. The fields that did not receive fall herbicides are much more of a concern as we try to adapt burndown programs to a delayed start that allows the overwintered weeds to create problems.”

In terms of addressing the weedy situation at hand, Loux has a number of suggestions.

“For many weeds, increasing the glyphosate rate to 1.5 pounds of acid equivalent or higher in mixtures with 2,4-D or Sharpen, will help compensate for larger weed size. This will not help with glyphosate-resistant marestail, and the other issue for marestail is that by the time we can finally plant, we will be unable to use 2,4-D rates higher than 0.5 pound, and this rate still requires a 7 day wait to plant. The mixture of glyphosate plus 2,4-D has become less effective over time in some fields for marestail control,” Loux said. “Recommendations to improve control have included application to smaller plants, and increasing the 2,4-D rate to 1 pound per acre, and so the current situation will probably introduce more variability in marestail control. In fields with larger marestail that did not receive a fall herbicide treatment, control could be improved by supplementing the glyphosate/2,4-D with another herbicide that has activity on emerged marestail, or replacing the glyphosate with another herbicide.”

Loux pointed out a couple of other burndown challenges, including a lack of labels allowing the addition of Sharpen to mixtures containing flumioxazin (Valor), sulfentrazone (Authority), or fomesafen (Reflex). In addition, there is a depleted supply of Liberty and it is primarily being reserved for POST treatments.

Loux offers additional tips on soybean burndown in this week’s CORN Newsletter.

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