Mid-Summer Weed Management

July 20, 2017

Weeds poking above canopied soybeans are more than just an aggravation for farmers. They could signal the need for weed management plan adjustments.

By mid-July, most South Dakota soybean fields have reached full canopy, limiting weed growth. Weeds that germinated before canopy closure could still be problematic and those that escaped herbicide treatment call for action.

South Dakota State University Extension Weed Scientist Paul Johnson says from mid-July to September, farmers have limited options for controlling weed escapes.

“For one to two weeks after their final spray application, farmers should look for any remaining weeds to see if they are injured or have resistance issues,” Johnson says. “From that point on, you may have to live with what’s in the field until preharvest, or hand rogue.”

Johnson says waterhemp and velvetleaf are commonly found mid-summer because they germinate year-round. With sufficient moisture and an open canopy, seeds will continue to sprout. Waterhemp and velvetleaf may be most evident in potholes or low spots where canopy was delayed or not achieved. Weeds that germinated before canopy closure will continue to grow and should stick out above the shorter soybean plants, making them noticeable when farmers scout for other summer pests like soybean aphids.

If weeds are evident in late July, Johnson says that’s an indication management changes may be needed.

“Either the chemicals didn’t work at the stage in which they were applied or there may have been herbicide-resistance issues,” Johnson says. “Either way, farmers need to look at changing their plan for next year.”

Johnson says farmers should review the chemistries and modes of action they used if there are weed issues at this time of the year. They also need to consider the season’s weather conditions because that can impact herbicide effectiveness, leading to trouble later in the season. A wet spring may mean pre-emerge herbicides didn’t last as long. In a dry spring, herbicides are not activated and may not work, or the program farmers used didn’t match what was in the field.

“In this part of the world, waterhemp is the number one weed, but if we narrow our plan to focus on waterhemp, other weeds may escape,” Johnson adds. “That’s why it’s important that farmers know what weeds they have.”

Johnson says there are two important times when farmers have an opportunity to assess how well their weed management plan worked. Around August 1 and again at harvest time, farmers will be able to gauge how many weeds have escaped, review their program and plan for next year.

“If farmers have weed escapes when soybeans are in full bloom, they don’t have a lot of labeled options,” Johnson says. “Once we get to September and pre-harvest, they may have some herbicide options depending upon which weeds are present.”

While it’s not high on the list of favorite things, Johnson says farmers shouldn’t rule out hand rogueing scattered weeds because that extra effort could reduce problems in the fall and in future years.

“If labor is available, that can be a relatively cheap approach to alleviate problem weeds,” Johnson says. “One full-sized waterhemp plant can produce half a million seeds that will be in the ground next year. Getting rid of those weeds now, especially if they’re resistant, can make future weed control costs a lot lower.”