Harvest-Time Weeds Yield Future Trouble

Waterhemp is an aggressive and often resistant weed. ( Sonja Begemann )

If you were one of the many farmers who started harvesting earlier than usual, be prepared for aggressive weeds. Bare fields are breeding grounds for weeds. 

“[An earlier harvest] definitely gives those fall/winter annual and biennial weeds an extended period of time to collect sunlight because they’re not in competition with the crop as long,” says Kent Bennis, market development specialist, Corteva Agriscience. “You will probably see larger weeds and at a greater density that need to be addressed.”

To gauge the potential severity of the problem, consider past weeds in a field and the weather forecast. Pay particular attention to no-till fields. 

“The practice of applying herbicides after crop harvest has increased in Illinois over the past few seasons,” says Aaron Hager, University of Illinois Extension weed scientist. “Herbicides applied in the fall often can provide improved control of many winter annual weed species compared with applications made in the spring.”

Before you create a herbicide mix, make sure you know what weeds are common in your area. 

“Our goal in the fall is to control marestail,” says Kyle Allen, a Channel seedsman in Hawk Point, Mo. “We know we can control it best with a fall spray application because that’s when it germinates. I suggest farmers use as many effective modes of action as possible—at the right cost.”

In his area 2,4-D, Roundup and Sencor are effective. In Illinois, Hager says to use higher application rates of products such as 2,4-D.

Tillage could also work, just make sure to use the right tool for the job. “If you use a chisel plow it will be sufficient for those winter annuals and biennials,” Bennis says. “If you’re just going to do a vertical till pass to process stalks, then it’s more likely you’re not going to achieve control over those weeds.

Marestail isn’t the only weed threat. Some parts of the country might still see waterhemp or Palmer amaranth. 

“As long as moisture stays and temperatures are favorable you’ll have waterhemp, and Palmer might try to grow,” says Terry Mente, AgriGold agronomist from eastern Iowa. He doesn’t expect many of the farmers in his area to make more herbicide passes, however. If they’re not tilling and a weed issue doesn’t exist, many will wait for a hard freeze to kill off weeds—a move he says makes more economic sense where he’s at.

“If you skip on doing something now, be prepared to make up for it later—you tend to pay for inactivity down the road,” Mente adds.

Scout diligently to see where a herbicide application could be needed. In some cases you might need residual to get through to spring.

“[Application] in the fall almost always results in better control at planting compared with targeting overwintered and often larger plants with lower rates of 2,4-D in the spring,” Hager says. Also consider using a soil-residual herbicide.

“Delaying the herbicide application until later in the fall [for example, mid-November timing] often diminishes the necessity of a soil-residual herbicide since most of the winter annual weeds have emerged and can be controlled with non-residual herbicides,” Hager explains.

If you’re concerned about weed size by November, consult herbicide labels or your chemical retailer to make sure they’re still effective. 

When using a residual herbicide, check plant-back restrictions, Allen says. In some cases, certain herbicides could damage future crops. 

“Work closely alongside your local chemical retailer or agronomist to line up the best possible residual for the crop you’ll be planting in 2019,” Allen adds.