Fall Burndown May Be Even More Critical With Early Harvest

With the 2018 crop year winding down, many farmers in the Midwest and Midsouth are looking forward to the possibility of an early harvest. But in the hustle and bustle of harvest, what may go overlooked is the late-season emergence of horseweed, Palmer amaranth and other weeds. Left unchecked, they can gain a toehold and either go to seed or carry over into the spring.

Weed scientists say that how you manage these early, fall-emerged weeds depends on a number of factors, including:

  • Geography
  • Cropping plans
  • Weed spectrum
  • Resistance issues
  • Timing
  • Weather

As an example of geography, farmers in the Midwest look to control post-harvest weeds for completely different reasons than those in the Midsouth.

Midwest farmers don’t want weeds to grow to an unmanageable size by the following spring.

In the Midsouth, growers don’t want late-emerging weeds to go to seed in the fall and add to the soil seed bank.

The scenarios are different but the basics of control still depend on common factors – timing and herbicide selection, among them. Approaches also must be weighed against cropping plans and other objectives, like erosion control.

Here are some considerations for post-harvest weed control for the Midsouth and Midwest from veteran weed scientists Jason Norsworthy of Arkansas and Aaron Hager of Illinois.

Midsouth And Palmer: Frosts Don’t Come Soon Enough

Hard-to-control populations of Palmer amaranth can begin emerging prior to harvest and will continue to emerge after harvest as long as weather is favorable, says Norsworthy, University of Arkansas Extension Weed Scientist. “In late summer, we already had a lot of corn coming out of the field, which means we still have plenty of growing season left for resistant Palmer amaranth (pigweed) to emerge, grow and set new seed.”

Where cotton will follow corn the following year, Norsworthy suggests one of three options. 

First, you can use tillage in front of reshaping beds.

Second, apply a fall tank mix of paraquat, a low rate of Cotoran (fluometuron) and a pint of Dual (metolachlor).

Third, apply 1 quart of 2,4-D plus 1 ounce of Sharpen, which should hold things until the first killing frost. Metolachlor also helps with ryegrass issues that may develop during the winter months.

“If you’re going into corn the next year, atrazine, paraquat and metolachlor will provide the knockdown as well as some residual to get you to the first killing frost,” he adds.

When planning a fall burndown program, consider the timing of weed emergence versus the average date for the first killing frost, Norsworthy says. This can be one of the finer points when scheduling that fall spray, especially if you’re trying to prevent any more plants from going to seed.

“A weed that emerges 30 or more days prior to a killing frost is likely to set seed,” he says. “That goes for pigweed and a number of annual weeds in general. However, those plants emerging less than 30 days before frost will not produce viable seed.”

Post-harvest or fall chemical applications can also put Midsouth producers ahead of the game for the following spring, since whatever weeds come up then should be easier to control, Norsworthy notes.

“From the standpoint of weed control, it’s definitely a lot easier when we invest in fall burndowns or tillage versus letting all that overwintering vegetation resume growth in early spring.”

In The Midsouth, Take Winter Cover Into Account

Norsworthy notes that fall herbicide applications do come with a couple of downsides, not counting the additional costs.

“If you use herbicide to control all fall-emerging weeds, you’re going to be left with bare soil for most of the winter, which could promote soil erosion. Also, applying Cotoran and Dual (metolachlor) in the fall means you can’t plant a cereal rye cover crop.”

Cover crops don’t necessarily help post-harvest weed seed management, Norsworthy adds. “They reduce weed emergence the following spring. Weed emergence in the spring is a function of soil temperature and soil temperature fluctuation. Cover crops minimize that daily fluctuation of temperature, and that helps suppress emergence of weeds like Palmer amaranth.”

Sowing a cover crop into an existing crop in late September or early October can help prevent some late-season weed emergence, Norsworthy adds. With no emergence, no new seed are set later. To fully gain that suppression, the ground must be clean when the cover crop is established, he emphasizes.

Post-Harvest Weed Options In The Midwest

Fall applications of herbicide aren’t for everyone or every situation, notes University of Illinois weed scientist Aaron Hager. “Number one, define your objectives, then scout to determine if you actually need a fall application.”

Make sure that weeds don’t get so big that they can’t be managed the following spring. That, says Hager, should be the main objective for farmers in his state when determining whether to apply fall herbicides.

Treatments will be far more effective ahead of planting next year if you’re not trying to take out weeds carried over from this fall.

“And fall is a really good time to control certain perennial or biennial species because translocation patterns in the fall are going to favor movement of the herbicide and kill off the roots,” Hager says.

Scout weeds carefully and keep a close eye on the fall weather and crop harvest timing, he stresses.

“For example, if you have an early harvest, you likely will have a lot of acres where nothing is shading the ground. If environmental conditions are right, you might see emergence of winter annuals. In Illinois, we may be looking at weeds like chickweed, henbit and horseweed.”

A Good Time To Go After Resistant Horseweed

A fall application may be advisable, especially if the horseweed is resistant to glyphosate, Hager adds.

“For years, a very common spring burndown program was applying glyphosate and something like 2,4-D,” he specifies. “But if glyphosate is not going to be as effective because of resistance, the game changes. The rates of 2,4-D we can use in the spring on horseweed are pretty marginal with weeds that made it through the winter and then resumed growing.”

At times, it makes sense to add a residual to a fall application, Hager says. But timing matters, he contends.

“If you’re making a fall application and are chasing the combine out of the field, then you could benefit from including a residual in the tank. But if you wait until just before Thanksgiving to spray, a higher population of winter annuals will have already emerged. With that kind of delay, you could see less benefit from a residual since the weeds are already up.”

Carefully weigh the residual option, Hager says. Some residual herbicides applied in the fall may carry over “if we get into a very hard, cold winter,” he points out. “The products simply can’t degrade fast enough to get the concentration in the soil low enough that it doesn’t affect the rotational crop.”