You may not come out chanting, ‘residual, residual, residual,’ if you hear Bill Johnson talk about 2016 weed control, but you will leave knowing where he stands. While adding residual herbicides back to weed control systems might increase costs, Johnson says it’s crucial to do so.

“If you want to control tough weeds today, you need help from residuals,” he says. Johnson is a Purdue University Extension weed control specialist. “The days of cheap weed control built around post applications of glyphosate are over. There are too many resistant weed issues now for that to work successfully in most situations.”

CAUSE FOR CONCERN: Palmer amaranth is one of the weeds that is best attacked with residual herbicides before it gets out of the ground.

Fortunately, there are a large number of residual herbicides which can still be used to control weeds, he says. No new chemistry is coming on board for 2016, but there will be some new formulations or new combinations of existing chemistries. Some of it is driven by the need to get in front of weeds with soil-applied chemicals before weeds emerge. Another factor is putting several active ingredients in one product to combat weed resistance - both resistant weeds already out there and the development of new types of weed resistance in the future.

New premix tools

One new product with a new name that falls in this category is Resicore from Dow AgroSciences. It’s a premergence and early postemergence herbicide for corn which contains three different active ingredients - all of which have been on the market for a long time.

Another is Acuron from Syngenta which includes four active ingredients and three modes of action for controlling tough weeds in corn. It got some limited use in 2015 and includes mesotrione, s-metolachlor, atrazine and bicyclopyrone (which is new, but also an HPPD like mesotrione). And Syngenta launched Acuron Flexi in mid-2015 without atrazine for areas of the country where atrazine levels are limited.

Related: Will residual herbicide hurt your cover crop?

Increased costs may cause some farmers faced with tight margins and nervous bankers to balk at adding residual herbicides to their weed control plans. Johnson lays it on the line.

“Yes, weed control will likely cost more if you’re including residual herbicides compared to what you’ve done in the recent past,” he says. “The alternative is weedy fields. It’s that simple. Once some of these resistant weeds get more than three to four inches tall, you can’t take them down with post spray applications. Depending on what they are resistant to, choices for post application are limited anyway. “

The top questions about residual herbicides answered - >>>

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A residual herbicide primer

Some of you may need to bone up on residual herbicides. Below are answers to five key questions you may have.

What are residual herbicides?

What causes residual herbicides to not work as well?

How quickly do soil-applied herbicides break down?

Why is plant-back restriction information on label important?

Which common weeds have resistance to key herbicides?

Residual herbicides defined

Most residual herbicides are soil-applied, although some can be either soil-applied prememergence or applied postemergence, or can be applied both pre and post. Wikipedia defines soil-applied herbicides: Herbicides applied to the soil are usually taken up by the root or shoot of emerging seedlings and used as preplant or pre-emergence treatments. In some cases, plant shoots (of weeds) absorb soil-applied herbicides while they are still underground, leading to plant injury or death. Trifluralin (active ingredient in Treflan) was one of the early soil-applied herbicides.

Factors that influence residual herbicide activity

Herbicide adsorption to soil colloids or organic matter often reduce the amount of active ingredient available for weed absorption. Fred Whitford, Purdue University director of Pesticide Programs, notes that active ingredients can even be tied up in spray water if it contains organic matter or sediment. This is more likely to happen if you’re pulling spray water from a creek. Even if you’re pulling it from underground wells, water hardness can affect how well a herbicide works. Labels discuss how you can condition water to solve this issue. Water pH is a separate issue from water hardness. It can also affect how well a herbicide performs. Refer to the herbicide label for information about proper pH range.

Breakdown factors for herbicides

Chemicals have half-lives. This is the amount of time it takes for half of the chemical to degrade, or break down. Chemicals break down at different rates based on their chemical structure. In addition, breakdown can be affected by amount of rainfall received after application.

Related: 5 steps to residual weed control

If the breakdown interval is too short, the residual herbicide may not control later flushes of weeds. If it is too long, it may affect how quickly various crops can be planted after application without injury.

As an example, atrazine doesn’t break down readily after application on soils of above neutral ph- soils with pH levels above 7.0. That means the herbicide will stick around even longer in high pH, or alkaline, soils.  

Residual herbicides could impact cover crops

Cameron Mills, Walton, Ind., checks herbicide labels for any product he’s considering using today. Labels list plant-back restrictions, or intervals before you can plant another crop. The interval varies by crop. He’s found that for cover corps, like annual ryegrass, the plant-back restriction may be significant.

“We like to seed cover corps into green soybeans and corn before the crops start turning,” he says. “Depending on the herbicide, or when it was applied, you may get seedling injury or even death caused by herbicide residue still there. “

Herbicide resistance issues

Visit weedscience.org and view the International Survey of Herbicide-Resistant Weeds. It’s not bedtime reading unless you are OK with nightmares! Based on a 2015 report, there were 461 unique cases of resistance globally. This means 461 unique combinations of weed species and site of action. There are 247 species which show herbicide resistance- 144 are dicots, such as broadleaves, and 144 are monocots, primarily grassed. Herbicide resistant weeds have been found in 86 crops in 66 countries.

In Indiana, key resistant weeds include giant ragweed, lambsquarters, waterhamp, Palmer amaranth and many more.