Target Weeds, Higher Herbicide ROI

That ugly patch of weeds the combine just tore through is costing farmers money. One Amaranthus species — waterhemp or Palmer amaranth being the most common — for every 3’ of row can reduce yield by up to 30%, according to experts at the Weed Science Society of America.

When a single weed can cause that much money lost on farms, imagine what large patches of uncontrolled weeds can do. Help farmers review notes, identify problem fields and make a plan for 2021 weed control and years to come.

“For the current year, look at when did the weeds escape, what control measures did you use?” asks Bill Johnson, Purdue University Extension weed scientist. “What weeds do you have?”

Let’s break it down, question by question.

When did weeds escape?

Depending on what stage of the crop’s lifecycle the weeds emerge, growers could see weaker, suppressed weeds that don’t steal as much yield, or strong, huge weeds that wreak havoc. Look back at field notes and remind your customers, even if they don’t steal yield this year, they could produce seeds.

“If it’s a late-season escape, one you’ll see while you are in the combine, think about future expenses,” Johnson says. “I’m looking at a minimum, averaged over the next few years, of $10 per acre per year or more for dense infestations of easy-to-control grasses and some broadleaf weed species. With something like waterhemp, it could be $20 per acre or more.”

The weed seedbank carryover, especially if it has resistance, spells more money in future weed mitigation. Farmers will see those expenses increase in the form of more or ‘rescue’ herbicide treatments, the need for higher rates of herbicides or, in extreme cases, the potential need for mechanical in-season control like a weeding crew.

Early season weeds can be incredibly damaging. The earlier the weed hits, the bigger it gets and the more precious nutrients, water and sunlight it steals from crops.

Influence of corn and soybean competition on waterhemp emerging at different times in relation to the crop:

1Hartzler, Battles and Nordby.  1998 to 2002.  Iowa State University.
2Biomass is reported as percentage of dry weight produced by plants emerging at the same time as the crop.

“Weeds that emerge at least four or more weeks after the crop are unlikely to impact yield unless they’re present at high densities,” says Bob Hartzler, Iowa State University Extension weed scientist. “The yield saved by preventing early season competition by spraying prior to the critical period will cover the cost of a second post application in most situations.”

In essence, the pre-emergent herbicide your customers choose is likely the most important weed control decision they’ll make all season. Make sure they find something effective, with residual that can carry them through this critical growth period — and be sure to account for slow germination from cool or wet springs.

What control measures did you use?

Finding notes on this should be easy. Farmers will have sale receipts and hopefully good knowledge of what herbicide was applied where. Use this information to determine: if the herbicide was applied in a timely manner, if they were using multiple modes of action, if the herbicide is effective against their problem weeds and if there is new or emerging resistance on fields that needs to be accounted for.

“It seems like a lot of farmers don’t spray at the optimum timing — they let weeds get to 6” or taller,” says Richard Zollinger, North Dakota State University Extension Weed scientist. “They’re losing the battle before they even started.”

Even an effective herbicide mode of action might not have good control on tall weeds. Remind farmers to get out before weeds hit 4” tall — about the size of a can of soda. If you see a variety of weeds that are still alive, but the smaller ones are dead, this could be a good indication the grower or applicator waited too long to get out there with herbicides.

“Some farmers pay $65 per acre for a rescue treatment and still don’t control all weeds,” says Bryan Young, associate professor of weed science at Purdue University. “It’s about how you use the herbicide, not how much herbicide you buy.”

Take a look at the herbicide packages farmers are using. Are the herbicides effective against problem weeds in your area? Are they using more than one effective herbicide in a tank mix to stave off resistance? If they’re not, they’re potentially wasting money on herbicide passes that are ineffective or costing themselves more money in the future by unintentionally promoting weed resistance.

Knowing the weed control measures farmers applied to each field can help you narrow down if there is application error, the wrong herbicide choices or if new resistance is popping up in fields. If you suspect there is new resistance, consider reaching out to Extension experts for confirmation and to help customers plan next year’s weed control.

What weeds do you have?

This will take some legwork — by either you or your farmer customer. Figure out what weeds are present, what might have escaped and what that means for the crop.

When you think about problem weeds, Palmer amaranth or waterhemp might be the first to come to mind, and they are troubling, but they are by no means the only weeds you might see and not the only ones that cause yield loss.

“Certainly, the poster child in the Midwest is waterhemp, and Palmer in the Midsouth,” Johnson says. “However, on a per-plant basis waterhemp is not as competitive as things like sunflower, giant ragweed or cocklebur.”

On a per-plant basis, sunflower, giant ragweed and cocklebur steal more valuable nutrients, sunlight and/or water from crops and can therefore have greater impact on crop yield. However, they don’t produce as many seeds as Amaranthus species.

“Escapes allow the weed seedbank to build up and can allow new herbicide resistances to develop,” Hartzler says. “The two simplest solutions are to go to narrower row spacing or include a residual herbicide with a postemergent application.”

Once farmers identify the weeds they had this past year, work closely with them to ensure the pre-emergent and any postemergent herbicide will be effective against these driver weeds. Don’t ignore the ones that seem ‘less problematic’ because every weed steals yield.