Below-label herbicide rate OK or wrong?

With farmers putting everything under the microscope in their quest to reduce costs of production, many have taken to applying weed control chemicals at below-label or half-label rates.

Is this wise?

Is it effective?

And what happens if it doesn't provide the control desired? Who's responsible then?

The answers depend on whom you're talking with. Most chemical company reps take the position that label rates have been established and proven through exhaustive research and trials and that the farmer risks loss of control if he significantly reduces the rate.

Some weed scientists and ag consultants say label rates are a one-size-fits-all prescription and that there are many situations in which cutting those rates is perfectly acceptable.

“You may get by for one year, but you have to ask: What are the long term consequences?” says Dale Shaner of the BASF Corp. If it fails, he said at a recent seminar of the Western Weed Science Society, the grower not only faces a possible rescue situation and yield loss, but “you suddenly are putting a tremendous number of weed seeds into the field's seed bank.”

While rates are calculated to provide acceptable control for label weeds 95 percent of the time under varying environmental conditions,” Shaner notes that “most labels allow lower rates for the most susceptible species” on the label.

There is also a time management factor to be considered, he points out. “Lower rates are going to require more careful management.” And, if below label rates are used and there is a weed control failure, “then there's a question of who's responsible for that failure.”

Neil Harker, Agriculture & Agri-Food Canada, says cutting label rates can result in “lower weed control and lower reliability,” but at the same time can offer lower selection pressure for weeds developing herbicide resistance, lower environmental impact, and lower costs.

More weeds in a crop can cause harvesting problems, he says, as well as yield loss and “worse weed infestations the next year — but the lower herbicide/application costs and lower selection pressures” may offset those disadvantages.

Some rates too high

“Some European studies suggest herbicide rates may be too high to start with and that reducing rates had no adverse effect on selection pressure or seedbank buildup.”

The competitiveness of the crop and the weeds may also be a factor, Harker says. “A competitive variety and increased seeding rates can make a big difference in the way a crop performs under lower herbicide rates. Once you get to a decent seeding rate, weed competition under a lower rate basically levels off.”

A chemical company's desire “to boast in its advertising that ‘We're No. 1,’” also seems to be a factor in setting rates, Harker contends. “Many are so intent on suggesting their products are perfect that they overlabel rates so they will work under any conditions. This type of thinking has led to moving rates higher and higher, rather than seeking the lowest effective rate.”

In an open discussion that followed, several WSSS members offered these comments, pro and con:

  • “Herbicide rates were set at levels that were shown to control weeds under most circumstances. If later a grower finds lower rates are effective, OK. But manufacturers can't start with labeled lower rates because if a lot of failures result, growers won't have confidence in their products.”

  • “We've got to such a high level of efficacy with today's herbicides that we don't need such high rates to reduce competition from weeds. Farmers have become fairly well educated to rates and what chemicals will do. We need to look at some of our management practices to see if we can manage the weed seedbank with lower rates.”

  • “If manufacturers were to set the price of a chemical based on control at lower rates, but non-ideal situations required more control and higher rates, could farmers afford it? When they set the price of a herbicide, they set it at a level they think will give the most efficient control under most conditions. There are studies showing that low rates worked great for the first two or three years, resulting in savings to the farmer — but in the fourth year, when everything went wrong, it became very pricey and control of the weed seedbank was lost. If you set rates based on the expectation of failure every four years — or 25 percent of the time — that's irresponsible.”

  • “What many farmers expect is the highest efficiency; they want 95 percent to 98 percent control. Often, they use only one chemistry, and resistance is a major problem.”

Pressure by region

  • “Companies also have to consider variations in weed pressures from region to region. Weed control in soybeans in the South, for example, is much different than in the Midwest. Southern farmers will accept weedier fields than Midwest farmers. We're trying to sell products that will give farmers the control they want at a price they can afford.”
  • “Growers don't want weed control failures. They want to know when they can use lower rates and when they need full rates. They need more information to quantify these conditions.”

  • “It's easy in retrospect to explain product performance, but how can you predict in advance weather and other variables.”

  • “You can use data to assess predictability. A lot of rates are set 50 percent higher than they need to be to compensate for variability.”

  • “Growers have to operate within a certain window of opportunity for their various operations, and being able to carry out a practice on an 8 by 10 research plot is a lot different from having to spray a thousand acres.”

  • “We have to ask what will be the long term cost to growers who use high rates continuously and find they have no effective herbicides left because of weed resistance.”

  • “Low rates may reduce resistance for many years — but the grower may have so many weeds in the process that he can't economically grow a crop.”

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