The Rate Debate

Cut the rates and accept the risk. That's not as much a threat as it is a fact. Canadian farmers face risks if they tinker with pesticide application rates that vary from what appears on the product label.

Posted: January 30, 2006

Technically it's illegal to use a lower rate of herbicide and/or water volume, if that latitude isn't specified on the product label. But in practice, no one is likely to be thrown in jail for applying less pesticide than called for by product registration.

In fact, the Pest Control Products Act says only that pesticides should not be applied in an unsafe manner. However, researchers point out that applying lower than label rates in most situations can hardly be considered as unsafe.

No warranty

So why not do it? The greatest producer risk from cutting rates, other than reduced weed, insect or disease control, is voiding the warranty, says Dr. Len Juras, with Dow Agro Science in Regina. "If there is a concern about control and the product has been applied properly, the farmer can expect protection as well as proper service and satisfaction from the product representative. But if the producer used an off-label application, there isn't much a manufacturer can do."

Whether to follow or bend rules on pesticide application rates has been debated for years. Ag chemical manufacturers state label recommendations reflect the broadest range of control a particular pesticide can offer. For best performance from the herbicide, insecticide or fungicide, it must be used within the recommended chemical and water application rates.

That's a difficult stand to defend when some sprayer equipment manufacturers, independent researchers and farmers, point to experience that shows reduced chemical and/or water rates can still provide effective control.

Research shows producers can reduce herbicide and water application rates, but it is a gamble, say researchers.

"Over a series of years, reduced rates may work just fine," says Brian Storozynsky, a long time sprayer technology specialist with Alberta's AgTech Centre in Lethbridge. "Say it works four out of five years, but that fifth year where it doesn't work could be very expensive."

Can be a gamble

In evaluating sprayer systems and nozzle designs over more than 20 years, Storozynsky says the factors that influence herbicide effectiveness are difficult to predict.

"Reducing herbicide and water rates can be done and it will reduce costs, but there will be years you'll run into reduced control," he says, pointing to recent field studies. "This past year is a good example. Whenever we reduced rates, we had reduced weed control, but the two previous years were fine."

Dr. Tom Wolf, a long-time Agriculture and Agri-Food Canada researcher in Saskatoon has a similar response. While Wolf specializes in technology and application methods that affect pesticide drift, he is familiar with the rate debate.

"Label application rates provide producers with the broadest latitude for optimum weed control," says Wolf. "While there are powerful economic arguments made for reducing rates, producers need to be aware of the risks. They won't be prosecuted, but they need to know they are on their own. If they have a complaint, there is no recourse."

Wolf's concern is that reduced rate practices often contribute to increased spray drift.

Science base

It's often believed that ag chemical manufacturers defend recommended herbicide and water application rates only to sell more product; however there is science behind the numbers on the labels.

"Rates are designed to provide the broadest range of forgiveness under a wide range of conditions," says Juras. "Not every acre and every weed can be sprayed under ideal conditions, or at the most optimum time.

"Most products have a certain amount of headroom that covers off the environmental conditions," he adds. "But, when too many variables come into play, producers can run into problems. If the crop is at the five leaf stage, for example, the wild oats will have two or three tillers. If you reduce the herbicide rate and cut the water in half, and have the sprayer equipped with a coarse nozzle, then there's real risk of a crash or an issue of non-performance.

"Research shows that using the recommended herbicide rates and water volumes, and paying attention to crop and weed staging, provides the best insurance for pest control."

Reducing chemical and water rates can affect the mechanics of how pesticides work. With systemic products that must make contact with the leaf surface and then translocate to the roots, high chemical concentrations can affect that process.

"These products need to translocate through the plant to reach the growing point," says Juras. "If you cut water from 10 to five gallons, for example, and double the concentration, the droplet can be so concentrated that it burns the leaf. The herbicide can't translocate because tissue is destructed at the site of disposition."

The effectiveness of reduced rates also depends on the target weed, he adds. A coarser, more concentrated spray may be effective on smaller weeds such as cleavers or wild mustard, but it is ineffective on a larger plant such as Canada thistle. "With a more concentrated five gallon rate, you are depending on an active ingredient such as clopyralid to translocate into the root system.

"If the spray is too concentrated and scorches or burns the leaf, the clopyralid could be tied up and its primary purpose is defeated," says Juras.

"A producer might figure if reduced rates hammered the buckwheat and wild mustard, it's bound to do a good job on the sow thistle or Canada thistle," he says. "The tops may go down because the spray was concentrated, but the weeds could regrow. Depending on the product, producers are looking for long-season control or perhaps even residual action the following year, but it just isn't there."