Practical tips for spray application

Every year, Ontario grain farmers learn a bit more about herbicides and tweak their practices to boost yields and save costs.

There are many moving parts in getting it all right, and among the toughest challenges is preventing crop injury due to soil retention. Growers can only guess how much active ingredient remains in the soil at planting time, which factors into what should and should not be planted, explains Dr. François Tardif of the University of Guelph.

At the Southwest Agricultural Conference in January, Tardif noted that in most cases, herbicides are absorbed by soil colloids and released back into the soil water, where they undergo degradation. This breakdown can occur chemically, through exposure to sunlight, and/or biologically by microbes. Degradation, he says, is “a phenomenon that is impacted a lot by temperature, a lot by the amount of water you have, by the pH and the chemical makeup of the herbicide, as well as the type of colloids you have.”

It would seem that good insight into how long it takes a given herbicide to degrade is provided in the half-life printed on the product label, but that’s not really the case. Tardif reminds us that these values are lab-based and, therefore, entirely theoretical. Because labels are legal documents, half-life recommendations also tend to be very careful.

Tardif also reminds us that half-life values present a curved relationship, not a straight one. That is, it is the time required for an active ingredient to reduce from 100 per cent concentration in the soil to 50 per cent, and if that’s 120 days, for example, it will take another 120 days for that 50 per cent to become 25 per cent, and so on. “Half-lives are useful to compare different products, but in terms of telling you what’s happening in the field, they’re less useful,” Tardif says. “That’s because, as we know, temperatures are not constant, and rainfall is not constant.

So, if we take those curves and say, well, after 120 days, it’s starting to get pretty cold…going to September, October, it gets colder and colder, and that slows down the activity of the chemical degradation and most of the time the microbial degradation.” He therefore cautions that overall, there can be a very big gap where a herbicide “may have in theory reached a low concentration level in the soil, but the reality is that the year after, it’s still pretty high.”

Soil pH also affects the degradation of individual products. Pursuit, for example, has higher persistence at lower soil pH because, under these conditions, it binds more tightly to soil colloids. (There could also be soil acidity effects on microbiome activity, Tardif notes). “In this case, you get half-life closer to 40 days under constant temperature of 25 °C and 50 per cent field capacity, but as the pH increases, getting closer to neutral, the half-life is reduced quite a lot to less than 30 days,” he said. However, at soil pH higher than 7.8, herbicides like Classic have a very long persistence.

AN EXAMPLE CASE: WINTER CANOLA

Winter canola is a growing crop in Ontario, and while some farmers have had no issues planting it after winter wheat sprayed with Infinity, other growers may see crop injury. Asked at the conference about this, Tardif did his best to give guidance. “If I were in that situation…I’d look at how much rainfall we had this year,” he said. “If there were lots of rain, then there would be lots of biological activity in the soil. Also, if I had a huge winter wheat crop with a huge canopy, high-yielding, the winter wheat would have absorbed a lot of that herbicide. But if the season was dry, we had some cold spells, my wheat is not that great, maybe I see that the weed control from that herbicide is not so good. This means that maybe hasn’t been activated in terms of the residual, so then I’d be more worried that those residues would have persisted.”

HERBICIDE EFFECTIVENESS

Activation was one of the focuses of Dr. Peter Sikkema at the University of Guelph- Ridgetown, who reminded growers at the same conference of the factors that affect herbicide effectiveness. There is more variability in weed control than one might think, he stressed, and farmers should keep this in mind in order to avoid wasting money and seeing less-than-ideal results. A particular herbicide’s effectiveness can greatly impact everything from weather and soil conditions to application issues.

Soil-applied products require water for activation, and Sikkema explains that it’s not just rain but soil moisture conditions at application that matter. “If the field is near its maximum [water-holding capacity], a quarter of an inch of rain will do for activation,” he explains. “If the soil is powder dry, an inch important not only to look at the local weather forecast but also to get a reading on your soil moisture levels (and in all areas of your field if there is in-field variation in soil type). At the same time, excessive rain after application of soil-applied herbicides can rapidly leach a portion of your product beneath the weed seed germination zone, Sikkema notes, also reducing effectiveness.

With post-emergence products, weather that’s dry and also too hot will also cause problems. Sikkema points to research showing that glyphosate absorption by common milkweed plants is 15 per cent lower when temperatures are up. With other products, effectiveness in these conditions can be lower by 35 per cent or more.

Soil characteristics also matter. “Most soil- applied herbicides are adsorbed to clean organic matter,” Sikkema explains. “Always remember that if a herbicide is bound to the soil, it provides zero weed control, and

there are more binding sites on a heavy clay, textured soil high in organic matter. So, the herbicide rate should be adjusted depending on your soil type and organic matter level.”

WEEDS—AGE, TYPE, AND DENSITY

The waxy cuticle on weed leaves varies in composition and thickness among weed species, but of course, younger plants usually have thinner and less-developed cuticles more permeable to herbicide. “Always remember young or small weeds are easier to kill,” Sikkema says. “If you’re going to make a mistake as a grower, always make the mistake of spraying too early rather than too late. If you apply too late, you’re going to get poor weed control with almost every post-emergence herbicide.”

Herbicide effectiveness is also dependent on species. Sikkema’s research with colleagues has shown that five times as much Roundup is required to kill a common ragweed plant than a green foxtail plant (but note that these study plants were large). “So, you need to know what weed species you have in each field,” he says, “and adjust your rate depending on species composition.”

Looking at weed plant numbers, Sikkema points to a study where a herbicide applied at the same rate had a control decrease of about 20 per cent in higher densities. “It is a fact with every single herbicide, whether it’s a soil-applied herbicide or a post-emergence herbicide, herbicides always work better in low-density environments,” he says. “You should do everything you can to reduce weed seed return to the soil.”

Read more practical tips for spray application at www.ontariograinfarmer.ca. •