Features Agronomy Tillage
Herbicide carryover may be a concern in 2009 drought areas

With the low levels of precipitation that fell on some areas of the Prairies in 2009, concern has been raised over the potential of herbicide carryover affecting crops planted in the spring of 2010. Eric Johnson, a weed scientist at Agriculture and Agri-Food Canada at Scott, Saskatchewan, says that although herbicide carryover is unpredictable, he has seen enough trends to offer some cautions for 2010. “Numerous mechanisms are involved in herbicide dissipation, but soil moisture content and temperature are critical factors for microbial degradation of herbicides,” explains Johnson. “I can’t say that you are going to have a problem with 100 percent confidence, or you’re not going to have a problem, but there are some things you can keep in mind.”

Johnson explains that the high risk conditions for herbicide carryover seem to be when a grower applies a herbicide one year, has a very dry July and August that year, a dry May during the following spring, and then a heavy rainfall in June when the crop is emerging. He also says that even though seedling injury can occur due to herbicide carryover, the amount of yield loss, if any, is dependent on environmental conditions experienced during the growing season of the rotational crop.

Johnson’s experience on herbicide carryover runs deep during the last 10 years, quite often in collaboration with Rick Holm and Jeff Schoenau at the University of Saskatchewan. He took over some of the work from retired weed scientist Ken Kirkland, who started work on the Group 2 “imi” herbicides (Odyssey). Johnson has also worked on Group 2 flucarbazone (Everest), stacking of Group 2 residual herbicides in back-to-back years, and Group 14 sulfentrazone (Authority) carryover. “Sulfentrazone isn’t being used much yet because it only has conditional registration on chickpeas, but when it gets full registration on flax and field peas, it will also be a residual herbicide that we have to watch,” says Johnson.

Johnson has just started trials on Velocity (pyrasulfotole + thiencarbazone-methyl + bromoxynil) and Simplicity (pyroxsulam; Group 2), but has not done work on Infinity (pyrasulfotole + bromoxynil).

Based on 2009 climate data, Johnson explains that there is a high potential for herbicide carryover in east-central Alberta, and some parts of the Peace River region of Alberta. West-central Saskatchewan, where precipitation was low in 2009 also has a higher risk.

Herbicide carryover risk based on 2009 monthly and total growing season precipitation (% long-term normal)

Source:  Eric Johnson; Environment Canada weather data.

Soil moisture and temperature are the main drivers
Many factors are involved in herbicide carryover such as soil organic matter, clay content of the soil, herbicide half-life and pH. How the herbicide adsorbs to the soil and whether it is bioavailable are also important. For example, Reglone (paraquat) has a long half-life, but because it is very strongly bond to the soil particles, it is not bioactive and so does not present a herbicide carryover risk to the crop, even though a chemical analysis of the soil would show high levels of paraquat.

Soil moisture, temperature and organic matter however, are the overriding factors because they influence soil microbial activity, the main process that occurs when herbicide residues are broken down.

Looking back is how we can predict the future says Johnson. He cites a few examples to illustrate how drought stricken farmers might plan their crops for 2010. Around Yorkton, Saskatchewan, in 2001, for example, there was a large drought area. During the 2002 growing season, the spring started out dry and emerging canola looked fine, but above-normal precipitation in June resulted in significant Sundance (sulfosulfuron) Group 2 residue injury to canola. Although Sundance is no longer registered, the experience provides insight into herbicide carryover.

Johnson explains that under dry conditions, the herbicide is tightly bound to the soil colloids, and is not available for microbial breakdown because it has to be in a soil solution for microbes to feed on it. Then a rainfall comes along, the herbicide becomes soluble and present in the soil solution where it can damage the crop. “The precipitation pattern that seems to stand out is that there is a dry June through August when most of the herbicide degradation should occur but doesn’t, followed up with a dry spring and then a wet June the following year,” explains Johnson. “It is an arbitrary figure, but based on a lot of discussion with companies, we think 150 mm or less during the growing season or year of application is a problem. So keep that in mind as you look at your own cropping history.”

Johnson compiled Environment Canada weather data for several locations in Alberta to assess the risk of herbicide carryover. Based on the precipitation data and when the rainfall occurred, he would rate parts of the Peace River and east-central Alberta as high risk, but cautions that rainfall can be very localized, “so this kind of data is a bit arbitrary.”

How far back do you need to look?
Another concern in east-central Alberta relates to Odyssey carryover from a 2008 application, and whether it might cause injury to canola in 2010. Johnson says this type of application sequence at Scott caused some concern in drought years, even though canola grown two years after application is a registered re-crop. “The weather patterns where we had canola damage followed either a wet-dry-dry cycle over the three years, or a dry-dry-wet cycle, where the active ingredient didn’t seem to break down enough in the year after application,” explains Johnson. “For growers who applied Odyssey on pea in 2008 and had a dry 2009 growing season, that might be in issue for canola in 2010.”

Johnson summarizes the potential registered re-cropping recommendations that might be a concern in the high-risk zones where precipitation was low during the 2009 growing season. Odyssey applied in 2008 on land going into canola and mustard in 2010 may be a concern. He says that even Solo applied in 2009 in dry conditions can result in some carryover the following year. With Everest and Curtail M, his only potential concern is field pea.

Johnson says there have been some issues with Infinity carryover on field pea in the Brown soil zone of Saskatchewan. Lauren Davis, manager of cereal crop herbicides for Bayer CropScience, says that a limited number of growers who used Infinity in 2008 and re-cropped to peas in 2009 were affected. The vast majority of the inquiries came from the Brown soil zone area of Saskatchewan. “Bayer did a lot of pre-registration research on the active ingredient pyrasulfotole at up to three times rate and we didn’t see any trouble,” explains Davis. “The cold dry spring of 2009 led to delayed pea growth and residue symptoms with several chemistries, not only pyrasulfotole.”

Davis says the residue symptoms appear to be related mostly to soil parameters and that the effects on peas were observed primarily in the Brown soil zone on very low organic matter soils with high pH. The symptoms were typically seen on eroded knolls where these conditions often occur. Symptoms were also visible on sandy soils where the organic matter was less than 2.5 percent and pH was greater than 7.5.

Because of the 2009 experience, Bayer is submitting an additional label statement to Pest Management Regulatory Agency to add the following statement to the Infinity label: “Field peas may be grown the year following Infinity herbicide application in all Black, Grey-Wooded and Dark Brown soil zones. Do not plant field peas the year following an Infinity application in the Brown soil zone where organic matter content is below 2.5 percent and where soil pH is above 7.5 percent.”

“We had 1.5 million acres of Infinity applied in 2008, and only had a few problems. We do however want to make growers aware of the new re-cropping recommendation in the Brown soil zone and encourage growers to contact their local Bayer CropScience rep if they have any questions” says Davis.