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Protecting winter wheat yield

The value of winter wheat continues to increase, so farmers are devoting more acreage in Ontario to growing it, and maximizing yield is an important concern. When deciding which mixtures of herbicides and fungicides are most effective in protecting their winter wheat crops, growers seek the advice of other growers as well as seed companies and chemical company representatives. “Before the regulations changed in 2009, growers could only mix chemicals according to labels, using only registered mixes,” notes Dr. Francois Tardif, an associate professor in the department of Plant Agriculture at the University of Guelph. “Now you can mix any of the chemicals that are registered for a given crop. By spraying mixtures of herbicides and fungicides and/or fungicides plus liquid fertilizers, a grower can avoid some trips across the fields, but doing so can cause enhanced crop injury and lead to potential yield losses.


In order to shed some light on whether some combinations of products cause injury, Tardif has completed a study with funding support from the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). With Professor Peter Sikkema, graduate student Melody Robinson, research associate Nader Soltani, Mike Cowbrough (OMAFRA field crop weed specialist) and technicians, he looked into how temperature and timing of application factor in winter wheat growth.


“We know that applying herbicides early in the season when weeds are small is a good practice,” notes Tardif, “but under conditions where night temperatures are low, crop injury can occur.” He also wished to gain some insight into the underlying mechanisms behind such injury, specifically, what is occurring in the physiology of the plant.


Field trials were conducted at four sites in Ontario in 2008-09 and 2009-10, along with growth cabinet experiments at the University of Guelph. “Overall, we found that herbicide injury is not enhanced by cold conditions, but just the opposite – in some cases, injury occurred under warmer conditions,” says Tardif. “We found that specific herbicide-fungicide mixtures cause injury, but that winter wheat is quite resilient to the injury and for the most part in our experiments, yield was not significantly affected.”

Specific mixtures
The research team observed that Estaprop Plus with Folicur consist-ently injured winter wheat, causing dark flecking of foliage at all locations with both early and late application timings. Buctril M with Folicur was the most injurious tank mixture, and often caused more injury when applied late rather than early; injury was also seen with Buctril M combined with Stratego and Buctril M with Quilt. “We are not sure why these particular mixtures caused more injury than others,” notes Tardif, “but we do know that tank mixtures including the fungicide Folicur and mixtures including the herbicide Buctril M were most likely to cause injury.”


He says crop protection products are designed so that crop plants generally show less sensitivity to them than weeds, but mixtures can feature “synergistic effects,” and things like quicker absorption can result. “Crop plants can generally handle chemicals when they absorb them slowly, but mixtures can do things like dissolve the natural waxy covering on leaf surfaces,” notes Tardif. “The chemicals enter the plant more quickly and injury results. Similarly, when mixtures are applied later, the plant is growing faster and the temperature is warmer, so quicker absorption can happen here too.” This was evident with Target herbicide, where “late” applications of Target alone resulted in reduced crop height and yield, while “early” and “normal” application timings caused no injury to winter wheat.


Whatever herbicide-fungicide tank mixtures were used, the winter wheat in the study recovered from injury, and there was no significant impact on yield. “The injury was striking visually and looked quite bad from a distance, but it was just the top leaves,” Tardif explains. “Wheat is more resilient than we think, and appearances were deceiving in this case.”


Cowbrough adds that while the observed crop injury didn’t impact yield, crop injury can lead to yield losses if accompanied by secondary and tertiary stresses on the plant, such as disease. “No one wants crop injury, so check with all the companies of the products you plan to use and make sure your mixture is cleared,” he says. “Also increase your water volume in your tank, and spray in moderate temperatures. A wide range of temperatures experienced in a short period of time can provide more risk of injury to the crop.”  


The most important part of the study may be somewhat unrelated to winter wheat.


Tardif and his team also observed that Buctril M, Achieve and Puma were the only herbicides that did not greatly injure or reduce the dry weight of underseeded red clover. “It’s a good agronomic practice to have clover to improve soil structure and fertility and to help prevent soil erosion, so farmers should take these particular results into consideration,” Tardif concludes.