There are many reasons to grow winter wheat. But one mark against this crop is its vulnerability to weeds. Winter wheat growers want to get in a herbicide application as early as they can during the growing season. This can be a problem. Ontario farmers have observed that applying too early can injure the growing plants. How soon can you get your sprayer into the field?
A group of agronomists from the University of Guelph, and the Ontario Ministry of Agriculture set out to answer that question. They recently published their results in the “Canadian Journal of Plant Science” with the title “Winter Wheat (Triticum aestivum L.) Response to Herbicides As Affected By Application Timing and Temperature.”
Given the popularity of winter wheat out here on the Prairies we got in touch with one of these researchers, Melody Robison, to ask her about lessons they learned from their study that can be used west of Ontario.
The scientists studied wheat growing in open fields between 2009 and 2010. This was an important advance because previous studies of cold weather herbicide treatments had been conducted in growth cabinets in laboratories and greenhouses. They tested a wide range of herbicides both by themselves and in combination in three different timings: early (before the emergence of the crop), normal, and late. The early timing was set to coincide with below freezing temperatures over the test fields.
The majority of the herbicides were not found to cause lasting damage to winter wheat. Only 2,4-D by itself and the two combinations of dicamba/MCPA/mecoprop and dichlorprop/2,4-D were harmful to winter wheat, causing visible injuries like necrosis, leaf bleaching, and curling, but these healed as the growing season went on.
Though the herbicides produced little injury to the winter wheat this is not a reason to be careless with cold temperature spraying. The most important reason is because cold temperatures not only slow the metabolism and growth of winter wheat, they also slow the metabolism and growth of weeds. Slower metabolism means less uptake of herbicides, and thus less herbicide effectiveness. Spraying during cold weather, while it may seem like a way to get a jump on weed growth, may often be just wasting product and with it money.
It is also important to be cautious when applying herbicides to winter wheat late in the growing process, particularly combination herbicides. Dicamba/MCPA/mecoprop is the most damaging when applied late, and did adversely affect yields. As with so many things, timing is of the essence when it comes to treating weeds. Farmers should strive, whenever possible, to apply herbicides at the prescribed times to their crops rather than early or late.
Spraying too soon risks wasting money; spring too late risks injuring the crops. Herbicide sprayings should be timed to the growth stage of weeds, to make sure there is maximum absorption and effect.
To help you make better decisions about when to apply herbicides consider learning more about the Zadoks scale, a measurement of the development of cereal crops. Optimal herbicide application time on winter wheat is between Zadoks 21 to 29, when the plants stem is beginning to elongate. You will likely be able to find Zadoks scale guides through your local ministry of agriculture, and a quick Google search for “Zadoks scale” will turn up many good sites on its use.