Herbicide Tolerance and Yield
By Mike Cowbrough, OMAFRA Field Crop Weed Specialist
Greg Stewart, OMAFRA Corn Specialist

Claims that highlight substantial yield gains for growing herbicide tolerant corn hybrids versus conventional hybrids makes one wonder where the yield advantage comes from. Is the yield advantage a result of better weed control, a wider window for product application, increased crop tolerance for the intended herbicide, or a combination of factors? And, if it is a combination of factors which one should producers consider to be the most important in their specific operation? Certainly it is often implied that by growing a herbicide tolerant crop, yields will be greater due to increased crop safety if using the intended herbicide over a conventional one.

One way of isolating this "herbicide safety factor" is to take weed control right out of the picture. If all systems could be kept weed free for the entire growing season, would it matter whether or not the herbicide tolerant hybrid was treated with its intended herbicide? This approach was taken in experiments conducted by the University of Guelph at the Huron Park, Ridgetown and Woodstock Research Stations under the direction of Dr. Clarence Swanton and Dr. Peter Sikkema in 2003. The aim of these trials was to see what impact herbicide treatments had on the growth, maturity and yield of three different Roundup Ready and Liberty Link corn hybrids. All herbicide treatments were maintained weed-free so that only the effect of herbicide treatment could be evaluated. Preliminary results indicate that the various herbicide treatments had no significant impact on grain yield, and that any slight differences in yield could be attributed to normal variability within the field (See Tables 1 & 2). Based on this preliminary data, it would appear that growers should keep their focus on the old standbys of:
o Selecting a product that will best control the species present in the field.
o Applying herbicides so as to have a weed-free crop during the critical period.

This is not to say that crop injury has no bearing on grain yield. Rather, it just reaffirms that if you are a grower who can get a herbicide program down in a timely manner and under favourable growing conditions, it doesn't really matter what herbicide program you pick provided it meets the above two objectives. However, if you are a grower with a lot of land and can't guarantee that every acre will receive a herbicide application when environmental conditions are good, then a herbicide tolerant system on a portion of your acres makes a lot of sense.

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