Does Herbicide Tolerance Mean Higher Yielding Hybrids?
By Greg Stewart, OMAFRA Corn Specialist and Mike Cowbrough, OMAF Weed Management Specialist

When evaluating corn hybrids, how concerned should growers be that herbicide tolerant hybrids were actually treated with the appropriate herbicide?

When trying to assess the impact that a herbicide program has on corn yield, there are a number of factors that a producer needs to keep in mind. Here is our list in order of importance:

1) On Target: a herbicide program needs to be well suited for the weed spectrum in the field.
2) On Time: herbicides need to be applied in a timely fashion so that weeds are brought under control early; by the time the corn plant has 3 leaves, your program better have weeds under control or yield may start to evaporate.
3) Crop Safety: herbicides should put minimal stress on the crop.
4) Smooth Harvest: herbicide programs provide control such that harvesting problems and/or weed seed returns are kept to a minimum.

In reviewing this list, it is apparent that herbicide tolerant hybrids and their associated weed control programs can have an influence on the way we view each of these four factors. When working with broad-spectrum, non-selective herbicides such as Liberty or Roundup, being on target has become less of an issue. In conventional herbicide programs, growers need to be more aware of the weed spectrum they have in their fields and be sure to select programs that meet those specific needs.

Weed researchers and the crop production industry have done a good job of defining the need for weed control to be ‘on time’. For the most part this has meant understanding the period of time within the crop’s life cycle where it is absolutely critical that weeds are not allowed to compete with the crop. In most cases this has translated into an increased concern over herbicide applications that are made later than the ideal time. With conventional chemistry, if you applied the product late you were often left with enough weed escapes to remind you right up until harvest that your untimely weed control probably cost you significant yield. Herbicide tolerant systems have changed that perspective somewhat. You can still apply the herbicide too late, your corn will still suffer from too much early weed competition, and yields will still be reduced. However, now when you come back into the field to harvest, it is absolutely weed-free because that 8-leaf stage application of glyphosate cleaned up everything! The need for on-time weed control with herbicide tolerant hybrids is still very important even if it is not as obvious.

Crop safety has also been looked at differently as the result of hybrids that have been genetically engineered to be tolerant of specific herbicides. Here is the key question raised on this issue: Do conventional herbicides cause more crop injury and yield loss than if that hybrid was managed under the applicable herbicide tolerant program? The answer here is a resounding – Maybe! Are you a producer who is stretching time and equipment as thin as possible and therefore spraying a lot of post-emerge herbicides in less than ideal conditions or at less than the ideal crop stage? If so, then yield loss from crop injury is a risk to deal with. Planting some of your crop with herbicide tolerant hybrids may be a good strategy to deal with part of this risk. If however, you successfully lay down a balanced attack of soil-applied herbicides and on-time post-emerge products, the possibility of boosting yields just because you switch to a herbicide tolerant program is minimal.
Herbicide tolerant hybrids or not, weed control
timing is critical!
Research on this issue can be a bit tricky to sort out since spraying different herbicides at different times may bring several factors into play, not just crop safety. There can be some ‘system’ type advantages that may boost yields in herbicide tolerant programs depending on some of the factors discussed earlier.

In some instances, a hybrid may perform poorly because of its sensitivity to a particular conventional herbicide. In other instances, extraordinary weather conditions at the time of herbicide application has more of an influence. But on the whole, comparing hybrid performance of conventional and herbicide tolerant hybrids within a trial where they all received carefully applied conventional herbicides should still give a very accurate reflection of a hybrid’s yield potential.

Also remember that when hybrids are grown in test situations such as the OCC Performance Trials, care is taken to use herbicides and application timings that minimize crop safety concerns. If weed control is not excellent and uniform, the trials are discarded.

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