Even Glyphosate Tolerance has its Limits

Applying at too high rates can still knock corn.

Published on: Mar 10, 2008

Roundup Ready corn offers glyphosate tolerance. But that doesn’t mean it’s a license to make gross over application errors of Roundup and still expect not to suffer a penalty form your mistake. The Corn Illustrated crew recently demonstrated that in a greenhouse experiment. Fortunately, it was in the greenhouse and only involved a few plants, not in the field last summer where acres could have been involved.

As a matter of fact, last year’s hybrids used in the Farm Progress Corn Illustrated experiments on the Jim Facemire farm, Edinburgh, Ind., did not contain GMO traits. SO the corn was not tolerant to glyphosate. Most of the fields did not need spraying postemergence. Bicep was applied pre-plant. However, in the high yield plot, an application of Stinger was necessary to knock back patches of Canada thistle that were providing serious competition to young corn plants, competing for water during the dry weather early on in the growing season last year.

This year’s plots will likely involve GMO-hybrids. That’s not necessarily because Facemire or the consultant for the Corn Illustrate plots, Dave Nanda, believes that all the traits are necessary at Facemire’s location. The overriding factor is that the best genetics available to Facemire happen to be in triple-stack hybrids. However, it’s still not clearly decided if he will be choosing Roundup Ready hybrids and applying glyphosate where necessary, or if the will go with Herculex hybrids and apply Liberty herbicide where needed instead. Liberty tolerance tags along with the Herculex trait since the Liberty Link gene was used as a marker in developing the Herculex Bt event into a commercial trait for control of insects.

Meanwhile, in the greenhouse recently, a barrel of corn plants containing both the male and female parent of a hybrid line were planted together for demonstration purposes. Only one of the parents contains the Roundup Ready trait. However, it is then passed to the hybrid formed by crossing these parents, making it a glyphosate- resistant hybrid.

The idea of this simple demonstration was to spray all plants in the barrel with Roundup from a ready-to-use spray bottle, then see which ones lived. Unfortunately, only one plant survived, even though about half of each parent were planted originally. The conclusion is that most likely, the rate reached by spraying by hand and covering the plant was much higher than what corn would see in the field. So field-type experiments don’t always translate easily to the greenhouse.

Obviously some of the plants that died contained the resistant trait. They did turn brown more slowly, but did not survive. The rate that the plants saw must have been much greater than what Roundup-Ready plants sprayed with glyphosate typically see in the crop field during an application of the herbicide.

The hybrid plant was sprayed at the same time in the greenhouse, and it did not show any ill effects from the application.