Thursday, January 27, 2011
Why Is That Glyphosate Resistant Weed In My Field?
Wherever you farm in the Cornbelt, you may have a patch of weeds that Roundup or glyphosate will not touch. You might as well be watering them with your lawn sprinkler. You deal with them year after year, hoping they will disappear, but just like the tax bill, they show up every spring. The first question that comes to mind is how do you eradicate them; but deeper down, you wonder why they are there if they are a mutant species.
Your patch of glyphosate resistant weeds may have originated from two potential sources. One is the development of resistance by a weed several crop years back, and its seeds germinated near grandpa’s homestead. Or a weed could have grown from a seed that came in from some physical source, such as a bird flying over, in some crusted soil from your field cultivator, or it dropped from a deer that might have been running through the field. It can also come from pollen that was spread on the wind and the cross between the mutant pollen and the pure genes in your weeds turned out to be a problem child. That is the assessment of Iowa State University weed specialist Bob Hartzler.
Hartzler says weeds had some resistance to herbicides before they were introduced to your field, stemming from their genetic makeup. The amount of resistance depends on the percentage of weeds in the field that possessed the resistance trait initially. When the same herbicide is used in the field year after year, some of those weeds that carried more of the resistance gene will prevail over the others and become the dominant specie. Hartzler says, “Evolution of resistance within a field is a direct result of the weed management program used within the field and the initial frequency.”
The weed specialist contends the initial frequency of resistance will vary from one class of herbicides to another. For example, resistance is relatively high with herbicides that are ALS inhibitors. He says those were prevalent in Iowa, and their spread was due to the survival of the resistant plants from native weeds that grew throughout Iowa. They had a foothold to infest your fields. On the other hand, glyphosate resistance is more rare, and Hartzler doubts if there were any resistant weeds when Roundup was initially introduced. He says glyphosate resistant weeds will only appear due to independent selection in those fields which are “unlucky enough to have one of the rare plants.
With some resistant plant surviving to produce seeds, their genetic material will be available to move into your neighborhood. This can result either from seeds that have traveled to your field or in pollen grains from your neighbor’s mutant weeds. The likelihood of that happening, says Hartzler, varies with the characteristics of the resistance trait, biology of the weed, and the weed management practices in the adjoining fields. He says the cross pollination of plant that is resistant to glyphosate with a plant that is susceptible will result in increased resistance in their offspring. And he says with the volumes of pollen shed by waterhemp and giant ragweed, the genetic material can travel at length on a windy day. He quotes research at the University of Illinois which found waterhemp pollen can travel three miles per year by wind dispersal. Hartzler does not want you to think every field within three miles of a resistant plant will become home to a new resistant population. It has to cross with a plant that is receptive to the mutant gene and the field must rely heavily on glyphosate. He says if the weed management program on that field includes herbicides with different wants to control the resistant weed, then the resistant trait will not survive.
Regarding the physical spreading of the seed, Hartzler says they are easily transported on farm machinery, and glyphosate resistant waterhemp was found 25 miles from its original field within 2 years of the identification of the resistance.
Summary:
Glyphosate resistance in waterhemp, giant ragweed, and horseweed is becoming more prevalent. The resistant trait can spread in several ways, including the physical movement of the seed on wild animals and on farm equipment. Additionally, the genetic material can be transferred by way of pollen grains which can blow within a 3 mile radius, but have to land on a susceptible plant. However, the initial resistance came from the fact that all plants have some resistance to herbicide chemistry and the continued use of the same herbicide will allow certain mutants to develop and prosper without any challenge from a different herbicide.
Posted by Stu Ellis on 01/27 at 12:03 AM | Permalink