Plants that develop resistance to herbicides are simply following the age-old process of “survival of the fittest.” This process acts on phenotypes, or the plants in this case, but the unit of inheritance is the gene.

Different versions of the same gene are called alleles. Individuals usually have two different alleles for each gene. An example of this in humans is blood type, where each parent contributes a different allele to the offspring.

Therefore, herbicide resistance can be defined as “an increase in frequency of resistant alleles in a plant population after exposure to herbicide selection.” There are three main sources of these resistant alleles: new mutations, immigration and standing genetic variation.

New mutations are rare and impossible to predict, so there’s really nothing we can do about those. Immigration occurs when a population is found in new territory, such as when Palmer Amaranth invades an area. Immigration also is considered to be fairly rare unless it’s introduced through an outside source.

The most common source of resistant alleles is from standing genetic variation. We’ve all seen studies of how this can happen. Herbicide is sprayed on a field where there is a high concentration of weeds. Most of those weeds die but a few survive. If not controlled with another method or herbicide, those few surviving weeds will produce seeds that will germinate the following year. If the same herbicide is used that next year, the population can increase dramatically. After multiple years of this practice, a tremendous weed problem exists.

When waterhemp became a serious problem in Illinois, Dr. Aaron Hager and his colleagues conducted a multi-year study on what factors contributed most to the occurrence of herbicide-resistant weeds. They took into consideration a wide range of factors within four main categories: Management, Weeds Present, Soil Properties and Landscape.

Three major findings resulted from the University of Illinois study:

1. How close you are to a neighbor who has resistant weeds is not a good predictor of when or if you would have them.
2. Overall management factors have the most impact on preventing weed resistance: How many different herbicides are used; How often pre-emergence herbicides are used; Crop rotation, etc.
3. Herbicide rotation is important, but mixing herbicides with different sites of action makes weed resistance 83 times less likely to occur.

My college professor in Principles of Plant Physiology said it quite simply, “A weed is a plant that is out of place.” I never thought to ask him just where he felt was the proper place for Waterhemp… or Kochia… or Palmer Amaranth.

Weeds have been a problem since the dawn of mankind. After Adam sinned in the Garden of Eden, God “cursed the ground” and told him that “thorns and thistles would be produced” because of what he did. (Genesis 3: 17-19)

Not only were thorns and thistles (i.e. weeds) produced, but they multiply rapidly. Did you know that Waterhemp produces as many as 1.5 times more seeds than most other pigweed species? Waterhemp plants generally produce about 250,000 seeds per plant. Some Waterhemp plants can produce 1 million or more seeds under optimal conditions in noncompetitive environments.

Resistance to herbicides results when weeds get sprayed but live through the application. This might happen because of improper rates, improper timing or just due to natural selection. That’s why we must learn from the past to ensure success in the future.

Herbicides were introduced in the U.S. after WWII with the marketing of 2,4-D. Atrazine was first registered for use in 1959, followed by a succession
of products designed to control weeds.

Farmers in the 1970s and ‘80s didn’t worry about weeds becoming resistant to herbicides being used because all they had to do was visit their local chemical retailer where they’d get the newest and greatest product to try.

The first herbicide in the U.S. that weeds developed resistance to was Atrazine in the early 70s. The alarm, however, didn’t really get sounded until resistance to Pursuit® and Scepter® occurred. These ALS herbicides were introduced in the mid-80s and weed resistance showed up very quickly.

When Roundup® (glyphosate)-tolerant soybeans were introduced in 1996, farmers believed they finally had the ultimate answer to weed control. Not only did this new system work, but it was very simple to use.

Roundup made good weed managers out of bad. Weeds too big before the field was sprayed? No problem… just add a little more! Don’t bother with any of those old herbicides… all you need is Roundup!

Sound familiar? We were assured weeds would NEVER develop resistance to glyphosate, but it happened. To make matters worse, the “endless supply” of new herbicide products stopped. The last new herbicide group of HPPD inhibitors was discovered in 1986.

There are 29 groups of herbicides, each with its own unique site of action, according to the Weed Science Society of America (WSSA). At the latest count, weeds have developed resistance to all but three of them.

It’s been well documented that weed resistance is a growing problem. In next week’s article, I’ll talk in more detail how weeds develop resistance to herbicides. The last article in this series will cover what (if anything) can be done.