Though pigweed resistance to glyphosate in cotton has been the big newsmaker recently, resistance of several weeds, including pigweed, to the ALS-inhibiting family of herbicides could be equally devastating for peanut farmers.
There are over 20 ALS-inhibiting herbicides currently on the market, including the most widely used broad spectrum material in peanuts — Cadre. ALS is an acronym for acetolactate synthase, which is an enzyme that is essential for plants to grow. By inhibiting the channels of transfer of ALS, herbicides in the same family as Cadre, Classic, Ally, Beacon, Strongarm and 20 or so more, effectively control a broad spectrum of weeds.
In peanuts, a widely used program consists of Prowl, followed by Cadre. Prior to availability of ALS-inhibitors, that same weed management program would have typically consisted of five herbicides, each with a different mode of action.
In cotton, prior to Roundup Ready technology, a vast array of chemicals, each with a different mode of action was commonly used to manage weeds and grasses. With so many different modes of action, there was less chance of resistance developing.
Herbicide resistance is not new. The first documented cases came in about the same time as broad spectrum herbicides were introduced into the market — in the 1960's. For 20 years or so not much happened with resistance. In the 1980's, growers saw a sharp increase in resistance to many pesticides, from sterole-inhibiting fungicides to ALS-inhibiting herbicides.
The practice of growing cotton in rotation with peanuts, often at the exclusion of corn, could have been a positive factor in reducing ALS resistance, because of the widespread use of glyphosate to control weeds and grasses in Roundup Ready cotton. Using ALS inhibitors in cotton to avoid glyphosate resistance may be the kiss of death for growers who rotate cotton with peanuts, because ALS resistance to weeds, especially pigweeds, is speeded up.
Though glyphosate is widely used in both Roundup Ready soybeans and corn, these crops present less of a problem for peanut growers. Soybeans is not a major problem because it is rarely grown in rotation with peanuts and the herbicide systems used on corn, especially atrazine, tend to break up the resistance cycle.
Jay Ferrell, a weed scientist at the University of Florida, speaking at the recent Southern Peanut Producers Association meeting, says that herbicide resistance problems in pigweed is both difficult and easy to explain.
On the one hand, he says it is simply a numbers game. One mature female Palmer Amaranth pigweed produces up to 500,000 seeds. Two mature female Palmer pigweed plants provide enough seed to produce a one in a million chance for a mutation to occur and resistance to follow. Even if the odds are one in a billion, 20 mature plants provide enough seed for a mutation to occur and a chance for resistance. By comparison, common cocklebur produce less than 10,000 seed and a peanut plant less than 200.
On the other hand, a complex set of biological changes have to occur at the precise time in the weed's growth for resistance to occur. The most common cause for resistance is changes that occur biologically in the weed that alter the site to which herbicides attach.
Ferrell compares such an altered site to trying to use a Phillips head screwdriver to attach a screw with a standard head. When such an altered site occurs, no amount of herbicide will attach to the weed's growth mechanism and it becomes immune to the chemical.
For cotton growers the great fear should be that Palmer pigweed is just the first weed to play the numbers game and win. For peanut growers that fear should be ALS chemistry. For both the great fear is that pigweeds have developed cross resistance to both herbicides.
Pigweed is the most prolific seed-producing weed that peanut or cotton farmers have to face, but it is not the only one to develop resistance to herbicides. Worldwide, 95 different weed species have been documented to have varying degrees of ALS-resistance. In the U.S., 38 species have been documented to have ALS-resistance.
“In the past, lack of pigweed control was usually due to poor timing of application. Applying herbicides to a two-inch versus a two-foot high pigweed are two different things,” notes University of Georgia Weed Scientist Eric Prostko. “Now, those two-foot weeds are likely to be resistant to the herbicide of choice,” he concludes.
Prostko notes pigweed is always going to be one of the most competitive plants in a field. In tests in 2006, he says Palmer amaranth pigweed grew two inches per day. In peanuts, it creates major problems by robbing nutrients during the growing season and by clogging up diggers at harvest time.
In Georgia, Prostko's research team treated 2,900 peanut plants from fields suspected of having resistance problems. Using a Cadre rate 10 times the lethal amount in the field, 2,400 of the plants survived in the greenhouse. The bottom line, Prostko says is those weeds should have died, and they didn't.
Growers who suspect they have ALS or glyphosate resistance should look at the history of herbicide use in these fields — not just in a particular crop. If there is consistent use of either family of herbicide across multiple crops over a period of years, that is a first red flag.
If small areas of weeds continue to grow in fields in which the grower is certain there has been good control on other weeds, there is reason to be concerned. If these plants are treated with labeled or higher rates of a herbicide and the expected result doesn't happen, the likelihood is high that there is a resistance problem.
“It used to be that we looked at resistance as a last resort reason for lack of control, now it's one of the first things we look at,” Prostko notes. “It would be nice to be able to walk out into a field and say with some confidence — you've got resistance problems. Unfortunately, it's not that easy to diagnose. To be sure of weed resistance to a herbicide requires harvested seed from the suspect plant, growing these seed in the greenhouse, apply varying rates of herbicide under controlled conditions. All this takes time,” the Georgia weed scientist stresses.
Four weed species have been documented to have resistance to ALS-inhibitors or glyphosate. These include rigid ryegrass, horseweed, common ragweed, and Palmer amaranth pigweed.
It is critical for farmers to understand that resistance starts out slow. It may occur in a field four or five years before numbers get high enough to really cause problems. However, by that time, it's essentially too late to do anything other than avoid using the target herbicide.
Palmer amaranth resistance, for example, will likely start with a few plants that survive in a cotton or peanut field sprayed with glyphosate or an ALS-inhibitor. There are hundreds of reasons, other than herbicide resistance for these escapes. However, with resistant pigweed in the fourth or fifth year, the number of resistant plants will explode and literally take over the field.
Ferrell explains that resistance starts in a field with one plant that plays the numbers game and develops resistance. When a grower comes back and sprays the field, he eliminates competition for that resistant weed. If that weed produces seed that are resistant, the next year that one resistant plant becomes several, then rapidly spreads as the grower continues to provide a near perfect environment by putting heavy pressure on those susceptible weeds in the field.
The key to living with pigweed resistance in peanuts is to understand the way every herbicide used in that field kills a weed, regardless of the crop planted. If ALS-inhibitors are used in peanuts, these materials should not be used, or at best used on a limited basis, on subsequent crops in that field.
In peanuts, using Valor has proven to be a good alternative to ALS-inhibitors. If Valor is used in peanuts, growers should remember that it is in the same family of chemistry as Cobra, Blazer and Reflex. Also, Valor is moisture activated, so for non-irrigated peanuts applying this herbicide under drought conditions would be a waste of money. Valor also is difficult to wash out of a sprayer and it can cause damage to other crops, particularly cotton.
In cotton fields where resistance to glyphosate is expected, a good alternative has been to use Reflex in the herbicide mix. If peanuts are grown in subsequent years in the same field, Reflex is the same chemistry as Valor, Cobra and Blazer. So, solving one problem in cotton may create a bigger one in peanuts.
Likewise, Prostko points out that all the attention given to Cadre resistance in peanuts in the Southeast is a little unfair. In most of these fields, cotton was grown for two or three years and Staple and Envoke, both ALS-inhibitors, were widely used to control weeds.
Rotating chemical families is critical to avoiding or reducing resistance problems, but using more chemicals requires more planning and more money. At $3 a gallon for diesel fuel, every additional pass through the field costs significantly more money.
Using older materials, like 2,4-DB and paraquat may be valuable in breaking up a herbicide monoculture. In tests in Georgia, Prostko notes that adding 2,4-DB, plus one percent agri-oil tank-mixed with Cobra improved control of 3-4-inch tall pigweed.
“When pigweed gets much taller than four or five inches, I don't know of anything that will kill it,” Prostko says. Using a pre-emergence material in peanuts, then coming back at cracking with a different family of herbicides, then spraying with Cobra or Ultra Blazer when the weeds are 2-3 inches tall is a workable program to deal with ALS resistance, according to Prostko. However, he warns that when pigweed gets 4-5 inches tall neither Ultra Blazer or Cobra will control it.
For Southeastern peanut growers, adding another expensive input is exactly what they didn't need. Managing herbicide resistance won't be easy or inexpensive, but it is workable, if growers approach it wisely.