It's Really a Matter of Time

Aaron Hager

Aaron Hager

Assistant Professor and Extension Specialist in Weed Science Department of Crop Sciences

Phone: (217) 333-4424


These days it seems as though no one ever has enough time to accomplish the myriad tasks at hand. We continue to search for and adopt new devices or technologies that allow us to complete various projects more efficiently and in a shorter amount of time. The media are replete with advertisements for the newest, the fastest, the most innovative inventions to come along in years, inventions that promise greater efficiency. Advisors realize how important time has become as a commodity, and they know with certainty that consumers are willing to purchase items that result in our having more time to spend doing the activities we enjoy most.

Time-saving technologies have been no stranger in agricultural circles. We watch with amazement as fields are planted in the spring with 24-row planters and harvested in the fall with behemoth-sized combines. In between these events, many are quietly thankful for products that free us from many laborious tasks associated with growing agronomic crops. For example, herbicides commonly are used to provide effective, broadspectrum control of weeds once controlled exclusively by preplant and in-crop tillage operations. As many people can attest, these tillage operations were often supplemented with miles of walking and hours of close contact with a hoe or weed hook.

The way in which herbicides are used in agronomic systems also has changed with the times. Nowadays, there don't seem to be as many fields in which trifluralin is twopass incorporated or Basagran is applied postemergence to control the common cocklebur that escaped through the Lasso plus Sencor soil-applied treatment. People with only a few years of experience wonder how we ever managed to control grass weeds postemergence in corn before Accent entered the market. The two-pass program of Prowl followed by Pursuit in soybean has been largely replaced by the single postemergence shot of glyphosate. We've tended to move away from the age-old practice of managing weeds before they interfered long enough to cause crop yield loss to the contemporary practice of controlling big weeds because we desire to make only a single pass across the field, and because tools now are available to control big weeds.

Single-pass weed control programs have been available for corn and soybean since the advent of herbicide premixes and tankmixes. Illinois farmers have tended to use single-pass soil-applied programs more frequently in corn, whereas single-pass postemergence programs have been more common in soybean. Basagran was one of the first postemergence herbicides widely adopted for use in soybean. It effectively controlled a wide spectrum of broadleaf weed species, but missed several important species such as pigweed, nightshade, and grasses. Tank-mixing Blazer and/or Poast with Basagran helped fill in many of the holes in its weed-control spectrum. Pursuit took the postemergence soybean herbicide market "to the next level," with a weed control spectrum as wide as many two- or three-way tank-mixes. Applying Prowl before Pursuit provided helpful control of common lambsquarters and annual grasses, and this two-pass system was used widely until waterhemp began to encroach on more and more Illinois soybean acres. Without doubt, waterhemp was a "driver weed" that helped usher in the widespread and rapid adoption of glyphosate-resistant soybean varieties.

Few would argue that glyphosate-resistant soybean varieties have significantly changed many facets of weed control. The use of soil-applied herbicides is no longer considered routine. Also, timely applications of postemergence herbicides are not consistently made before weed interference results in soybean yield loss. In-crop cultivation after an initial postemergence herbicide application has lost favor to narrow crop rows and easy resprays. Weeds such as waterhemp and common lambsquarters, species that were widely considered an "easy kill" with glyphosate in the late 1990s, now frustrate farmers and custom applicators almost annually. Retail margins for soybean herbicides such as glyphosate are more than tight, and screening for new soybean herbicide active ingredients by the principle herbicide manufacturers is approaching a standstill. Weed resistance to glyphosate, once predicted by some in the manufacturing industry to be a "non-issue," has occurred in the United States and around the world (Table 1).

Although the recent history of soybean weed control can be recited by almost all weed control practitioners, the story of corn weed control currently is being revised. The adoption of glyphosate-resistant corn hybrids has been increasing in Illinois over the past several years, and probably will increase significantly during the coming seasons. Will this technology be embraced similarly to the use of glyphoste-resistant soybean varieties? People are quick to offer their speculation to this question, but time will provide the most accurate answer. Perhaps a more pressing question is, "How will farmers use this technology?" Again, time will provide the most accurate answer, so much of the remainder of this paper will provide considerations related to the timing of weed control practices in corn.

Regardless of the hybrid planted or the herbicide program used, perhaps the most critical difference in weed control between soybean and corn is weed interference begins to adversely affect yield sooner in corn than in soybean. As a general guideline, pervious research suggests that if weeds are removed from soybean within 3 to 5 weeks after emergence, it is unlikely that soybean yield will be adversely affected. The interval shortens to 2 to 4 weeks for corn. Without question, corn is more sensitive than soybean to early season interference from grass weeds. This fact has been at least partially responsible for the long-practiced application of soilresidual herbicides before corn and weeds emerged. Some of the more expensive herbicides (in particular those soilresidual products applied for annual grass control) are still applied on most of the corn acres in Illinois to allow the crop to emerge without excessive competition from grass weeds.

One may speculate whether the adoption of glyphosate-resistant corn hybrids will transform corn weed control programs into something resembling contemporary soybean weed control programs. In other words, will total postemergence weed control programs in corn become as common as they are in soybean (Figure 1)? Without much doubt, there will be some who attempt to manage weeds in glyphosateresistant corn in a manner similar to way they manage weeds in glyphosate-resistant soybean, that is by using a single postemergence application of glyphosate. Under the "right" conditions, it is altogether likely that excellent weed control and corn yields can be achieved with this type of program. But this type of program also includes the greatest risk for significant yield loss if weeds are allowed to compete too long, or if significant weed populations emerge after the initial glyphosate application and are not controlled. Most weed scientists agree that some type of sequential weed control program in corn provides a higher level of consistency of weed control across years and locations, compared with single-application programs. However, it is reasonable to assume that there is less agreement among weed scientists regarding what constitutes the best scenario for a sequential weed control program. Some argue that a sequential program consisting of multiple applications of postemergence herbicides is equivalent to a sequential program of a soilapplied product followed by a postemergence herbicide. In reality, the "best" program depends upon the individual farmer, field, and intended purpose.

Recent introductions in the corn herbicide market include premixes or recommended tank-mixes that "guarantee" season-long weed control with a single soil application. Although many of these programs provide excellent weed control in most instances, soil-applied herbicides all have the same "Achilles heel"—when applied to the soil surface, these products require either mechanical incorporation or precipitation to move them into the soil solution. Dry soil conditions and lack of timely precipitation in many areas of Illinois in 2005 highlighted this critical limitation. Also, it is becoming increasingly difficult to achieve a satisfactory level of weed control with a single soil-applied product when the Illinois weed spectrum includes species, such as waterhemp and giant ragweed, with prolonged periods of emergence. Total postemergence weed control programs are viable options, but close and careful attention must be given to the timing of the initial application, as well as to accurately assessing the need for subsequent applications.

Sequential weed control programs that spread the risk of weather conditions adversely affecting weed control should be considered. A soil-applied product followed with a postemergence herbicide (either planned or applied as needed) provides several important advantages, including spreading the risk. Additionally, using herbicides with different sites of action will help slow the selection for herbicide-resistant weeds.

Many people who are contemplating growing a glyphosate-resistant corn hybrid have asked what rate of a soil-residual herbicide should be used. Recommendations vary depending on how the conversation is couched. Some (including a few herbicide manufacturers) suggest that rates of soil-residual herbicides should not be reduced so as to reduce the likelihood of selection for a glyphosate-resistant weed. Others (again including a few manufacturers) suggest that rates of soil-residual herbicides can be reduced by approximately 30 to 50 percent when such applications are followed by a postemergence application of glyphosate. It may be best to consider this question from at least two perspectives: 1) What rate should be applied based on agronomic considerations, and 2) What rate should be applied to reduce the selection for glyphosate-resistant weeds?

Glyphosate-resistant Weeds

Since the commercialization of glyphosate-resistant crops, the question of whether glyphosate-resistant weeds will or will not be selected has been extensively discussed by individuals involved in virtually all phases of production agriculture. Those in academia have (generally) agreed on at least two points: 1) The potential for selecting weed biotypes resistant to glyphosate is less than the potential for selecting weed biotypes resistant to other herbicide families, and 2) Never say it will never happen. Whichever position you might have taken on this question during the early years of glyphosate use incrop, the facts are: Glyphoste-resistant weed populations have been selected; these resistant populations represent more than one weed species; weed scientists from several states, from the Midwest to the Atlantic coast, have reported instances of glyphosate-resistant weeds (Figure 2); and we have no evidence to suggest that Illinois will be "immune" to this phenomenon.

Researchers at the University of Missouri recently reported two populations of waterhemp that have consistently survived after glyphosate applications under field and greenhouse conditions. News releases from Missouri reported that some of these waterhemp plants survived as much as 6 lb acid equivalent glyphosate, a rate approximately equivalent to 170 fluid ounces of Roundup Original Max. If glyphosate-resistant waterhemp can occur in other states, it seems likely that it can occur in Illinois.

We continue to stress several points related to glyphosate-resistant weeds and glyphosate stewardship:

  1. A selection pressure for herbicide-resistant weeds occurs each time the same herbicide is applied to a particular field.
  2. Increased adoption of glyphosate-resistant corn hybrids, with a concomitant use of glyphosate to the exclusion of other weed management tools, will speed the selection of glyphosate-resistant weeds.
  3. Rotating herbicides (sites of action) or tank-mixing herbicides will help slow the selection of glyphosateresistant weeds, but it is unlikely to prevent their selection. Keep in mind that it is nearly impossible to make blanket statements about how effective a particular alternative herbicide or tank-mix partner will be in slowing the selection of glyphosateresistant weeds.
  4. Stewardship of glyphosate herbicide is an easy concept to discuss, but more difficult to implement. Manufacturers often have differing messages about stewardship, so it may be wise to ask why a particular manufacturer seems to be concerned with stewardship of glyphosate.

In summary, the preponderance of evidence suggests that it is only a matter of time until glyphosate-resistant weeds begin to occupy places in the Illinois agronomic landscape. Farmers and retailers alike will likely experience adverse financial consequences as a result of continued heavy reliance on a single herbicide active ingredient.

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