No. 2 Article 7/April 7, 2006

It's Really a Matter of Time

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 let us complete various projects more efficiently and more quickly. The media is replete with advertisements for the newest, the fastest, the most innovative inventions to come along in years that promise greater efficiency.

Time-saving technologies have been no stranger in agricultural circles. We watch with amazement as fields are planted with 24-row planters and harvested with behemoth combines. In between these events, numerous products are available to free us from many laborious tasks associated with raising agronomic crops. For example, herbicides are commonly used to provide effective, broad-spectrum control of weeds once controlled almost exclusively by preplant and in-crop tillage. As many 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 herbicides are used in agronomic systems also has changed over time. Nowadays there don't seem to be as many soybean fields in which trifluralin is two-pass incorporated or Basagran is applied postemergence to control common cocklebur that escaped through a soil-applied treatment. The two-pass program of Prowl down followed by Pursuit in soybean has been largely replaced by the single postemergence shot of glyphosate. In some respects, 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 many desire to make only a single pass across the field.

Few would dispute that glyphosate-resistant soybean varieties have significantly changed many facets of weed control. No longer is the use of soil-applied herbicides considered routine, nor are timely applications of postemergence herbicides consistently made before weed interference results in soybean yield loss. In-crop cultivation following an initial postemergence herbicide application has lost favor to narrow crop rows and easy resprays. Weeds such as waterhemp and common lambsquarters, species considered a "good fit" for glyphosate in the late 1990s, are nowadays frustrating farmers and custom applicators on an almost annual basis. Retail margins for many soybean herbicides are "narrow," and screening for new soybean herbicide active ingredients by the principal herbicide manufacturers is approaching a standstill. Weed resistance to glyphosate, once predicted by some manufacturers to be a "nonissue," has occurred in the United States and around the world (more about this topic next week).

While the "recent" history of soybean weed control can be recited by most all weed control practitioners, the story of corn weed control is currently being "revised." The adoption of glyphosate-resistant corn hybrids has been increasing in Illinois over the past several years and is poised to make significant increases during the coming seasons. Will this technology be embraced similarly to glyphosate-resistant soybean varieties? People are quick to offer speculations, but time will provide the most accurate answer. Perhaps a more pressing question is how farmers will use this technology. Again, time will provide the most accurate answer, so much of the remainder of this article 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 to remember between soybean and corn weed control is this: Weed interference begins to adversely affect yield sooner in corn than in soybean. As a general guideline, previous research suggests that if weeds are removed from soybean within 3 to 5 weeks after emergence, soybean yield is unlikely to be adversely affected. That interval shortens in corn, perhaps to as soon as 2 to 4 weeks after emergence. These intervals should be considered guidelines, as many factors can influence exactly when weed interference begins to reduce yield potential. For example, a dense stand of giant ragweed will reduce corn yield potential sooner than a dense stand of prickly sida, weed interference in dry soil conditions often reduces yield potential sooner compared with more optimal soil moisture conditions, and so on. In other words, since we cannot always predict if weed interference will begin to reduce corn yield potential on day 14 or day 28, it might be to the farmer's advantage to control weeds sooner rather than later.

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 today? Without much doubt, some will attempt to manage weeds in glyphosate-resistant corn in a manner similar to 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 it also carries 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 left uncontrolled. Thus, total post-emergence 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.

Most weed scientists would agree that some type of sequential weed control program in corn will provide more consistency across years and locations than a single-application program. However, it's reasonable to assume less general agreement among weed scientists on what constitutes the best scenario for a sequential weed control program. Some argue that a sequential program of multiple applications of postemergence herbicides is equivalent to a sequential program of a soil-applied product followed by a postemergence herbicide. In reality, the "best" program may depend on the individual farmer, field, and intended purpose. Sequential programs that spread the risk of having weather conditions adversely impacting weed control should be considered. A soil-applied product followed with a post-emergence herbicide (either planned or applied as needed) provides several important advantages, including spreading risk. Additionally, using herbicides with different sites of action will help slow the selection for herbicide-resistant weeds.

Recent introductions in the corn herbicide arena include premixes or recommended tank mixes that "guarantee" season-long weed control following a single application to the soil. While many of these programs provide excellent 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 during the 2005 growing season 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.

Many who are contemplating growing a glyphosate-resistant corn hybrid have asked what rate of a soil-residual herbicide should be used. It seems that 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 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% when followed by a postemergence application of glyphosate. It may be best to consider this question from at least two perspectives: What rate should be applied based on agronomic considerations, and what rate should be applied to reduce the selection for glyphosate-resistant weeds?

In summary, Illinois farmers are poised to plant a significantly higher percentage of their corn acres in 2006 with glyphosate-resistant corn hybrids compared with previous years. While this technology offers some enticing advantages, it's unlikely to solve all perceptible weed management problems. Keep in mind that, although this technology represents a new tool for weed control, the principles of weed control in glyphosate-resistant corn hybrids are the same as in conventional corn hybrids.--Aaron Hager

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