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Issue No. 11, Article 6/June 4, 2004

Musings About Postemergence Herbicide Programs

Weed management practices have changed greatly since the time when hand removal of weeds was the most common method used in crop husbandry. Illinois producers, as well as producers in most other midwestern states, have come to depend on herbicides as their primary weed management tool. Some have suggested the "chemical age" began with the discovery of the herbicidal properties of 2,4-D in the 1940s, gained momentum when products such as atrazine, alachlor, and metolachlor became dominant players, took a big leap forward with the adoption of postemergence applications that allowed herbicide selection to be based on the weed spectrum present in a particular field, and was "revolutionized" with the commercialization of glyphosate-resistant soybean varieties.

As each new technology found its fit in the marketplace, we learned more and more about how the chemicals work, how they persist or why they don't, what factors are needed to achieve good weed control, and how weeds can eventually adapt to repetitive use of the same management program. We've made great strides forward in our understanding of how weeds and crops interact and how the biology of a given weed species may determine the best way to manage that species; and we've come to recognize that the weed spectrum is not necessarily static but rather can be quite dynamic.

Many of these discoveries occurred during a time when herbicide manufacturers were actively developing new active ingredients to introduce into the marketplace. Times have changed, and we now find ourselves in an era when the introduction of novel herbicide active ingredients is not occurring with any great frequency. Currently, a diversity of weed management practices are still used for corn production, but many fewer practices are used for soybean production. It's difficult to remember a time previous when so many soybean acres were treated with the same active ingredient.

As long as the technology remains acceptable to producers, it seems logical to assume it will continue to be used on a large scale. We generally think we know best how to utilize this technology, but are we using it as we should? What have we learned since the "revolution" of glyphosate-resistant soybean, and perhaps as important, are we overlooking some important weed management principles? Arguably, glyphosate has been a tremendous benefit to many soybean producers, and its effective longevity may be influenced by how it's used today. With this in mind, let's review some of the principles governing the effectiveness of postemergence herbicides.

How long should I wait to spray?

Most producers are aware that weed control is implemented so that the weeds do not deprive the crop of resources needed for maximum productivity. Removal of weeds may occur before the weeds emerge, after the weeds emerge, or by a combination of these. The governing principle of postemergence herbicide programs is that the crop and weeds can coexist for some critical period of time without causing a detrimental loss in crop yield. Numerous research trials conducted over many years have demonstrated that if weeds are removed before this critical period, crop yield is generally not adversely affected. Obviously, the real "trick" is determining when this critical period occurs, especially when considering that the critical period may change somewhat each year based on the crop grown, weed species present, and environmental conditions. As a general guideline, previous research suggests that if weeds are removed from the crop within 3 to 5 weeks after emergence, crop yield is unlikely to be adversely affected.

Apart from preserving crop yield, another advantage of removing weeds within 3 to 5 weeks after emergence is that small weeds are usually much easier to control than large weeds. Several factors "govern" this principle, but needless to say, a 4-inch-tall waterhemp plant is much easier to control than a 14-inch-tall waterhemp plant. So, yes, size does matter. During the early adoption of postemergence applications, targeting small weeds was the norm because most postemergence broadleaf herbicides were contact herbicides and did not translocate within the plant. Thorough spray coverage of the target weeds was necessary for effective control, and spraying small weeds with 15 to 20 gallons of carrier usually resulted in good coverage and control. Many times, a postemergence soybean herbicide was used following a soil-applied herbicide, which often meant target weeds were relatively consistent in size.

What happens when the postemergence application is delayed?

Glyphosate is an exceptionally effective herbicide for control of a broad spectrum of grass and broadleaf weed species. Rates can easily be adjusted to match weed spectrum and size, additive selection is relatively simple, and tank-mixing issues are not as complicated as with other herbicides. Glyphosate does not provide any soil-residual control of weeds that emerge after application. That characteristic can be either an advantage or a disadvantage, depending on the question at hand. Following the widespread adoption of glyphosate-resistant soybean, there has been a subtle trend toward delaying the initial postemergence application longer than was once common. Because glyphosate provides no residual weed control and application rates can be adjusted to match weed size, producers hope that delaying the initial postemergence application will allow enough additional weeds to emerge so that a second application will not be necessary. So, what should be expected when delaying postemergence applications to allow more weeds to emerge becomes common practice? Three disadvantages of delaying a postemergence application include weeds become larger and more difficult to control, crop yield loss is more likely to occur, and other critters may take up residence inside the large weeds.

The first disadvantage (that is, large weeds are more difficult to control than small weeds) has been understood for many years and should not be too surprising to anyone. Ample evidence exits that demonstrates that the longer weeds compete with the crop the greater the crop yield will be depressed. The weed spectrum in many Illinois fields has become populated with species that tend to have a prolonged emergence period during the growing season; thus it is becoming increasingly more difficult to achieve success with a one-pass weed control program. If you are waiting for the last flush of waterhemp to emerge before you spray, you may be waiting until the middle of July! While the application is delayed to allow more weeds to emerge, crop yield loss may be occurring due to interference from the existing (and growing) weeds.

Finally, larger weed stems become attractive to various insect species. Insect feeding/tunneling within the stem tissue of several weed species was very noticeable in some areas of Illinois during 2003 and may have contributed to poor weed control following the application of a postemergence herbicide. Weed species that "harbored" these insects included waterhemp, giant ragweed, horseweed/marestail, annual smartweed species, common ragweed, and common lambsquarters. To date, researchers have identified insects in the orders Lepidoptera (Papaipema nebris, Ostrinia nubilalis, Epiblema spp.) and Coleoptera (Rhodibaenus tredecimpunctatus, Lixus spp., Dectus spp., Hippopsis lemniscata) present in these weed species as either larva or adults. Research is currently under way to examine many important questions related to this phenomenon, such as how extensively herbicide translocation is reduced by insect tunneling and at what stage of plant growth do these insect species begin their association with the weeds.

Is insect tunneling in weed stems only a recent (within the last 3 years) phenomenon? The answer depends to a great extent on how you define "recent." Some (including myself) speculate a weed/insect relationship has existed for as long as weeds and insects have coexisted. Aside from mere speculation, researchers at the University of Illinois have previously investigated the relationship between insects and weeds. One of the more interesting results reported was that most stem-boring insects preferred large-diameter stems more than small-diameter stems. In other words, it was difficult to find an insect tunneling within the stem of a 4-inch-tall giant ragweed but much easier to find one tunneling in the stem of a 12-inch-tall giant ragweed. In case you are curious, this work was published in July 1976.

Herbicide-Resistant Weed Biotypes

Beginning during the latter half of the 1980s, herbicide resistance was becoming more common in many states and around the world. However, by 1993, researchers in Illinois had documented herbicide resistance in only three weed species biotypes, and resistance was to only one herbicide family. A decade later, the list of herbicide-resistant weed biotypes in Illinois had grown to include 10 species, and resistance had spread to include four herbicide families (Table 2). The most recent addition to the list of herbicide-resistant biotypes is a waterhemp population with resistance to three herbicide families (triazine, ALS inhibitors, and PPO inhibitors). Researchers at the University of Illinois are continuing to examine this biotype to determine the mechanisms responsible for its herbicide-resistance characteristics.

The majority of herbicide-resistant weed biotypes in Illinois are broadleaf species, but concern currently exists that there may be an increase in grass species (giant foxtail, in particular) demonstrating resistance to ALS-inhibiting herbicides. This type of resistance has been documented previously in Illinois (Table 2); however, recently we have received several requests to screen giant foxtail and shattercane biotypes for resistance to ALS inhibitors. The increasing occurrence of herbicide-resistant weed biotypes in Illinois continues to reduce the effectiveness of many herbicide options.

Late-Season Observations from 2003

During late September and early October 2003, we traveled across a large area of Illinois collecting samples of waterhemp from soybean fields. These travels also provided an opportunity to survey the weed species present at the end of the growing season. To no surprise, waterhemp was the predominant weed species that appeared in fields of mature soybean. The next most common weed species tended to be region specific. Giant ragweed was very common in northern and northwestern Illinois, while common lambsquarters was frequently noted in other areas. One observation that was somewhat surprising was the number of fields with mature horseweed/marestail plants. From mere anecdotal observations, we could not determine whether these plants had simply survived an early-season burndown herbicide, emerged later during the spring, or survived a postemergence herbicide. Regardless of the reason, we should remain watchful toward this weed species, as glyphosate-resistant biotypes of horseweed/marestail have been discovered in the border states of Indiana and Kentucky.


The weed spectrum will eventually change in response to the perpetual use of glyphosate. The magnitude of these changes is unknown, but change appears to be inevitable. Producers will be forced to address these changes when they occur, but before that time we should remember that the principles governing the effectiveness of postemergence herbicides are not likely to change.Aaron Hager

Aaron Hager

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