No. 4 Article 4/April 20, 2007

Timing Is Critical

While the following statement is a gross simplification of a complex system, agronomic cropping systems in Illinois and other Midwestern states essentially strive to grow a single plant species (usually corn, soybean, or wheat) that ultimately yields a marketable commodity for the farmer. Years of research by agronomists and plant breeders across the United States and around the world have contributed innumerable advancements and improvements that have dramatically increased crop yields. Additional research (some as old as recorded time) has identified numerous "barriers" to maximizing crop yield that, if not adequately managed or controlled, can reduce the amount of harvested commodity. Weeds are one example of these "barriers," as weeds utilize finite resources (water, nutrients, sunlight, etc.) to the detriment of the crop. Farmers have a multitude of tools available to control weeds, but we should remember that even the most effective weed control tool does not increase crop yield; rather, these tools control weeds so as to preserve the inherent yield potential.

So how and when should weeds be managed in order to reduce their ability to compete with the crop and to (ultimately) reduce yield? It would be very difficult to make an all-encompassing statement that adequately answers this question because a long list of crop, weed, and environmental factors all interact to determine when and by how much crop yield is reduced through weed interference. However, most researchers who study crop and weed interactions would probably agree that weed interference is more likely to reduce yield potential in systems that rely exclusively on a single tactic compared with multiple tactics.

Weed management programs that consist of a single postemergence herbicide application are becoming increasingly common in Illinois agronomic crops. This approach has been more historically prevalent in soybean, but total postemergence weed control is becoming more commonplace in corn. While this tactic can provide effective and near-complete weed control, a weed-free field at harvest does not necessarily indicate that crop yields were not reduced by weed interference. Weed removal must be done before weed interference begins to reduce crop yield potential; once interference has reduced potential, simply controlling the weeds will not restore the lost yield.

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 for post-only systems, previous research suggests that if weeds are removed from soybean within 3 to 5 weeks after emergence, significant soybean yield is less likely. That interval shortens to perhaps as early as 2 to 4 weeks for corn. Corn is more sensitive to early-season interference from grass weeds than is soybean. This fact has been at least partially responsible for the long-practiced use of soil-
residual herbicides applied before corn and weeds emerge. These herbicides (in particular soil-residual products applied for annual grass control) usually allow the crop to emerge without excessive competition from grass weeds.

One potential problem with "general" guidelines that describe when weed interference reduces corn yield is that they are not very precise, and they likely vary from year to year or even field to field. Previous research has reported that weed interference began to reduce corn yield as early as the 2-leaf stage to as late as the 14-leaf stage; some reports even have indicated that season-long interference had only a minimal impact on corn yield. However, it is altogether likely that weed interference does, at some point, reduce corn yield on most acres each year. Keep in mind that with the current relatively high corn prices, any corn yield loss caused by weed interference reduces revenue more this year than in most previous years. For example, when corn is $2.50 a bushel, a yield loss of 5 bushels per acre caused by weed interference will reduce a farmer's revenue by $12.50 per acre: at $4-per-bushel corn, that same yield loss of 5 bushels an acre now reduces revenue by $20 per acre.

We suggest utilizing multiple tactics to manage weeds before they reduce crop yield potential. A weed management program that uses soil-residual herbicides in conjunction with postemergence herbicides greatly reduces the potential for crop yield loss compared with one-pass postemergence programs. Farmers may be interested in doing their own on-farm comparison between two-pass and single-pass programs, and we would be interested to know your results.

How would you go about conducting such a comparison? Two suggested approaches include a split-field comparison and a within-field replicated treatment comparison. In a split-field comparison, one half of a field is treated with a soil-applied herbicide (select a product that will adequately control the weed spectrum present in a particular field and apply no less than 2/3 the rate suggested for that soil type) followed with a postemergence treatment. The other half of the field is treated postemergence only. In a within-field replicated treatment comparison, multiple strips across the field are treated with the sequential program (soil-applied followed by postemergence), while other strips are treated postemergence only. The more information collected for each field (crop stand counts, weed species and densities, percent weed control provided by each application, etc.), the better. Crop yield will often be the final measurement of treatment efficacy, but please bear in mind that yield (i.e., treatment) comparisons made across multiple years will provide a more accurate assessment than results obtained from any single season.

In collaboration with Dr. Jeff Bunting, herbicide technical specialist with Growmark, we have embarked on an effort to establish these types of comparisons across Illinois. If you are interested in establishing a similar comparison on your own farm, please get in contact with either me or Jeff and we'll provide you with more details.

Per-acre weed control expenditures in 2007 may not differ much from recent years, but the potential revenue loss caused by weed interference will be greater this year than in recent years if crop commodity prices remain high. Integrating multiple weed management tactics reduces the likelihood that weed interference will adversely impact crop yields.--Aaron Hager

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