Issue No. 2, Article 7/April 6, 2007
Weed Management Considerations in Continuous Corn
Numerous indications point to an increase in the acres of corn planted in Illinois during the 2007 cropping season. Some have asked if weed management practices will need to be altered for continuous (second-year) corn as compared with a corn-soybean cropping rotation. Only minor adjustments are likely to be needed; here are a few points to consider:
- Control all existing weed vegetation before planting. Preplant tillage (in some form) is practiced on the majority of Illinois corn acres, but reduced tillage or no-till practices have gained popularity in recent years. Whether you plan to use tillage or herbicides before corn planting, be sure to plant corn into weed-free conditions. A practice that has become common in soybean production is no-till planting into existing weed vegetation that is not sprayed until some time following soybean emergence. Glyphosate is then applied to control existing winter annual weeds and early-emerging summer annual weed species. This practice is very unadvisable in corn for several reasons, perhaps the foremost being that corn is not as competitive with early-season weed interference
as is soybean. In other words, early-season weed interference can reduce yield potential sooner in corn than soybean. Also, fields with weedy vegetation may be more attractive oviposition sites for insects (such as black cutworm) compared with fields devoid of weeds.
- Be cautious about adopting total postemergence weed control programs. It is altogether possible that glyphosate-resistant corn hybrids will be planted on the majority of Illinois corn acres in 2007. However, while some weed control tools can be used only in conjunction with glyphosate-resistant corn hybrids, the principles of weed interference and management are identical in conventional and herbicide-resistant hybrids. In other words, weed interference will reduce corn yield potential whether the hybrid is herbicide-resistant or not. The key to preserving the inherent crop yield potential is to not allow weeds and crop to compete too long. This can be accomplished with timely postemergence herbicide applications, but the specific date after which crop yield is reduced due to weed interference cannot be predicted with great precision. Thus, including multiple weed management tactics (including soil-residual herbicides, cultivation, etc.) introduces a higher probability of preserving crop yield potential than does relying exclusively on any single weed management tactic.
- Existing crop residue and soil-residual herbicides. When including soil-residual herbicides in continuous corn systems, keep in mind that existing surface crop residue may (at least initially) reduce the amount of herbicide available for weed control. This residue may physically prevent some of the applied herbicide from reaching the soil surface, potentially allowing weeds to emerge before the herbicide is moved off the corn residue. Precipitation and mechanical incorporation are two methods that can move the applied herbicide into the soil solution; precipitation can occur either before or after planting, whereas mechanical incorporation is usually best accomplished before planting. Aggressive row cleaning attachments may displace enough herbicide-treated crop residue or soil to allow weeds to emerge within the crop row. A potential solution would be to apply the soil-residual herbicide after planting instead of before.
- Control of volunteer corn in field corn. It's unlikely that the weed spectrum will dramatically change or shift during only the second year of corn production on a given parcel of land. However, one unique "weed" that may be present in second-year (or continuous) corn is volunteer corn. In conventional corn, no selective corn herbicide will selectively control volunteer corn. If second-year corn is planted, control of volunteer corn in the 2007 corn crop can be enhanced with the use of herbicide-resistant/tolerant hybrids that allow the application of herbicides that normally control corn. Table 3 lists some options for volunteer corn control in the 2007 corn crop