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Issue No. 21, Article 3/September 9, 2011

Take Time to Assess the Effectiveness of Your Weed Management Program

The pace of crop harvest soon will quicken across most areas of Illinois. The elevated view from the combine cab is an excellent vantage point for surveying and assessing the effectiveness of your weed management programs. A field free of weeds during harvest is very desirable, and it represents an outcome that will require increased management as weeds continue to adapt to modern crop production practices.

"Windshield time" around the state has found waterhemp and horseweed (aka marestail) appearing very frequently in corn and soybean fields. Recent field visits have indicated seed production to be successful on these mature plants, suggesting that a preharvest herbicide application (discussed in issue 20, August 25) may do little to reduce the viability of these mature seeds.

There are many reasons why these species have successfully completed their life cycles in corn and soybean fields. One reason is herbicide resistance. Glyphosate resistance is known to occur in Illinois waterhemp and horseweed populations, and we suspect it to become increasingly common. Resistance to five different herbicide site-of-action herbicide families has also been documented in Illinois waterhemp populations. In all instances except one, these resistance traits can be transferred by movement of both pollen and seed.

Small patches of an individual weed species may indicate the presence of a herbicide-resistant population.

Removing seed-bearing waterhemp plants before harvest can reduce movement of seed within and among fields.

Generally, herbicide-resistant populations do not completely infest a field over the course of a single growing season. Rather, they usually begin as a small number of plants (perhaps as few as one individual plant) that survive to produce seed. The next season the herbicide-resistant population may exist as a patch of weeds encompassing a small area in the field. Rogueing these plants before they produce viable pollen and seed can help slow the spread of the herbicide-resistant population within the field and reduce the movement of the resistance trait to other fields. Female waterhemp plants can produce more than one million seeds per plant, although that number is usually much smaller when the plants have grown under competitive conditions. If you notice a few surviving female waterhemp plants in a field, it might be a good investment of time to remove (cut, pull, etc.) these plants before they enter the combine. As illustrated by the photo below, combines can spread weed seeds across a field and transport seed from one field to another.

Harvesting equipment can disperse weed seeds within and between fields. The inset is a close-up of the weeds in the green rows.

Avoiding weed patches during harvest operations can reduce transport of seeds on harvesting equipment.

If you suspect herbicide-resistant waterhemp is present in a particular field, you might consider, if feasible, harvesting that field last to prevent introducing the seed from the resistant plants into the combine and their subsequent movement to other fields. Another option would be to simply avoid patches of weeds during harvest operations.--Aaron Hager

Aaron Hager

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