Department of Crop Sciences University of Illinois at Urbana-Champaign

Agronomy Day 2006

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Weed Watching 101

Aaron Hager Aaron Hager
Assistant Professor of Weed Science
Department of Crop Sciences

University professors who have considerable experience in the classroom will often relate how their classes and students have changed over time. Hair and clothing styles often can be benchmarks for distinctive eras of our past, such as when bell-bottom pants were the rage or when flip-flops became as common as tennis shoes. Students with cell phones seem to be in the majority, while very few students stroll across campus these days with a slide-rule protruding out of their shirt pocket. Lectures using blackboards and chalk largely have been replaced by lectures using high-tech audiovisual equipment.

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Dense populations of butterweed are now common in many fields during early spring months.

Akin to these sociological observations, university professors who have considerable experience in the field will often relate how agronomic production practices have changed over time. We watch with amazement as fields are planted in the spring with 24-row planters and harvested in the fall with behemoth-sized combines. In between these events, many are quietly thankful for products that 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 Dense populations of butterweed controlled exclusively by preplant and in-crop tillage operations. As many can are now common in many fields attest, these tillage operations often were supplemented with miles of walking and hours of close “interaction” with a manually-operated hoe or weed hook.

The spectrum of weeds growing in Illinois farm fields has changed in multiple ways over time. Weed species not known to occur in agronomic production fields have become more prevalent and problematic; repeated applications of specific herbicides have selected for weed biotypes resistant to that particular herbicide; the biology or growth characteristics of one or more weed species has changed in response to changes in agronomic practices. Collectively, these changes have contributed to weed problems that continue to challenge Illinois farmers.

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Biennial weed species such as poison hemlock are becoming increasingly common in reduced-tillage corn and soybean fields.

Nowadays, questions about how to control common cocklebur in soybean are rare, whereas questions about what to apply in the fall for control of winter annual weed species are common. Waterhemp has gone from virtual obscurity as a weed problem prior to the 1990s to the weed species considered by the majority of Illinois farmers as most problematic in corn and soybean. Glyphosate has become the most widely used postemergence herbicide in soybean, yet its consistency in controlling certain annual weed species (such as common lambsquarters) does not appear to be as great today as it was immediately following the commercialization of glyphosate-resistant soybean varieties. Biennial weed species (such as poison hemlock) are finding a home in fields where reduced tillage row crop production is practiced. Giant ragweed continues to emerge early in the season, but many populations demonstrate multiple emergence events lasting into mid-June. Weed size when postemergence soybean herbicides are applied has increased from the traditional 2-to-4 inches to contemporary 8-to-12 inches, suggesting weed control has become more predominant than weed management. Expect additional changes to occur in the weed spectrum if cost control takes precedence over weed control.