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Agronomy Day 2010

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Tour A

Contemplations Concerning a Continuing Conundrum

Aaron G. Hager
Aaron G. Hager,
Associate Professor,
Weed Science,
Department of Crop Sciences
Dean Riechers
Dean Riechers,
Associate Professor,
Weed Physiology,
Department of Crop Sciences

Waterhemp is a broadleaf weed species common to many areas of Illinois and  much of the Midwest.  Although indigenous to Illinois, waterhemp was not considered much of a problem weed species in agronomic crops until it began to spread across the state beginning sometime during the late 1980s or early 1990s.  Today, waterhemp populations continue to infest additional acres of Illinois farmland, aided by several adaptations that allow the species to thrive in contemporary agronomic crop production systems. 

Controlling waterhemp with postemergence herbicides in corn and soybean has become common practice.  Applications made after crops and weeds have emerged allow for species identification and an assessment of waterhemp density so herbicide selection, application rate and spray volume can be tailored for optimal control.  Postemergence herbicide applications minimize the interactions of the herbicide with factors associated with soil (such as soil texture and organic matter content), but often magnify interactions between the herbicide and prevailing environmental conditions or even with other herbicides.

In order to achieve waterhemp control with postemergence herbicides, the herbicide must come in contact with the target, be retained on the leaf surface prior to absorption into the plant, be able to reach the site of action within the plant, and induce a phytotoxic response that (hopefully) leads to lethality.  If for any reason one or more of these steps is restricted or limited, the level of weed control can be expected to decline and the plants may survive.  Surviving waterhemp can produce up to a million seeds per plant, which contribute to the soil seed bank for future years.

Plant age and size, relative humidity, soil moisture, and air temperature are other factors that influence the activity of postemergence herbicides.  Younger, actively growing plants are easier to control than larger plants that have experienced various types of stresses.  Many postemergence herbicide labels recommend applications be made when target weeds are small and caution of reduced effectiveness if applications are made to larger plants.  Labels of postemergence herbicides may also suggest users delay applications if weeds are under “adverse environmental conditions”.  Examples of such adverse environmental conditions may include prolonged periods without significant rainfall (dry soil) or low air temperatures before spray applications are made. 

An unfortunate consequence of using a limited selection of herbicide active ingredients to control a particular weed species is the selection for herbicide-resistant biotypes.  Waterhemp has demonstrated a great propensity to evolve biotypes resistant to various herbicide families.  The initial publication of herbicide resistance in Illinois waterhemp occurred in 1997, in which the selection of waterhemp populations resistant to ALS-inhibiting herbicides was chronicled.  Since that initial declaration, Illinois waterhemp populations resistant to triazines, PPO inhibitors and glyphosate also have been confirmed.  While resistance to any one herbicide family can introduce management challenges, biotypes with resistance to more than one herbicide family are becomingly increasingly common and limit postemergence options for waterhemp control. 

The conundrum known as waterhemp will continue to plague Illinois farmers well into the future.  Selection for herbicide resistance continues to reduce the effectiveness of many herbicide families, and few new herbicide active ingredients from novel chemical families are expected to enter the marketplace in the foreseeable future.

Soybean by G. tomentella hybrid	plant growing in culture in the	laboratoryFigure 1. Effectiveness of postemergence herbicides can be influenced by several environmental factors and stresses.
Two soybean plants that have	different numbers of G. tomentella chromosomesFigure 2. Waterhemp biotypes resistant to one or more herbicide families will become increasingly common across much of Illinois.
Agronomy Day 2010