ISSUE  10   July 12, 2007


In last week’s Crop & Pest Report approved crops to plant the year after acetochlor application were listed. Sunflower was not listed among these crops. Sunflower is approved for planting the year after acetochlor application.



The following is from a report submitted by Dave Stoltenberg, Chris Boerboom, and John Gaska, Weed Scientist, Extension Weed Scientist, and Senior Outreach Specialist, respectively, Univ. of Wisconsin. Note - With the number of common lambsquarters misses by glyphosate increasing, the same information would hold true for North Dakota.

In Wisconsin, weed resistance to glyphosate has not been confirmed, but there have been many reports of variable or inconsistent responses of common lambsquarters to glyphosate in Roundup Ready soybean fields. Re-treatment of common lambsquarters escapes with glyphosate has typically resulted in adequate control, but the cause of less than optimal control, and in some cases poor control, has yet to be fully explained. Numerous factors may affect glyphosate efficacy including weed stage of growth, environmental conditions (rainfall, temperature, dust) preceding or following glyphosate application, time of day, glyphosate formulation, additives to the glyphosate spray solution, and potentially, weed resistance.

We’ve investigated some of these factors and others are currently under study. We conducted several experiments in 2004 and 2005 to determine how common lambsquarters growth stage and glyphosate rate affect control. We found that at a given glyphosate rate, common lambsquarters control (% biomass reduction) was typically greater when applied to 3- to 4-inch tall plants than to 7- to 8-inch tall plants (Figure 1). Plus, control was typically less variable when glyphosate was applied to 3- to 4-inch tall plants than to 7- to 8-inch tall plants. Although these results showed that glyphosate rate and common lambsquarters stage of growth both affected control, other factors are likely involved, including inherent variability among common lambsquarters populations to glyphosate.

Weeds graph 1        Weeds graph 2
Figure 1.
Common lambsquarters response in the field to
glyphosate rate applied at two stages of growth in 2004-05.

Since March 2006, we’ve tested 40 populations of common lambsquarters to determine their response to glyphosate under greenhouse conditions; seeds were collected from plants in or near the edge of Roundup Ready soybean fields on farms across Wisconsin. In an experiment conducted from March to May 2006, the response of 4-inch tall common lambsquaters to glyphosate applied at 0.75 lb ae/acre varied among the 29 populations (Figure 2). Although shoot biomass of treated plants was typically between 20 to 35% of non-treated check plants for most populations, shoot biomass of treated plants from four populations was 50% or more of non-treated check plants, suggesting that these four populations were less sensitive to glyphosate relative to the response of other populations.

Weeds graph 3
Figure 2.
Response of 29 common lambsquarters populations
(4" tall plants) to glyphosate (0.75 lb ae/acre) 21 days after
treatment in the greenhouse (March-May 2006).

In Fall 2006, seeds were submitted from an additional 11 populations and tested for seedling (4-inch tall plants) response to glyphosate in a greenhouse experiment conducted from March to May 2007. The response was compared to a population from the UW Arlington Research Station (ARL). Among these 11 populations, common lambsquarters response to glyphosate also varied (Figure 3). When sprayed with the low rate of glyphosate, all common lambsquarters populations were injured and some populations appeared more sensitive than the population from Arlington after 28 days. At the high glyphosate rate of 1.5 lb ae/acre, all of the populations were injured severely or killed and plant survival was low (10% or less of treated plants). However, for two of the populations, plant survival was high (up to 75% of treated plants) and injury was only moderate, suggesting that these two populations were less sensitive to glyphosate among these 11 populations.

Weeds graph 4
Figure 3.
Response of 11 common lambsquarters populations
(4" tall plants) to glyphosate applied at 0.375 and 1.5 lb ae/acre
at 28 days after treatment in the greenhouse (March – May 2007).

So among the 40 populations that we’ve tested so far, greenhouse results suggest that six of the populations may be relatively less sensitive to glyphosate. In some instances, it appears that less sensitivity may be associated with a field history of previous glyphosate use, but in other instances, it does not; that is, some common lambsquarters populations may be inherently less sensitive to glyphosate than other populations. It should also be kept in mind that the relationship between results of greenhouse experiments and control experienced in the field may not be strong with glyphosate; that is, it may be that reduced sensitivity to glyphosate demonstrated by some of these populations in the greenhouse, may not occur in the field. In the field, the glyphosate rate used may be adequate to control lambsquarters that is less sensitive under favorable environmental conditions. However, these results should serve as a reminder of the importance of glyphosate application to common lambsquarters at earlier rather than later growth stages, and the potential value of integrating glyphosate with other tactics, including rotation with other herbicide modes of action and mechanical methods.

Richard Zollinger
NDSU Extension Weed Specialist

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