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Sublethal Herbicide Effects on Weeds
by Bob Hartzler

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June 1, 2000 -  You would never guess it when looking at herbicide advertisements, but 100% control is rarely achieved with a single herbicide application.  A decision many managers must make is whether the additional expense of a second trip is warranted to clean up weeds that survive the initial herbicide application.  Although few persons base decisions on whether a specific weed density needs to be controlled on 'hard' economic thresholds, most make decisions using some type of subjective threshold rather than striving for complete control.   These subjective thresholds are often based on what the field will look like at harvest rather than an actual projection of yield losses caused by the surviving weeds.   A factor that may frequently be overlooked in making these decisions is the relative competitiveness of the weeds that survive a herbicide compared to weeds not exposed to the herbicide.  Failing to consider the herbicide's impact on surviving weeds may lead to an overestimation of the severity of the weed problem.  This article will summarize studies investigating the competitiveness of velvetleaf plants that have survived a herbicide application.

Researchers at the Michigan State University studied the effect of preemergence atrazine applications on velvetleaf (Schmenk, R. and J. J. Kells.   1998.  Effect of soil-applied atrazine and pendimethalin on velvetleaf competitiveness in corn.  Weed Technol. 12:47-52).  Velvetleaf were planted 3" to the side of a corn row and atrazine  was applied at either 0.5 or 1.0 lb per acre.  The soil was a loam with 3% organic matter and a pH of 7 in 1992 and 2.3% organic matter and pH of 6.5 in 1993.    In the 0.5 lb treatment velvetleaf were thinned to three plants per foot of row whereas at the 1.0 lb atrazine rate the velvetleaf were thinned to two plants per foot of row.

Velvetleaf height and seed production was reduced by both rates of atrazine in both years (Table 1).  The 1.0 lb atrazine rate reduced velvetleaf height by 80% in 1992 and 64% in 1993 whereas 0.5 lb caused reductions in growth less than 40%.  Reductions in velvetleaf seed production due to atrazine were similar to or greater than the height reductions.

Table 1.  Growth of velvetleaf surviving preemergence atrazine applications.

Velvetleaf Growth Parameter 0.5 lb atrazine 1.0 lb atrazine
  1992 1993 1992 1993
Height  (% reduction) 39 13 80 64
Seed production (% reduction) 35 49 84 72

Source:  Schmenk, R. and J. J. Kells.  1998.   Weed Technol. 12:47-52.

Atrazine treatments also reduced the impact of velvetleaf on corn yields (Figure 1).  At the high atrazine rate corn yields were not affected by velvetleaf at a density of 2 plants per foot of corn row.  Untreated velvetleaf reduced yields approximately 25% at this density.  Velvetleaf ( 3 plants per foot of row ) treated with 0.5 lb atrazine reduced corn yields in both years, and the herbicide's effect on competiveness varied between the two years.  In 1992 untreated velvetleaf caused more than seven times the yield loss as weeds surviving atrazine, whereas in 1993 atrazine reduced the competitiveness of velvetleaf by less than 40%.

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Source:  Schmenk, R. and J. J. Kells.  1998.   Weed Technol. 12:47-52.

 

We have been conducting similar studies in Roundup Ready soybeans looking at the competitiveness of velvetleaf that survives postemergence Roundup Ultra applications.  Velvetleaf that emerged within one week of soybean planting were tagged and their growth monitored throughout the growing season.  Roundup Ultra was applied at 1 pt and 2 pt per acre at three sizes of velvetleaf (4", 6" and 10").  Experiments were conducted at three locations during 1997 and 1998.   For simplicity, data presented are averaged over the three locations and the three application dates (Table 2).

Table 2.  Effect of Roundup Ultra applications on velvetleaf growth in Roundup Ready soybeans.

Roundup Ultra Rate Velvetleaf survival
(%)
Dry wt
(% reduction)
Seed production
(% reduction)
0 95 0 0
1 pt 67 86 82
1 qt 27 93 90

Source:  Hartzler and Battles, ISU.   Unpublished data.

Greater than 50% of velvetleaf plants survived the 1 pt Roundup rate, whereas only 27% of treated plants survived 1 qt of Roundup Ultra.   Although significant numbers of velvetleaf survived the herbicide treatment, biomass accumulation and seed production of surviving plants was reduced by more than 80% at both herbicide rates.  The experimental design did not allow us to measure the impact of surviving plants on soybean yields.  However, the reductions in velvetleaf growth at either Roundup rate were equal to or greater than the reductions caused by 1.0 lb of atrazine in the Michigan State studies.  In the corn studies velvetleaf with growth reductions of at least 80% did not reduce corn yields, and I feel that the velvetleaf plants that survived the Roundup would not have affected soybean yields.

Both studies found that sublethal herbicide rates greatly impaired the growth, and therefore, competitiveness of velvetleaf.  Similar results have been reported for other species and herbicides.  The reduced vigor of surviving plants should be taken into account when determining whether a secondary control strategy is required.  Weeds surviving herbicides are unlikely to affect crop yields unless at such high densities that the need for retreatment is obvious.  While velvetleaf surviving 0.5 lb of atrazine did reduce corn yields in Michigan, keep in mind that 0.5 lb atrazine would not be expected to provide effective control of velvetleaf even under the best of conditions.  Even though weeds surviving herbicides may not reduce crop yields, in most situations these plants will produce seed.  The need for additional control strategies should be based on the impact of seed production on future weed populations, taking into account the reduced seed production by these plants.

 

Acknowledgement:  The author appreciates the suggestion of Brad Van Kooten (Pioneer Hi-Bred agronomist) for an article on this topic.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
http://www.weeds.iastate.edu
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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.