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Is Your Weed Management Program Reducing
Your Economic Return?

Bob Hartzler

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November 21, 2003 -  The flexibility Roundup Ready™ soybeans provide in herbicide application timing is undoubtedly the greatest advantage of this system compared to conventional weed control programs.  No other product provides as wide an application window to control the major weeds of soybean as glyphosate.  This large application window is increasingly important for farmers and custom applicators who must cover large acreages with limited resources (manpower and equipment), and is beneficial to smaller operators in seasons with adverse weather conditions.  However, it is important for farmers to recognize that the simplest weed management program is not always the most profitable. 

There are several reasons why farmers control weeds, including:

            1) protect crops from yield loss due to competition with weeds,

            2) minimize future weed problems by reducing weed seed production,

            3) prevent interference and loss of efficiency during harvesting,

            4) satisfy personal or landowner desire for clean fields.

Many growers consider a weed management program successful if their field is relatively free of weeds at harvest.  However, a clean field only indicates that the last three objectives have been satisfied, it does not guarantee that weed-related yield losses have been prevented.  This article will describe the importance of application timing in maximizing crop yields and profits when relying on postemergence control tactics.

Postemergence weed control programs are based on the principle that crops can tolerate weeds for a portion of the growing season without suffering yield losses.   Weeds that emerge at the same time as the crop are most damaging to yields, and at high densities these weeds can begin to reduce yields as early as the V2 to V3 stage of soybean.  As control of these weeds is delayed, the yield lost to competition increases.

Weeds that emerge after the soybean are much less damaging than those that emerge with the crop.  Late-emerging weeds are shaded by the crop canopy, and while these weeds may survive to produce seed, they are much less likely to affect yields.  For example, the dry weight and seed production of waterhemp plants emerging at the V2, V4 and V6 stage in soybean was reduced by 80, 93 and 98%, respectively, compared to waterhemp emerging with soybeans.  Under most situations, weeds emerging after the V3 stage of crop development are unlikely to affect yields.   If maximing crop yields is the primary goal for a weed management program, then application timing should be based on the effects of early-emerging weeds on crop yields.

The primary reason for delaying application of postemergence herbicides is to eliminate the need for a second application to control weeds that emerge after the initial treatment.  Delaying applications until soybean reach the V4-V5 stage reduces the survival and competitiveness of weeds emerging after the herbicide application.  The assumption is that net returns will be increased by eliminating the need to make a second herbicide application.  However, the value of the crop lost due to delayed herbicide applications may exceed the cost of a second herbicide application.  In these situations, a one-pass program will provide less net return than a weed management program utilizing two herbicide applications.  The cost of the weeds in the one-pass program is hidden, since both weed management strategies can result in relatively clean fields at harvest.

Research at land-grant universities has documented that weeds emerging with the crop may begin to impact soybean yields as early as the V2 or V3 stage of development.  Weeds that emerge with the crop typically would be approximately 3 to 6 inches tall at this time.  If soybeans are planted no-till into existing weeds, competition may begin even earlier in the growing season.  Once weeds begin to impact soybean yield, each additional day they are allowed to compete can result in yield losses up to 1% per day. 

Attempting to reduce production costs by implementing a one-pass program can reduce net profits.  For example, assume a field has a yield potential of 50 bushels and the weeds begin to reduce yield at the V3 soybean stage.  A farmer might scout the field at this time and see that the weeds are only 4 inches tall and the soybeans have not yet canopied over the row.  Because of concerns for late-emerging weeds he chooses to delay application until the V4 stage (5 days later) when weeds are 6-8 inches tall.  If the soybeans have a value of $6.50 per bushel the decision to delay application may reduce the gross return per acre by $16.25 (Table 1).  The cost of a glyphosate application (product plus application cost) is approximately $12 per acre, thus in this scenario the yield gained by spraying earlier would cover the cost of a second glyphosate application and provide an additional $4.25/acre income.   If the farmer received $7.50 per bushel for the beans, the decision to rely on a one-pass program could cost the farmer $6.75 an acre ($18.75 - $12.00).  The cost associated with delayed application also increases quickly as the yield potential of a field increases.

Table 1.  Influence of yield potential and soybean price on economic losses to
weed competition with delays in herbicide application.  Data based on a 1% yield loss
each day weeds are allowed to compete after yields begin to be affected.


                   Dollars Lost

40 Bushel

50 Bushel

60 Bushel




























A second risk associated with single postemergence application is that weather conditions may cause unplanned delays in the herbicide application.  It is not unusual in Iowa to encounter wet or windy periods that keep sprayers out of fields for several consecutive days.  In the earlier scenario the value of the yield loss associated with treating at the V4 stage (5 days after weeds began to impact yields) was slightly greater than the cost of a second herbicide application.  However, the return per acre would continue to decline rapidly if field conditions kept the sprayer out of the field after the beans reached the V4 stage.

In planning weed management programs it is important to recognize that what is best for one field may not be appropriate for others.  The examples described above assume a heavy weed infestation that emerges at the same time as the crop.  Fields with a history of good management are likely to have lower weed densities that would not begin to impact yields until later in the season.   Narrow row spacing enhances the competitiveness of soybeans and provides greater flexibility in application timing.  The risks associated with  one-pass programs are reduced in fields with low weed densities or drilled soybeans, increasing the potential for these systems to provide both optimum yields and acceptable weed control.  However, research has documented that planned two-pass programs provide more consistent returns than one-pass programs in most situations.

Some farmers may be hesitant to adopt programs relying on two postemergence applications due to limitations with labor or equipment.  An alternative to the two-pass postemergence program would be to apply a preemergence herbicide at or prior to planting and follow it with a single postemergence herbicide.  The goal of the preemergence herbicide is not to provide full-season weed control, but rather reduce the competitiveness of early-emerging weeds so that the postemergence program can be delayed until later in the season.

In summary, the high efficacy of glyphosate may allow farmers to achieve effective weed control with a single herbicide application made after the crop canopy has developed sufficiently to minimize the impact of late-emerging weeds.  However, the ability of weeds to impact soybean yields early in the season may have severe economic repercussions when using this tactic in certain situations.   Weed management programs should be developed that both produce clean field and minimize the impact of weeds on crop yields.

This article was prepared for the 'Issues in Weed Management for 2004'  insert sponsored by the Iowa Soybean Association.  The article will appear in the January issue of Iowa Soybean Today .

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
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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.