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Critical Periods of Competition
Bob Hartzler

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March 4, 2003 -  Weeds negatively affect crop production efficiency in several ways, including reducing yields, reducing harvest efficiency, and contributing to future problems through weed seed production.  A clean field at the end of the season is an indicator of a successful year of weed control for most farmers.  However, a clean field at harvest can be misleading in guaging the success of weed management due to the ability of weeds to affect yields early in the growing season.   This article will review the factors affecting weeds impact on crop production and how farmers can use this knowledge to design effective management programs.

Weeds that emerge with the crop have the greatest potential to affect yields.   The yield loss associated with this group of weeds is strongly influenced by how long they are allowed to remain in the field.   The critical period is defined as the length of time these weeds can be allowed to compete with the crop before yields are affected.  To obtain maximum yields, farmers relying on postemergence herbicides must control weeds before the critical period is reached.   The decision of when to apply postemergence herbicides was often dictated by the effectiveness of the herbicide, rather than the critical period.  For example, Basagran should be applied before velvetleaf reaches a height of six inches to insure consistent control.  The need for early application reduced the risk of yield losses since velvetleaf was unlikely to affect yields when less than six inches tall.  However, many of the herbicides available today (e.g. Roundup UltraMax, Steadfast, etc.) can be applied long after the critical period has been exceeded yet still provide excellent weed control.   Thus, farmers need to understand factors affecting competition between crops and weeds to avoid significant crop yield losses when relying on postemergence herbicides.

It would be nice to have a simple rule to determine when to apply postemergence herbicides.  Examples of such guidelines could be:  1)  apply within the first three weeks after crop emergence; 2) apply at or before the V3 crop stage; or 3) apply before weeds reach a height of six inches.   Unfortunately the critical period varies widely depending on the specific conditions in each field.  For example, in a series of seven similar experiments investigating early-season competition in corn, the time required for weeds to cause a 5% yield loss ranged from 12 to 52 days after planting (3 to 12 leaf corn).   Thus, no simple guideline will accurately predict the critical period for all situations.

In the absence of simple decision making tools, farmers must make subjective decisions based on a sound understanding of biological principals.   The two factors that have the biggest impact on the critical period of competition early in the season are:  1) the density of weeds, and 2) the relative time of weed emergence.  Obviously, the more weeds present, the sooner they will affect crop growth.  Research in central Iowa found that corn yields were reduced within two weeks of corn emergence with high weed densities (40 weeds / ft2) .  In situations with low weed densities (< 5 weeds / ft2) yields were not affected until five weeks after corn emergence.

Relative time of emergence of the crop and weed also has a significant influence on the critical period.  Relatively small delays in weed emergence can result in a much longer critical period.  Research in Nebraska found that the critical period occurred at the V4 corn stage when weeds emerged at the same time as corn, whereas a seven day delay in weed emergence extended the critical period until the V7 corn stage. 

Since weeds can begin to reduce yields early in the season, an effective scouting program is needed to monitor weed populations.  Fields should be scouted when the crop is at the V1-V2 stage to assess the weed population.   Evaluating the percent cover provided by weeds at this time can provide a reasonable estimate of both weed density and relative time of emergence.  If little soil or crop residue is visible at this time due to weed growth there could be significant risks in delaying control tactics.  However, if the weed growth provides 50% or less coverage of the soil surface the risk of delaying application for 7-10 days should be relatively small.

The method described above only takes into consideration weed density and emergence time.  Weather conditions and cultural practices also influence the critical period and should be taken into consideration.  The relationship between several factors and the critical period is provided in Table 1.  For example, as weed density increases the critical period decreases, whereas the length of the critical period decreases as soil moisture decreases.  While any factor that effects the growth of crops and/or weeds will influence the critical period, weed density and relative time of emergence probably have the greatest and most consistent impact on early-season competition. 

Table 1.  Effect of various factors on critical period for early-season competition.


Relationship between factor and critical period (CP)


Weed density


As density increases, CP decreases

Relative time of emergence

As weed emergence is delayed, CP increases

Row spacing

As row spacing narrows, CP increases

Soil moisture

As soil moisture becomes limiting, CP decreases

Crop stress factors

As crop growth is reduced, CP decreases.



Once the critical period has been reached, the yield loss can increase fairly rapidly if weed control is delayed.  For example, removing weeds at the V4 soybean stage resulted in a 20% yield loss compared to 15% loss with removal at the V3 stage in 30 rows (Figure 1).   Narrow row spacing reduced the rate that yield losses occurred.  A systematic scouting program is needed to monitor the spcecific weed population encountered in each field managed by a farmer to avoid these types of losses.


The prior discussion has focused on weeds that emerge with or shortly after crop emergence.  These are the weeds of greatest economic importance since they can cause devastating yield losses.  A second group of weeds to consider are those that emerge after control tactics have been implemented.  These weeds are at a competitive disadvantage with the crop due to their delayed emergence, yet they are still capable of causing economic damage.  The economic impact of late-emerging weeds decreases rapidly as their emergence is delayed by at least three weeks after crop emergence.  For example, common ragweed emerging 2, 4 and 6 weeks after soybean emergence caused 17, 1 and 0% yield loss.






As with early emerging weeds, numerous factors influence the impact of late-emerging weeds.   The competitiveness of late-emerging weeds is strongly influenced by how quickly the crop canopy develops.  Narrow-row spacing and high crop populations may reduce the impact of late-emerging weeds due to more rapid canopy development that shades these weeds.  Factors that reduce crop growth early in the season (poor drainage, nematodes, herbicide injury, etc.) may increase the impact of late-emerging weeds.



Although more difficult to quantify than crop yield losses, an added impact of late-emerging weeds is seed production.  Most weeds are capable of producing seeds even when at competitive disadvantages with the crop.  Waterhemp emerging with soybeans can produce several hundred thousand to more than a million seeds per plant.  The reduction in seed production due to delayed emergence is shown in Figure 2.  Seed production of late emerging plants is of greater concern with prolific seed producing species (i.e. waterhemp, lambsquarter, foxtail, etc.) than for weeds with limited seed production  (i.e. cocklebur, velvetleaf).







Figure 2.  Effect of delayed waterhemp emergence on seed production


It is not necessary to maintain fields weed-free for the duration of the season to achieve maximum yields and economic returns.  However, failure to account for the competitive relationship between weeds and crops can result in yield losses even though the field is weed-free at the end of the season.  This can occur when applications of postemergence herbicides are delayed beyond the critical period in an attempt to minimize problems with late-emerging weeds.  To avoid this situation, farmers need to understand factors that influence the critical period and implement  a systematic scouting program that allows them to respond rapidly to specific weed infestations in their fields.


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.