Iowa State University

When Did That Weed Emerge?
by Bob Hartzler

April 4, 2003 -  Late-emerging weeds have become a significant problem as the use of residual herbicides and lay-by cultivation in corn and soybean has declined.  Although late emergence is an effective mechanism to escape control tactics, it does place the weed at a competitive disadvantage since the crop is provided a head start.  Weeds that emerge after a specific time frame in the growing season are no longer economically important since they are incapable of competing with the crop or producing significant seed.  This point of time is called the critical period, and control tactics are not warranted for weeds that emerge after this time period. 

A common problem facing persons involved in weed management is determining exactly when a population of weeds emerged.  Unfortunately, most fields are not scouted frequently enough to know whether weeds emerged at the VE, V2, or V4 stage of crop development.  This information is critical in accessing the economic impact of a weed infestation and determining whether a control tactic is warranted.  A recent article in the journal Weed Science discusses the reliability of predicting weed emergence timing based on weed leaf number (Weaver, S. E.  2003.  Correlations among relative crop and weed growth stages.  Weed Sci.  51:163-170).  The research was conducted by Susan Weaver at the Agriculture and Agri-Food Canada research center in Harrow, Ontario¹.  Corn (30 inch rows) and soybean (24 inch rows)  were planted in mid-May.  Several species of weeds were seeded at 0, 2 and 4 weeks after crop planting, and thinned so that they would not compete with other weeds.  Dates of emergence were recorded for the weeds and leaf number was determined every three days during the early part of the growing season.  Research was conducted in 1997, 1998 and 1999. 

Results were fairly consistent during the three years of the study.  There were differences in growth rates of VE weeds in corn during 1999 compared to the other two years, but the development of weeds emerging at the V1 and V2 stage were similar in all three years.  No differences were seen among years in the research conducted with soybean.  In soybean four weed cohorts were identified (VE, V1, V2 and V3), whereas in corn three cohorts were monitored.   Crop growth varied depending upon weather, but days required for corn and soybean to produce a new leaf (e.g. V2 →V3) typically ranged from 4 to 8 days.

Figure 1.  The competitiveness of giant foxtail, and other weeds,  is strongly influenced
                 by when they emerge in relation to corn emergence.
 

At the V3 soybean stage, weeds that emerged at the same time as the soybean (VE) averaged 7.7 to11.4 leaves (Table 1).  When emergence was delayed until between the V1 and V2 soybean stage (V2 cohort), the weeds averaged between 3.2 and 6.7 leaves per plant.  Common lambsquarter exhibited the smallest decline in leaf number with delays in emergence.  The V3 cohort of redroot pigweed had a 26% reduction in leaf number compared to the VE cohort.  This reduction in leaf number due to delayed emergence correlates fairly well with the reduction in end of season biomass of waterhemp due to delayed emergence  (ISU results).

Table 1.  Average leaf number of  weeds emerging at different times in soybean. 
Leaves counted at the third trifoliate soybean (V3) stage.

Results of the corn research are presented in Table 2.  The growth rate of VE weeds was slower in 1999 than in the other two years the research was conducted.  The author speculated that a dry period at this time may have slowed the development of shallow-rooted weeds in 1999.  The growth rate of the weeds and the effects of delayed emergence in corn were fairly similar to the response in soybean.

Table 2.  Average leaf number of  weeds emerging at different times in corn.
Leaves counted at the eight collar corn (V8) stage.

The consistency of the results across years indicates that leaf number can be a reliable predictor of relative emergence dates of weeds in corn and soybean.  I suspect most people base management decisions on weed height rather than leaf number, and therefore they may question the usefulness of these results.   The author cited research supporting a strong correlation between leaf number and weed height and other research that found variable height response depending upon growing conditions.   This research suggests that leaf number may be a more reliable method of accessing the age of a weed, but I suspect that using weed height would provide information with sufficient accuracy to base management decisions.   Regardless of the method used, it is important to consider the relative emergence date of weeds with crops when determining the need to implement a control tactic.  The information from this research can be used to accurately estimate when weeds emerged, and therefore determine their competitiveness and whether control tactics are warranted.

¹For those questioning whether research done in the Great White North is pertinent to Iowa, Harrow, Ontario is at the same latitude as Ames, Iowa.

Related articles:
    Critical periods of competition
    Waterhemp research results

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