Waterhemp Research Results
by Bob Hartzler
December 14, 2001 -  The North Central Weed Science Society held its annual conference recently in Milwaukee, WI.  The focus of the meeting is presentation of weed science research results, either in the form of oral presentations or posters.  The following is an adaptation of a poster displayed at the meeting.  The poster summarizes research that has been presented on this web site earlier.  Although this website reaches a much broader audience than attends the conference, hopefully the information is presented in a manner that can be understood by persons without formal training in weed science.  Fecundity is a scientific term for seed production, a word I would not use in typical extension settings.  An abstract from the Proceedings is also available.

Effect of Delayed Waterhemp Emergence on Waterhemp Growth and Fecundity
Bob Hartzler, Bruce Battles and Dawn Refsell.  Iowa State University, Ames

Background

     Waterhemp (Amaranthus tuberculatus) developed into one of the most problematic weeds of the northern Cornbelt in the mid-80's.  While many factors contributed to this increase, the prolonged emergence pattern of waterhemp is one of the most important (Figure 1).
    A significant number of seedlings frequently emerge after typical post-emergence application dates.  These late-emerging plants are at a competitive disadvantage due to the crop’s head start.  While the impact of late-emerging waterhemp on soybean yields may be greatly reduced, these plants may maintain the weed seed bank, therefore contributing to future problems.
     The objective of this research was to evaluate the effect of delayed emergence on waterhemp growth and fecundity.

Methods

   Four experiments were conducted in central Iowa during the 1998 and 1999 growing seasons.  Glyphosate resistant soybeans were planted in 76 cm rows for all experiments.  Four waterhemp emergence cohorts were established in each experiment:  1) plants emerging soon after planting (~14 days after planting (DAP), 2) V2 soybean (~27 DAP); 3) V4 soybean (~40 DAP), and 4) V6 soybean (~50 DAP).  A total of 60 plants per cohort was selected in each experiment and their fate monitored throughout the growing season.  Other weeds were controlled with directed applications of glyphosate and hand weeding.
     At maturity, waterhemp plants were harvested at the soil line.  Plants were dried at 40º C, weighed, and then seed were separated and seed number per plant determined.

Results

    ANOVA indicated that waterhemp survival in response to emergence date was similar at all locations, thus survival data were pooled across experiments.  Waterhemp growth and fecundity was similar at three sites, and data from these sites (COMB) were pooled.  Data from the forth experiment, conducted at Stratford, IA, are presented separately.

Survival   
  Survival of the first waterhemp cohort was 91%, and declined linearly to 19% for plants emerging 50 DAP (Figure 2).  Variability in survival among locations increased as emergence was delayed.

Biomass
     At three sites (COMB), the first emergence cohort averaged 300 g dry matter per plant, whereas at Stratford this cohort produced more than 1300 g dry matter.  The Stratford site was located in a poorly drained area of the field, which may have favored waterhemp growth.  The largest decline in biomass at the combined sites occurred between 14 and 27 DAP (Figure 3).  Biomass was reduced by 79, 95 and 99% when emergence was delayed until the V2, V4 and V6 stage of soybean. 
   At Stratford, biomass accumulation did not decline as rapidly with delays in emergence as at the other locations (Figure 4). Biomass  of waterhemp emerging 27 DAP was reduced 46% compared to the first cohort at Stratford, whereas at the other locations a 79% decrease was observed.

Fecundity
   At the combined sites, waterhemp emerging shortly after planting produced more than 300,000 seeds per plant (Figure 5).  At Stratford, the first cohort of waterhemp averaged more than 2 million seeds per plant (Figure 6).  The most prolific plant produced nearly 5 million seeds.  Declines in seed production in response to delayed emergence were similar to declines in biomass.  Fecundity was closely correlated with biomass (Figure 7). 

Summary

- 91% of waterhemp emerging shortly after planting survived to maturity.  A mortality rate of approximately 2% per day was observed with delayed emergence.

- Productivity of plants declined rapidly with delays in emergence.  Plants emerging after the V4 stage of soybean are unlikely to affect soybean yield due to low biomass.

- Fecundity was closely correlated with biomass.

- Waterhemp emerging as late as the V6 stage are able to maintain, or increase, the weed seed bank.

Aknowledgment:  This research was supported by the Iowa Soybean Promotion Board.

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