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Late-emerging waterhemp - how big of a problem?
by Bob Hartzler and Bruce Battles
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December 2, 1998 - Several factors have led to the increase in waterhemp problems, one being the adoption of weed management systems that favor late-emerging weeds such as waterhemp.  The prolonged emergence of waterhemp allows a significant percentage of the population to escape control tactics made early in the season; however, these late-emerging plants are at a competitive disadvantage with the crop since the crop has a head start on the weed.  At some point of time late-emerging weeds become non-economic due to their lack of competitiveness.  The Iowa Soybean Promotion Board is funding a project to determine the impact of waterhemp plants emerging at different times during the growing season.  This paper will present some of the results of this project.

Studies were conducted during 1998 on cooperator fields in Story and Webster counties.  Roundup Ready soybeans were planted in 30" rows using conventional tillage in fields with heavy waterhemp infestations.  Roundup Ultra was used to select waterhemp that emerged at different times during the growing season.  Treatments included a single postemergence Roundup application at the V2, V4 or V6 stage of soybeans.  A control plot where waterhemp was allowed to emerge with soybeans was also included.  Waterhemp plants that emerged shortly after the Roundup applications were flagged and the growth of these plants was monitored for the duration of the growing season. Other weeds were removed by hand or with spot applications of Roundup. Results from these studies is presented in Table 1.

Table 1.  Effect of time of waterhemp emergence on growth and competitiveness of waterhemp in soybeans at two Iowa locations during 1998.


Waterhemp emergence date
(soybean stage)

% survival


Dry wt

Seed production

% reduction in soybean yields2


At planting

90 a

87 a

1300 a

2500 a

42 a



85 a

78 a

630 b

1600 a

24 b



72 a

65 b

220 c

490 b

6 c



27 b

59 b

120 c

250 b

2 c


At planting

85 a

71 a

240 a

480 a

12 a



63 b

53 b

85 b

100 b

7 a



22 c

22 c

25 b

12 b

6 a



5 c

8 d

12 b

0 b

4 a

1Means within a column and location with same letter are not significantly different at the 0.05 level.
2 All soybean plants within a 1 sq. m area of the waterhemp plant were hand harvested. Yields were compared to yields from a similar-sized area with no weeds present to determine yield loss.

Emergence date of waterhemp relative to soybean had a large effect on survival and growth of waterhemp. There was no difference in waterhemp survival at the three earliest emergence dates at the Story county study, with survival ranging from 72 to 90%. However, survival of the latest emerging waterhemp (V6) was reduced to 27%.  Waterhemp dry weight and seed production were more sensitive to delays in emergence than was plant height. Although the height of waterhemp was reduced from 87 inches when the weed emerged with soybeans to 59 inches when emerging at the V6 stage, late-emerging plants eventually broke through the soybean canopy late in the season. Waterhemp at this site were extremely productive, with plants emerging with soybeans producing on average more than 2,000,000 seeds per plant. One plant at this location produced over 5,000,000 seeds.  Waterhemp emerging at the V6 stage of soybean still produced approximately 250,000 seeds per plant. Delays in waterhemp emergence greatly reduced competitiveness, with plants emerging later than the V2 stage having no effect on soybean yields.

Survival of waterhemp emerging at the V4 and V6 trifoliate stage was only 22 and 5 percent, respectively, at the Webster county site. Plants emerging at the later two stages remained within the soybean canopy for the entire season. Waterhemp that emerged with soybeans accumulated 240 g of dry matter, which was less than 20% of the biomass produced by waterhemp at the Story county site.  Delayed emergence greatly reduced seed production by waterhemp, but plants emerging at the V4 stage still produced 12,000 seeds. No female plants emerging at the V6 stage survived, thus we were unable to determine seed production at this time of emergence.  Soybean yields were not significantly affected by waterhemp competition, regardless of emergence time.

This research is providing important information concerning waterhemp management. Waterhemp plants emerging later than the V2 stage produced less than 20% of the biomass as plants that emerge with soybeans and have not reduced soybean yields. However, late emerging plants may impact soybean production in other ways. Under certain conditions these weeds can produce enough biomass to interfere with harvest efficiency when present at high populations. These late-emerging plants also have the capability to produce large quantities of seed that will increase waterhemp problems in future years. Thus, although it is unlikely that waterhemp emerging later than the V2 stage will reduce yields, there are costs associated with these plants that may warrant implementation of control tactics.

The other important finding of this study is the high variability in waterhemp growth with changes in environment.  Similar studies were conducted 1997, and in three experiments no waterhemp emerging later than the V2 stage of soybean survived until maturity.  In 1998, there was tremendous variation in the growth and competitiveness of waterhemp at the two locations. The most logical explanation for the differences in waterhemp growth seems to be soil moisture availability early in the growing season.  In 1997, rainfall during May and June was significantly below normal at the research sites, whereas in 1998 both locations were above average for rainfall during this period. Waterhemp at the Story county site accumulated more than five times as much biomass than at the Webster county site. The most apparent difference between the research sites was soil drainage. In Story county, the site was positioned in a low-lying, poorly drained area of the field, whereas in Webster county the site was placed in a well drained area of the field. The native habitat of waterhemp is wet areas, and waterhemp typically is more of a problem in years or fields with abundant soil moisture. This variability in growth complicates management decisions, but the variability must be considered when determining the need to treat late-season weeds.   These studies will be conducted for another year, plus we plan to look more closely at the effect of soil moisture on waterhemp competitiveness.

Acknowledgements:  The Iowa Soybean Promotion Board is funding this research.   We appreciate their support.

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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