Waterhemp: biology and control

A ten-leaf hairless seedling plant with narrower, wavy margined leaves compared to other pigweed species.


INDEX

    1. Waterhemp identification
    2. Ontario counties with confirmed resistant populations (July 2020)
    3. Testing services to identify waterhemp and if herbicide resistant
    4. Waterhemp control in corn
    5. Waterhemp control in soybean
    6. Waterhemp control in winter wheat
    7. Influence of cover crops on waterhemp
    8. Influence of tillage on waterhemp
    9. Acknowledgments


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SPECIES INFORMATION
Lifecycle: Annual
Propagation: Reproduces by seed, dioecious, has separate male and female plants.
Emergence: Spring, typically after lamb’s-quarters, emergence continues through late summer or early autumn depending on weather conditions.
Habitat: Not native to Canada. Now found in several counties throughout Ontario in agricultural fields. Historically it was found mainly along waterways and beaches. Prefers full or partial sun, moist or wet conditions.
Competitiveness: In University of Guelph research trials, poor control of waterhemp has resulted in over 80 percent yield loss in both corn and soybeans.

IDENTIFICATION CLUES
Seedlings
Cotyledons: Linear, narrow, purple-red underneath.
Mature leaves: Lance-shaped (tapered and typically with a pointed tip), with a wavy margin.
Mature Plant
Stem: Alternate leaf orientation; smooth with no hairs; variable colouring (green to red/purple); up to 3 m in height.
Seed heads: Long, spikey clusters (somewhat similar to ragweed) that can vary in colour from green to red to purple. Only the female plants produce seed.
Roots: Taproot

OFTEN MISTAKEN FOR
I know it’s NOT … because…

Redroot pigweed … The stem of redroot pigweed is covered in dense short hairs, whereas waterhemp is smooth and hairless. The tips of waterhemp leaves are typically not notched.

Green pigweed … The upper stem of green pigweed has a cluster of dense hairs whereas the stem of waterhemp is smooth and hairless. Waterhemp will typically have narrower leaves with wavy margins and without a notched tip.

Eastern black nightshade … The leaf margins of young eastern black nightshade leaves are hairy and the older leaves have wavy toothed margins whereas waterhemp leaves are hairless with wavy margins.

A ten-leaf hairless seedling plant with narrower, wavy margined leaves compared to other pigweed species.
A ten-leaf hairless seedling plant with narrower, wavy margined leaves compared to other pigweed species.
A close-up of a mature and narrow waterhemp leaf
A close-up of a mature and narrow waterhemp leaf
The completely hairless and smooth stem of waterhemp
The completely hairless and smooth stem of waterhemp
The very long, fingerlike spikes of the female seed head
The very long, finger-like spikes of the female seed head
Male (left) and female (right) plants with reddish-purple flowers in a Chatham-Kent county soybean field
Male (left) and female (right) plants with reddish-purple flowers in a Chatham-Kent county soybean field
Waterhemp plants found in a soybean field in Norfolk county during the 2019 growing season. They were eventually confirmed to be resistant to group 2, group 9 and group 14 herbicides.
Waterhemp plants found in a soybean field in Norfolk county during the 2019 growing season. They were eventually confirmed to be resistant to group 2, group 9 and group 14 herbicides.
Waterhemp in a soybean field weeks after glyphosate had been applied. All other weeds died.
Waterhemp in a soybean field weeks after glyphosate had been applied. All other weeds died.


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Waterhemp populations confirmed as resistant to herbicide groups in Ontario counties.

CountyWSSA Group 2 ResistantWSSA Group 5 ResistantWSSA Group 9 ResistantWSSA Group 14 Resistant
BrantYesYes
BruceYesYes
Chatham-KentYesYesYesYes
ElginYesYes
EssexYesYesYesYes
HaldimandYesYesYesYes
HuronYesYesYesYes
LambtonYesYesYesYes
MiddlesexYesYesYes
NorfolkYesYesYes
NorthumberlandYesYes
WellingtonYesYesYes
WentworthYesYes
Group 2 herbicides include: Pursuit, Classic, Pinnacle, Refine SG, Peak etc.
Group 5 herbicides include: Aatrex 480, Sencor etc.
Group 9 herbicides include: glyphosate (Roundup Transorb, Crusher, Credit Xtreme)
Group 14 herbicides include: Blazer, Reflex



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Testing services to identify waterhemp and determine if herbicide resistant

There are two testing solutions available in Ontario. The plant bioassay service offered by the Tardif lab at the University of Guelph requires that mature seed is collected in the fall and submitted. Plants are then grown from that seed and sprayed at various doses of the herbicide(s) suspected. Results are provided typically by March, the service is provided at no cost to Ontario farmers. The molecular testing service, requires plant tissue that is roughly “loonie sized”, so a single leaf is often adequate. Results can be obtained in a matter of days, but it is a fee based service.

  1. Free plant bioassay testing by the University of Guelph
  2. Fee based molecular testing by Harvest Genomics (private lab)


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CONTROL IN CORN

Control of waterhemp is best achieved with a two-pass herbicide program where the first pass is with a pre-emergence (PRE) soil applied herbicide and the second with a postemergent (POST) herbicide. Consistency of waterhemp control with soil applied herbicides has varied and one should not expect perfect season long control. However, a soil applied herbicide will eliminate the majority of early seedlings and will reduce the risk of having large, less susceptible seedling plants emerged at the time of the post-emergence herbicide pass.

Waterhemp control (% out of 100) with various preemergence (PRE) corn herbicides

Trade NameTimingControl
(% out of 100)
WSSA Group #Corn Hybrid Trait
AcuronPRE9127,27,15,5All
IntegrityPRE8214,15All
Aatrex 480PRE655All
Broadstrike RCPRE402All
Callisto + Aatrex 480PRE8227,5All
Converge XTPRE8227,5All
dicamba (e.g. Engenia)PRE424All
Dual II MagnumPRE7315All
EngardePRE722,27All
Frontier MaxPRE7115All
MarksmanPRE764,5All
Primextra II MagnumPRE7815,5All
ZiduaPRE7415All
The table provides a summary of waterhemp control at 8 weeks after the application of corn herbicides prior to the emergence of waterhemp and corn (PRE). Source: Dr. Peter Sikkema, University of Guelph (Ridgetown campus)


Waterhemp control several weeks after a preemergence application of Acuron herbicide. (Source: Dr. Peter Sikkema, University of Guelph - Ridgetown)
Waterhemp control several weeks after a preemergence application of Acuron herbicide. (Source: Dr. Peter Sikkema, University of Guelph – Ridgetown)
Waterhemp control several weeks after a preemergence application of Integrity herbicide. (Source: Dr. Peter Sikkema, University of Guelph - Ridgetown)
Waterhemp control several weeks after a preemergence application of Integrity herbicide. (Source: Dr. Peter Sikkema, University of Guelph – Ridgetown)

Waterhemp control (% out of 100) with various postemergence (POST) corn herbicides

Trade NameTimingControl
(% out of 100)
WSSA Group #Corn Hybrid Trait
Shieldex + Aatrex 480POST9727,5All
Callisto + Aatrex 480POST9227,5All
MarksmanPOST874,5All
2,4-DPOST854All
Armezon + Aatrex 480POST8327,5All
dicamba (e.g. Engenia)POST824All
DistinctPOST7419,4All
Vios G3POST692,27All
Pardner + Aatrex 480POST686,5All
Liberty 200 SNPOST6210Liberty Link
Aatrex 480POST605All
Peak + dicambaPOST542,4All
Permit + Agral 90POST312All
The table provides a summary of waterhemp control at 8 weeks after the application of corn herbicides after waterhemp and corn emergence (POST). Source: Dr. Peter Sikkema, University of Guelph (Ridgetown campus)


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CONTROL IN SOYBEAN

Control of waterhemp is best achieved with a two-pass herbicide program where the first pass is with a pre-emergence (PRE) soil applied herbicide and the second with a postemergent (POST) herbicide.

Waterhemp control (% out of 100) with various preemergence (PRE) soybean herbicides

Trade NameTimingControl
(% out of 100)
Active WSSA Group #Soybean Variety Trait
FiercePRE9214, 15all
TriactorPRE862,5,14all
Sencor 75DFPRE865all
BlazerPOST8614all
ValteraPRE8514all
Authority SupremePRE8314, 15all
Boundary LQDPRE815, 15all
Dual II MagnumPRE8015all
ZiduaPRE7715all
Frontier MaxPRE7315all
IntegrityPRE7214,15all
AuthorityPRE6514all
Eragon LQPRE4614all
Prowl H2OPRE243all
PursuitPRE182all
The table provides a summary of waterhemp control at 8 weeks after the application of soybean herbicides prior to the emergence of waterhemp and soybean (PRE) . Source: Dr. Peter Sikkema, University of Guelph (Ridgetown campus)

Waterhemp control several weeks after a preemergence application of Fierce (left) compared to an un-sprayed strip (right). (Source: Dr. Peter Sikkema, University of Guelph - Ridgetown)
Waterhemp control several weeks after a preemergence application of Fierce (left) compared to an un-sprayed strip (right). (Source: Dr. Peter Sikkema, University of Guelph – Ridgetown)

Waterhemp control (% out of 100) with various postemergence (POST) soybean herbicides

Trade NameTimingControl
(% out of 100)
Active WSSA Group #Soybean Variety Trait
Reflex + TurbochargePOST8914all
BlazerPOST8614all
Roundup Xtend (high rate)POST804RR 2Y Xtend
Enlist Duo (high rate)POST802,9E3 (Enlist)
Liberty 200 SNPOST4710ES (Enlist)
Liberty Link
Pinnacle*POST332all
Classic*POST262all
FirstRate**POST222all
Basagran FortePOST156all
The table provides a summary of waterhemp control at 8 weeks after the application of soybean herbicides to emerged waterhemp and soybean (POST). Source: Dr. Peter Sikkema, University of Guelph (Ridgetown campus)


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CONTROL IN CEREALS

Overall, the experience in Ontario has been that winter cereals are a good rotational crop for reducing waterhemp populations. A limited amount of research has been completed in Ontario primarily because waterhemp has not been a problem in winter wheat thus far. This could be, in part, because the fall seeded winter wheat crop tends to out-compete spring emerging summer annuals like pigweed species which include waterhemp.

During a plot tour of Dr. Sikkema's waterhemp control studies, a participant, Joe Vink, observed how there was little waterhemp in the neighbouring winter wheat crop compared with the soybean plots.
During a plot tour of Dr. Sikkema’s waterhemp control studies, a participant, Joe Vink, observed how there was little waterhemp in the neighbouring winter wheat crop compared with the soybean plots.

However, if waterhemp recruitment is a concern, Eragon LQ has been cited as being effective on waterhemp (Knezevic et al., 2010) and can be applied pre-plant to winter wheat. Otherwise, it would be logical to assume that cereal herbicides that are the most effective at controlling pigweed, would also be effective at controlling waterhemp should it show up in your winter wheat crop.



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INFLUENCE OF COVER CROPS
An Illinois study found that fall seeded cereal rye that was crimped prior to planting soybean consistently reduced biomass of waterhemp over a 3 year period compared to vetch or no cover crop (Davis, 2010). The main challenge with using cereal rye as a weed suppressing cover crop is that its thick residue can negatively affect soybean stand establishment which can result in variability of soybean yield as observed by Davis (2010) and Liebel et al., (1992). This should not be viewed as a deal breaker but rather the importance of residue management at the time of planting which has become easier with improvements in equipment units like strip tillers and row cleaners.



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INFLUENCE OF TILLAGE
Multiple studies have shown that waterhemp emergence is greater in no-till cropping systems compared to ones that include tillage (Refsell and Hartzler, 2009). An Iowa study demonstrated that duration of waterhemp emergence was 26 days longer in a no-till cropping system compared to one that used a chisel-plough (Refsell and Hartzler, 2009).



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Acknowledgments: Dr. François Tardif co-wrote  the identification section and Dr. Peter Sikkema provided the herbicide efficacy data.

Further reading: Waterhemp was first identified in an Ontario field in 2003, when Dan Foster came upon some odd looking pigweed in a soybean field. You can read more about the “early days” of waterhemp in Ontario in this article.