Waterhemp: biology and control

INDEX

1. Waterhemp identification
2. Ontario counties with confirmed resistant populations (Dec 2023)
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 up to 99 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 3.5 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.

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Ontario Counties with herbicide resistant waterhemp

A total of 195 waterhemp populations have been tested since 2002. Over 90% of populations are resistant to one or more of the following herbicide groups: WSSA groups 2, 5, 9, 14 and 27. The majority of populations are resistant to 2 or more different modes of action, with resistance to as many as 5 modes of action occurring.

The following image provides an overview of the number of waterhemp populations by county or district and the number of modes of action that they are resistant to.

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The image below provides a broad overview of where herbicide resistant waterhemp has been found in the province.

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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 TurnKey Genomics (private lab)

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A harmonized monitoring protocol for waterhemp has been developed to enable coordination and information-sharing across provinces. This collaboration will contribute to knowledge of the distribution of herbicide-resistant populations of waterhemp in Canada, which will help inform management programs for this problem weed to limit its impact and further spread in Canada.

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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.

CORN: PREEMERGENCE HERBICIDES

Herbicide	WSSA Group(s)	Control (%)	Corn Hybrid Trait
ACURON	5,15,27,27	92	All
INTEGRITY	14,15	91	All
CONVERGE XT	5,27	89	All
CALLISTO + AATREX	5,27	86	All
PRIMEXTRA II MAGNUM	5,15	85	All
ACURON FLEXI	15,27,27	82	All
HARNESS (acetochlor – not in Canada)	15	82	All
MARKSMAN	4,5	79	All
ZIDUA SC	15	76	All
ENGARDE	2,27	73	All
FRONTIER MAX	15	72	All
DUAL II MAGNUM	15	69	All
AATREX 480	5	61	All
ENGENIA (dicamba)	4	49	All
BROADSTRIKE RC	2	44	All

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CORN: POSTEMERGENCE HERBICIDES

Herbicide	WSSA Group(s)	Control (%)	Corn Hybrid Trait
ACURON	5,15,27,27	96	All
LAUDIS + AATREX 480	5,27	94	All
ACURON FLEXI	15,27,27	92	All
SHIELDEX + AATREX 480	5,27	92	All
CONVERGE XT	5,27	92	All
CALLISTO + AATREX 480	5,27	91	All
HALEX GT + AATREX 480	5,9,15,27	89	All
ENLIST DUO	2,9	89	E3 (Enlist)
ENGENIA (dicamba – high rate)	4	84	All
PRIMEXTRA II MAGNUM	5,15	83	All
ARMEZON + AATREX 480	5,27	82	All
MARKSMAN	4,5	81	All
PARDNER + AATREX 480	5,6	80	All
DESTRA IS	2,27	78	All
2,4-D	4	77	All
ENGENIA (dicamba – low rate)	4	71	All
DISTINCT	4,19	69	All
LIBERTY 200 SN	10	64	E3 (Enlist), Liberty Link
VIOS G3	2,27	62	All
AATREX 480	5	62	All
PERMIT	2	27	All

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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.

SOYBEANS: PREEMERGENCE HERBICIDES

Herbicide	WSSA Group(s)	Control (%)	Soybean Variety Trait
FIERCE	14,15	90	All
VALTERA	14,15	83	All
TRIACTOR	2,5,14	82	All
BIFECTA	5,14	81	All
AUTHORITY SUPREME	14,15	80	All
TAVIUM	4,15	79	RR 2Y Xtend
BOUNDARY LQD	5,15	78	All
CANOPY PRO	2,5	76	All
ZIDUA	15	74	All
DUAL II MAGNUM	15	72	All
INTEGRITY	14,15	66	All
FRONTIER	15	65	All
SENCOR	5	60	All
OPTILL	2,14	56	All
AUTHORITY	14	55	All
ERAGON LQ	14	47	All
ENGENIA (dicamba)	4	43	RR 2Y Xtend
FREESTYLE	2,2	25	All
PROWL H2O	3	24	All
PURSUIT	2	17	All
CLASSIC	2	10	All

SOYBEANS: Postemergence herbicides

Herbicide	WSSA Group(s)	Control (%)	Soybean Variety Trait
TAVIUM	4,15	80	RR 2Y Xtend
ENLIST DUO	4,9	75	E3 (Enlist)
ENGENIA (dicamba)	4	74	RR 2Y Xtend
HURRICANE	6,14	62	All
BLAZER	14	59	All
REFLEX	14	56	All
LIBERTY 200 SN	10	45	E3 (Enlist), Liberty Link
PINNACLE	2	33	All
CLASSIC	2	26	All
FIRSTRATE	2	22	All
CLEANSWEEP	2,6	21	All
BASAGRAN FORTE	6	20	All
PURSUIT	2	5	All

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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.

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.

A trial evaluating winter wheat herbicides was conducted by the Tardif lab at the University of Guelph in 2023, results are provided in the following table. An interesting finding from this work is that the active ingredient fluroxypyr, which has been added to a number of herbicides to provide control of certain problem weeds (e.g. vetch) provides no activity on waterhemp.

WINTER WHEAT: Postemergence herbicides

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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.