Know Thine Enemy

Weather, weed size can greatly influence grass herbicide applications in small grains

By Tracy Sayler

Weather and weed size can greatly influence the success of herbicide applications. Ideal temperatures for applying most post herbicides are between 65 and 85 degrees, when weeds are actively growing.  Speed of kill may be slow when temperatures remain below 60°, while some herbicides may injure crops if applied above 85° or below 40°, according to the North Dakota State University Extension Service.

Early season conditions are also the reason why certain weed species dominate in a field in one year and not another. If it’s warm early, green and yellow foxtail often germinate aggressively and get established with wild oat, and often before the crop is up. Cool temperatures and drier conditions typically favor wild oat, which can germinate from deep in the soil profile. It is important to be aware of how weather affects the development of weed populations so you can choose the best herbicide for the conditions.

Application timing is also critical for weed control.  The application window varies by herbicide, but generally the smaller the annual weed, the easier it is to kill. Lower application rates also have better success when weeds are smaller.

“Granted, with herbicide products that don’t have soil residual, you’re not going to catch later weed flushes. But in many cases, eliminating the early season weed competition preserves more yield than waiting to catch later flushes.  Once there’s canopy closure, later emerging weeds are much less competitive,” says Kirk Howatt, assistant professor of weed science at NDSU.

Howatt says it’s important to rotate different herbicide modes of action and chemistries to help prevent weeds from developing herbicide resistance. Two common modes of action for grass herbicides are ACCase inhibitors and ALS enzyme inhibitors.  These modes of action (the names of which refer to how they kill weeds) are further broken down into chemical families (see table below).

Herbicide Classification and Mode of Action Common Grass Herbicides for Small Grains

Action Mechanism	Common Name	Herbicide Trade Name
ACCase Inhibitor Aryloxypenoxy proprionic acids “Fops” Cyclohexandiones “Dims”	clodinafop-P fenoxaprop-P tralkoxydim	Discover Puma, Dakota Achieve
Imidazolinones “Imi” Sulfonylureas “SU” Sulfonylamino- carbonyltriazolinone “SACT”	imazamethabenz mesosulfuron flucarbazone propoxycarbazone	Assert Silverado Everest Olympus

Profile of Key Grassy Weeds in Small Grains
Wild oat - - Spring wheat’s most aggressive competitor (barley is more competitive with wild oats than spring wheat) with high reproductive capacity, long term viability in the soil, and an ability to adapt to a wide range of environmental conditions. A single plant can produce up to 1,000 seeds, threatening crop returns for years to come. This weed displays an interesting mechanism called “differential dormancy” whereby seeds which are shed first have a longer dormant period than those which are shed later - this allows all the seed to become viable at the appropriate time.

Wild oat cotyledons have a strong deep-green color and the youngest leaf appears “left-rolled.”  Further, leaves tend to twist counter-clockwise, as viewed from above.

Postemergence wild oat herbicides require application at precise leaf stages – application timing is key. An accurate leaf count is important for optimum control. Leaf number on wild oats is determined by counting the leaves on the main stem and disregarding the tillers. The youngest leaf is counted as a full leaf only when another leaf becomes visible. Lower leaves which may have died from various stresses, such as frost, should also be counted in the total leaf number.

Green foxtail -- Features rather wide leaves that gradually taper to a point. Green foxtail lacks hairs over the whole plant and has rounded stems. Green foxtail has the same short, cylindrical flower cluster as yellow foxtail but the seeds are smaller, and each seed has only 1 to 3 bristles beneath it.  Foxtail species may germinate early, before wild oat, when the weather is warm and before spring wheat is seeded.

Yellow foxtail -- Has relatively broad, limp, dull blue-green leaves with a white middle stripe on mature leaves. Long hairs located at the leaf base are highly predictive of this species. The stems are usually flattened and may be reddish in color. Yellow foxtail has larger seeds than green foxtail, and each seed has 4 to 12 bristles beneath it.  With optimal conditions, this annual grass can produce up to 800 seeds per plant, and like other foxtail species, germinates in warm conditions. ACCase inhibitors tend to offer better control of foxtails than ALS inhibitors, and herbicide performance can even vary between green and yellow foxtail.

Barnyardgrass -- Common in wet areas; be on the lookout for this weed in fields plagued by wet conditions in the Red River Valley. The plants tend to be flattened and purple near the base making them easy to identify at an early stage. Leaves have a distinct white midvein that becomes keeled toward the basal potions of the leaf. The seeds are rough with tiny bristles pointing towards the tip. A key characteristic of barnyardgrass is the absence of a ligule, the thin outgrowth at the junction of the leaf and leafstalk, present in most other grasses.

Persian darnel -- This annual grass is becoming a troublesome weed for spring and durum wheat growers west of Bismarck and west through Montana. It germinates very early in the spring, even in relatively dry areas, usually before wild oat. Seedlings grow in dense, highly competitive patches and research has shown that as a competitor for moisture and nutrients, Persian darnel is similar to wild oat. The weed leaves have no hair and are waxy on the underside, giving the leaves a glossy appearance.

Brome species -- Downy brome and Japanese brome (sometimes referred to as “cheatgrass,” though Japanese brome is not a true cheatgrass species) are increasing in some western areas of the Northern Plains. Downy brome tends to prefer drier conditions than Japanese brome. They are winter annuals, meaning they can germinate and grow in the fall or in the spring. Japanese brome may have a higher tendency to be found in winter wheat crops or no-till environments. While awns of mature downy brome are straight and longer than the reddish-purple seed, awns of Japanese brome curl at maturity and are the same size or shorter than the tan seed.  Brome grasses are easiest to control early; they are harder to control once the plants go into reproductive stages.

Weed Control Information Online
See herbicide product labels for information on application rates, timing, crop/weed stages , adjuvants, tankmixing capabilities, crop rotation, crop safety, and other specific information. An NDSU “one-stop shop” web site for online weed control information is www.ag.ndsu.nodak.edu/weeds.   There, you’ll find links to the 2005 NDSU Weed Control Guide, online weed publication and identification, herbicide resistance, and product label search.  Another resource is the University of Minnesota Weed Science Department, http://appliedweeds.coafes.umn.edu , also with links to herbicide resistance and weed ID information, and the 2005 U of M weed control guide.

How Weeds Can Affect Wheat

• Ten wild oats or wild mustard plants per square foot will reduce wheat yields 10-20 bushels per acre or 35%.
• Two to three kochia plants per square foot can reduce yields 30%.
• Canada thistle patches often reduce yields by 60%.
• Green foxtail can reduce yields 10-15% when wheat is planted late. (Source: NDSU)

How Weeds Can Rob Wheat Yields

Weeds/Sq. Yard	Foxtail	Wild Oat
% wheat yield reduction
10	0	8-9%
50	4-5%	18%
75	6-7%	25%
100	8-9%	34%
150	15%	40%

Strategies To Minimize Herbicide Resistant Weeds

• Scout fields regularly and identify weeds that escape herbicide treatment. Monitor changes in weed populations and restrict spread of potentially resistant weeds that match the field history and herbicide pattern.
• Rotate herbicides with different modes of action.
• Apply herbicides in tank-mix, prepackage or sequential mixtures that include multiple modes of action. Two or more herbicides in the tank-mix must have substantial activity against potentially resistant weeds. Most commercial premixes do not contain herbicides that target the same weed species.
• Rotate crops, particularly those with different life cycles; for example, winter annual crops (winter wheat), perennial crops (alfalfa), summer annual (spring wheat, corn or beans). Don’t use herbicides with the same mode of action in the different crops unless other effective control practices are also included.   

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New Bayer CropScience Grass Herbicides

Bayer CropScience has followed up its release of Silverado with two more grass herbicides,

1. Olympus, with labels for use in the Dakotas and Montana (no label in Minnesota)
2. Osprey, labeled for use on winter wheat in South Dakota, Montana, and Pacific Northwest States.

Olympus offers cost-effective grass control, including wild oat and winter annuals such as cheatgrass, downy brome, and Japanese brome. It has a wide application window in fall or spring treatments, and good tankmix flexibility with common broadleaf products such as MCPA and 2,4-D. Activation under low rainfall conditions means better weed control with less risk, with less soil persistence than other herbicides that allows for more flexible rotation.

Osprey is specially developed for use in winter wheat for postemergence control of a broad spectrum of difficult weeds such as all types of annual (Italian) ryegrass, wild oats, Persian darnel, and certain broadleaf weeds. It has good tankmix options, a wide window of application, and allows for full flexibility to rotational crops that follow winter wheat.

Silverado has a broader label registration, for postemergence control of wild oat (and ACCase resistant wild oat) mustard control, and green foxtail suppression in winter, spring , and durum wheat in Minnesota, South Dakota, North Dakota, and eastern Montana. It provides multiple broadleaf weed control options to accomplish one pass weed control.

For more information about these and other BayerScience products, go to www.cerealexperts.com