Problematic Weeds in Northern Utah

    Problematic Weeds in Northern Utah

    Weeds in a northern Utah Field





    Looking Back to Plan Ahead for Weeds
    Each year of crop production can be a learning experience based on management successes and failures.  Reflecting on the previous year provides an opportunity to adjust weed management approaches to increase success the next year.  Obviously, environmental conditions vary widely between years and can have large impact on the species and density of weeds in the early spring.  Two species that were problematic in northern Utah in 2019 were dodder in alfalfa and invasive annual grasses in irrigated pasture.  These two species will be the focus of this article because potential management approaches between the two are similar.
    Dodder is an annual parasitic weed that looks like orange to yellow twine or string.  Being parasitic, it does not generate its own energy but draws all its nutrition by attaching to host plants (alfalfa in this case), penetrating the stems and stealing the host plants nutrition.  Dodder steals enough energy from alfalfa that its growth is severely stunted, impacting yields.  Left uncontrolled, dodder produces large amounts of seed that persist in the soil for decades.  Once dodder becomes attached to alfalfa plants, removal is difficult since the parts that have penetrated the stem remain imbedded even if the twining stems are removed.  Dodder can re-sprout from those parts left imbedded in the stem.  Mowing has been utilized to set back dodder but is challenging as some of the attachment points are near the soil surface.  Seed growers will desiccate infested plants by spraying them with a contact herbicide that dries out all the foliage and then burn those alfalfa plants with a propane torch prior to harvest to ensure the dodder seed is not picked up by the combine.  An effective management approach for dodder is to apply a preemergence herbicide that will control the dodder as it tries to germinate.  Products such as trifluralin (Treflan) or pendimethalin (Prowl H2O) can be effective in controlling dodder when applied at the right time and rate. In fields with known heavy populations of dodder, the planting of Roundup Ready alfalfa and the use of glyphosate (Roundup) can provide effective dodder management.  Preventing seed production of dodder is critical for reducing future problems with this challenging weed.  If you do not have dodder on your farm, purchasing certified seed (dodder-free) is important to ensure you don’t bring this pest to your fields.
    Invasive annual grasses in irrigated pastures
    Another weed management problem in 2019, was the increase of invasive annual grasses in irrigated pastures.  In many cases this occurred in pastures with high water tables and may be interrelated with prolonged soil saturation and salt accumulation, which reduced desirable perennial grass health.  One infestation was identified as Mediterranean barley (Hordeum hystrix), which is closely related to other grasses that are problematic in some pastures in Utah (hare barley, foxtail barley, etc.).  Obviously, it can be challenging to control a grass weed in a grass pasture.  With annual grasses, management approaches include preventing seed production and inhibiting germination.  While few herbicide options exist, recent label changes may offer opportunities to control annual grasses in grass hay and pasture settings.  Research has shown that low rates of glyphosate (Roundup) can control annual grasses with minimum impact to perennial grasses when applied while the perennial grasses are dormant in late fall or very early spring.  The annual grasses are actively growing during the winter and are quite susceptible.  This type of treatment should be made according to the label and with great care as higher doses than recommended may seriously injure the perennial pasture grasses.  Another approach, similar to that for dodder control in alfalfa, is the use of preemergence herbicides.  The challenge to this approach is that the invasive annual grasses are generally germinating in the fall.  Preemergence herbicides would need to be applied prior to germination in order to be effective.  We have not had opportunity to test either approach on the invaded fields and hope to have more information in the future.

    Key Point: Control of both species will require management decisions to be made prior to the appearance of the weeds in the field. So please plan ahead!

    - Dr. Corey Ransom (USU Extension Weed Specialist)

    Published on: Jan 23, 2020

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