Some hay producers have been unpleasantly surprised in the past when cressleaf groundsel infestations became evident in their hay fields in May prior to first cutting. Cressleaf groundsel in hay or silage is toxic to animals, and infested areas of the field should not be harvested and fed. Groundsel is a winter annual, emerging in late summer into fall, when it develops into a rosette that overwinters. Growth restarts in spring, with stem elongation and an eventual height of up to several feet tall. The weed becomes evident in hay fields when in becomes taller than the alfalfa/grass and develops bright yellow flowers in May. The problem with passively waiting until this point to discover that the hay is infested with groundsel is that: 1) it’s too late to control it with herbicides; and 2) hay from infested areas has to be discarded instead of sold or fed, and large plant skeletons are still toxic even if herbicides were effective on them. Groundsel plants finish their life cycle in late spring, once they flower and go to seed, so it should not be problem in subsequent cuttings.
The solution to this is scouting of hay fields in fall and early spring to determine the presence of cressleaf groundsel, when it is small and still susceptible to the few herbicides that can be used. We expect groundsel to be more of a problem in new August seedings, since it would be emerging with the new stand of alfalfa/grass. A well-managed established and uniform hay crop should be dense enough to largely prevent problems with winter annuals although there have been exceptions. Groundsel will be most easily controlled in the fall while in the rosette stage. Controlling plants in the spring is more difficult, because of cold conditions in early spring when plants are still small, and increased tolerance to herbicides as stems elongate.
Herbicide options are more effective in pure grass hay stands or grass pasture compared with legumes, and it’s not possible to control groundsel in a first-year legume/grass seeding without incurring injury to the grasses. Any treatment containing 2,4-D should be effective in grass hay or pasture. Apply 2,4-D (1 qt/A) in late October or early November. Low-volatile ester formulations can be more effective than amine formulations, but the latter are less likely to volatilize and injure nearby sensitive broadleaf vegetation. This treatment can also be effective in spring if applied in late March or early April when the rosettes of groundsel are less than several inches in diameter. Larger plants are more tolerant of 2,4-D, and effective control will require a mixture of dicamba (e.g. Banvel, Clarity, Sterling) and 2,4-D. Desirable legumes in the pasture will be injured or killed by any of these treatments.
In alfalfa fields, the most effective treatments are:
- Metribuzin (1.3 lbs/A of 75% DF) or Velpar (2 to 3 qts/A) applied in late February when alfalfa is still dormant. These herbicides can be applied to established alfalfa only (more than one year old). Metribuzin can be used in fields that have established grasses in addition to the alfalfa. Do not use Velpar in fields with desirable grasses or fields that will be rotated to another crop within the next two years.
- Pursuit (2.16 oz/A) can suppress groundsel when applied in late fall or early spring. Fall applications are likely to be most effective. Plants should still be in the rosette stage and less than 3 inches tall at the time of application. In the spring, apply during periods of relatively warm weather – daytime temperatures above 60 degrees F and nighttime temperatures above 50 degrees F. Include the appropriate spray adjuvants per the herbicide label. Pursuit can be used in seedling or established alfalfa, but alfalfa seedlings must have at least two trifoliate leaves at the time of application. Do not use this treatment where desirable grasses are present unless injury to grasses is acceptable.
- Glyphosate (0.75 to 1.5 lb ae/A) can be applied in fall or spring to Roundup Ready alfalfa. Fall applications will be most effective. Plants should be small at time of application – no more than several inches tall. Spring applications will be most effective during periods of warmer weather.
- Glyphosate can be applied as a spot treatment in the spring in any legume or legume/grass hay field. This treatment will injure or kill all vegetation in the treated area and should be used only when all other control measures have failed.
The links below provide more information including information on toxicity, biology, and control and photos of various plant stages for ID purposes: