By Mark Loux, Ohio State University Extension herbicide specialist
The next month and a half or so is an ideal time to control a number of weeds that cause problems in hayfields and pastures, and also certain weeds in fencerows and other areas adjacent to fields. We discussed scouting and fall control of cressleaf groundsel in a C.O.R.N. article last fall, to avoid problems with the toxicity of this weed in hay next year. Many of these weeds are most problematic in new hay and forage seedings, since the crop may not yet be dense enough to suppress them without the help of herbicides. A number of winter annuals fit into this category — mustards, marestail, pennycress, chickweed. For biennials such as wild carrot, poision hemlock, burdock, and teasel, the low growing plant after the first year of growth, which is present now, is more susceptible to control with herbicides compared with plants with elongated stems in spring. And it’s certainly a good time to go after dandelion, Canada thistle, and curly dock.
Fall herbicide options for grass hay and pastures, and non-crop areas, are considerably greater in number and often also effectiveness than those labeled for use in a first-year legume or legume/grass stand. For example, herbicides for a new stand of pure alfalfa include 2,4-DB (Butyrac), Pursuit, Raptor, and clethodim. The mixture of grasses and legumes removes all of these options except 2,4-DB, which we have sometimes characterized as “almost an herbicide on a good day.” A bit of an exaggeration, but it has a very limited spectrum of control and weed size range. In an established stand, dormant application of metribuzin or Velpar can also be an effective option. Glyphosate is of course an option in a stand of pure RR alfalfa (if you can get it). There are a number of more effective options in grass hay and pasture. Most of the herbicides in the pasture section of the OH/IN/IL Weed Control Guide can be used for grass hay also, as long as they specify a minimum interval between application and cutting for hay. The absence of legumes allows use of products and premixes containing 2,4-D, dicamba, metsulfuron, triclopyr, and aminopyralid. Be sure to understand the restrictions on feeding or grazing aminopyralid-treated hay or areas prior to use.
Poison hemlock deserves specific mention here because it got a lot of press in Ohio this year. While it has substantial toxicity when ingested, and can cause reactions on skin of sensitive individuals, it’s otherwise fairly benign. It has been fairly endemic to southern Ohio for a while, and is apparently creeping north.
In addition to toxicity to animals when ingested, cressleaf groundsel and poison hemlock share the property of being weeds that appear to “all of a sudden” show up in spring, when they were really present the previous fall. Herbicides are more effective on these weeds in the fall, but there is a general lack of awareness and scouting for them at that time of the year. Waiting until spring to control them, when they become clearly evident, increases the difficulty of control. And killing sizable plants in spring results in dead plants that are still toxic, which does not resolve issues in hay. Herbicides containing triclopyr (Remedy Ultra, Garlon, numerous others) or triclopyr plus 2,4-D (Crossbow) are most effective in controlling poison hemlock. Other herbicides that provide adequate control when applied at the proper timing are dicamba (Clarity, numerous others), metsulfuron-methyl (Escort XP), metsulfuron-methyl plus dicamba plus 2,4-D (Cimarron Max) and clopyralid plus 2,4-D (Curtail).