Now is the time to scout hay and pasture fields for the presence of winter annual and biennial weeds, especially those that are poisonous to livestock such as cressleaf groundsel. These weeds are resuming growth that started last fall and they are most effectively controlled with herbicides while still small. In addition to cressleaf groundsel, weeds of concern that should be treated soon include the following: poison hemlock, birdsrape mustard (aka wild turnip), wild carrot. Herbicides are most effective on these weeds in the fall, but they can be controlled in spring, preferably when still in the rosette stage. Control becomes more difficult once stem elongation (bolting) starts.
Options for control in pure legume stands:
- Pursuit – 3-6 oz
- Raptor – 4-6 oz
- 2,4-DB (Butyrac etc) – 1-3 qts
- bromoxynil – (first-year alfalfa only) – 1-1.5 pts
- glyphosate (RR alfalfa) – 0.75-1.5 lb ae
- Extreme (RR alfalfa, glyphosate + Pursuit) – 2.2 – 4.4 pts
Notes: higher rates are going to be generally more effective; poison hemlock or wild carrot cannot be controlled in nonRR alfalfa; combination of Pursuit/Raptor + 2,4-DB will be most effective on birdsrape mustard and cressleaf groundsel in nonRR alfalfa (and control of groundsel will likely be variable).
Options for control in mixed legume/grass stands:
- 2,4-DB (Butyrac etc) – 1-3 qts
Note: 2,4-DB is almost an herbicide on a good day when applied alone, and will not control any of the weeds mentioned here. Check ratings in the OSU Weed Control Guide before spending the money.
Options for grass hay and pasture are more numerous and should be generally more effective. Information on all of these, along with ratings, can be found in the “Permanent Grass Pastures/CRP/Grass Hay” section of the weed control guide. A few comments:
- Most herbicides labeled for these uses have activity on winter annuals, including mustard species, and we assume would be effective on birdsrape mustard.
- With regard to cressleaf groundsel, there is a lack information on effectiveness of many grass hay/pasture herbicides. A study at the University of Illinois across 7 environments determined that dicamba was ineffective on groundsel, while 2,4-D was effective only when applied in the fall. Fall or spring application of site 2 sulfonylurea herbicides, chlorimuron + tribenuron, was effective. While this mixture is not labeled for use on grass hay and pastures, another sulfonylurea herbicide – metsulfuron – is a component of some pasture herbicides. Metsulfuron is effective on a broad-spectrum of winter annual and biennial weeds, so we assume it would be effective on groundsel as well.
- Poison hemlock is most effectively controlled with products that contain triclopyr, such as Crossbow and Remedy Utra. Cimarron Max and products containing higher rates of dicamba also have activity.
Other weeds that can be treated in mid to late spring include bull thistle, a biennial, and spotted knapweed, smooth bedstraw, and Canada thistle, which are pernnials. None of these are effectively controlled in legume fields, with the possible exception of glyphosate used in RoundupReady alfalfa. They can be controlled in grass hay and/or pastures, although options may be most numerous in pasture due to restrictions on hay harvest for certain herbicides. We conducted a study of smooth bedstraw control in grass hay at two sites in eastern Ohio, with herbicides applied in mid-April to early May after full leafout. Crossbow (1 qt) provided the most effective control and reduction in bedstraw population the following year. Application of metsulfuron (0.4 oz), Cimarron Max, and the combination of dicamba + 2,4-D controlled the topgrowth through the season, but these treatments were less effective at permanently reducing the population. Spotted knapweed control options include:
- Stinger/Transline – after basal leaves up to bud stage
- Milestone/GrazonNext HL – fall or spring, rosette to bolt stage
- 2,4-D + dicamba – early bolting stage
- Curtail – fall or spring – rosette to mid-bolting