Spring Hay and Pasture Weed Issues

May 30, 2019

When controlling weeds in forages, herbicide application (or mowing) timing is critical and can make the difference between an effective tactic and a waste of time and money. Once the weeds are in the “setting seed” stage a herbicide application will not be effective and the seeds will most likely continue to develop and be viable. Winter annuals including yellow rocket and many of the other mustard species, common chickweed, marestail, fleabane, annual bluegrass, downy brome, etc. are growing rapidly and have already or will begin to flower and set seed very soon. Biennials including musk and plumless thistle, burdock, wild carrot, etc. should be treated before they begin to bolt (they are bolting now or very soon) and the smaller the better. (Late fall or early spring is really the best time to treat them.) And finally, control summer annual weeds as soon after they emerge as possible when they are most sensitive to chemical control. The most common herbicides used for control of many broadleaf weeds in grass hay/pasture this time of year are the plant growth regulator herbicides such as 2,4-D, dicamba (Banvel, Clarity, etc.), triclopyr products (Crossbow, Garlon, etc.), and clopyralid (Stinger, PastureGard, etc.). In addition, products containing metsulfuron (Cimarron, other generic formulations, etc.) can provide good control of many broadleaf weeds in the spring. (Be cautious, if forage grasses were recently seeded and are not yet established many of these herbicides can cause severe crop injury.) See Table 6-11 " Relative effectiveness of herbicides used in grass pasture, hay, and CRP grassland" from the Penn State Agronomy Guide or the Mid-Atlantic Weed Management Guide for additional details. Below are some guidelines to provide a quick management summary for some common weeds of pasture.

Management guidelines for some problem weeds of pastures.
Winter annuals (Mustard species, common chickweed, etc.)

Mow after bolting to prevent seed production.
Apply an effective herbicide in fall or spring prior to bolting.
Most winter annuals emerge by late fall - a smaller percentage will emerge in early spring
Prevent seed production to prevent spread.

Summer annuals (Pigweed species, common lambsquarters, common ragweed, etc.)

Keep pasture full and competitive.
Mow after bolting to prevent seed production.
Apply an effective herbicide in early summer.
Prevent seed production to prevent spread.

Biennials (common burdock, bull and musk thistle, poison hemlock, etc.)

Mow after plants have bolted but before seed set to prevent seed production.
Remove or dig individual plants by hand.
Apply an effective herbicide to rosettes in the spring or fall.
Prevent seed production to prevent spread.

Creeping perennials (Canada thistle, horsenettle, etc.)

Mow to suppress vegetative growth and prevent seed production.
Spray with an effective systemic herbicide at bud to bloom stage or in early fall prior to frost.
Most perennials spread by both seed and vegetative structures.

Woody perennials (multiflora rose, autumn olive, etc.)

Mow to suppress and prevent seed production - remove roots by hand or with heavy equipment.
Spray with an effective systemic herbicide at bud to bloom stage or in early fall.
Rose rosette disease has spread as far North as Central PA and may impact this weed in the future.

Multiflora rose control in pastures.

As spring progresses, multiflora rose begins its growth and eventually will bloom in late May/early June. Certain herbicides can provide good control of multiflora rose, especially when applied during the bud to bloom growth stages. Several foliar applied herbicides are suggested for late-spring/summer including Cimarron Plus/metsulfuron, Crossbow/Remedy, and glyphosate.

Herbicides in new grass and legume seedings.

Herbicide selection for new forage grass and/or legume seedings are limited. Most herbicide labels for grasses like orchardgrass, timothy, etc. state that the grasses should be well established with at least 4-5 inches of growth. Some labels are more restrictive than this. The metsulfuron label states that grasses should be established for at least 6 months prior to an application. This ensures that they are developing a solid root system that could tolerate potential stress from the herbicide. Herbicide selection, formulation (ester vs. amine) rate, and environmental conditions at application will all impact the potential for injury. Herbicide use in new legume seedings has similar concerns. In general, legumes should have several trifoliate leaves and be 2 to 3 inches tall before making an application. The Pursuit and Raptor labels state that alfalfa should have 2 trifoliates or larger at application; two of the most lenient products for new alfalfa seedings. Also, keep in mind that while we want our crop to have sufficient growth, our targets are generally “small” annual weeds which are easier to kill – a bit of a paradox. Finally, products are even more limited with mixed seedings. Pursuit is labeled for established alfalfa-grass mixtures as is metribuzin (Dimetric, TriCor, etc.); MCPA and Buctril are labeled on legume/small grain companion seedings, and several other products have labels for CREP. Prowl H2O now has supplemental labels for use in cool- and warm-season forage grasses to control certain annual grasses and broadleaf weeds. Keep in mind, this herbicide only controls weeds before they emerge. If weeds have already emerged an effective foliar herbicide will need to be tank-mixed with Prowl H2O. However, there are no herbicides labeled to control existing weedy grasses in grass hay or alfalfa/grass mixes. Additional details can be found at: Cool-season grass forage label.

Poisonous pasture weeds and livestock

Even though we have had moist conditions for good pasture growth thus far, we are receiving calls about concerns for poisonous weeds in pastures and hay. Livestock may be forced to graze on weeds that normally they would not, or they may eat weeds out of curiosity. Scout your pastures and remove these weeds before they cause livestock health problems. Keep in mind there are numerous poisonous plants that could invade an area or pasture. Many plants contain potentially poisonous substances that may be toxic to livestock if consumed. In addition, certain plants may be problematic because of mechanical irritation when eaten, photosensitization, and disagreeable tastes or odors in meat, milk or milk products. If you suspect livestock poisoning, call your local extension educator or veterinarian immediately. If death occurs, the stomach contents should be examined for consumed herbage. Identify the suspected plants and remove livestock from the grazing area until all poisonous plants have been removed or destroyed. The table below lists only some common weeds and their poisonous properties; many other plants can be toxic to livestock.

Key points about weed forage quality and poisonous plants:

Some weeds have excellent nutritive quality.
Weeds in the vegetative stage of development usually are more nutritious than more mature weeds.
Regardless of weed quality, livestock may avoid grazing certain plants because of taste, smell, or toxicity.
Some plants contain potentially poisonous substances that may be toxic to livestock if consumed – properly identify potential problem weeds and consult with a veterinarian if necessary.
A productive pasture is important to reduce the potential incidence of toxic weed exposure to livestock. Remember to soil test and maintain the proper lime and fertility levels. If possible, routinely mow or spray to manage weed problems within and around pasture area.

Source:psu.edu