Spring Weedy Grass Control in Grass Hay and Pasture

The forsythia is starting to bloom in certain areas of the state. This is a great natural indication that it is time to make applications of residual herbicides to control germinating summer annual weeds, especially grasses. We are receiving more calls about how to control weedy annual grasses such as crabgrass, foxtails, panicum, Japanese stiltgrass, etc. and others in grass forage settings. Unfortunately, these weedy grasses start to germinate at different times in the spring. Japanese stiltgrass can germinate in early spring, a couple weeks or more before crabgrass and the other species, thus an early application of pendimethalin will be necessary for initial control. Large crabgrass and the other mentioned grass species typically start to germinate around 200 GDD (growing degree days); however, Japanese stiltgrass can begin its germination earlier. Currently, pendimethalin (Prowl H2O and Satellite HydroCap) is an option to consider for controlling certain annual grasses and broadleaf weeds in cool and warm season forage grass settings.

Prowl H2O or Satellite HydroCap may be applied to established perennial forage grasses (including Kentucky bluegrass, bromegrass, tall fescue, orchardgrass, perennial ryegrass, timothy, switchgrass, and others) grown for forage, green chop, silage, hay production, and/or grown in pastures for livestock grazing.

• Apply at a broadcast rate of 1.1 to 4.2 quarts of product per acre in a single application or sequential applications made 30 or more days apart. Herbicide must be applied before weed germination in spring, or in-season between cuttings, otherwise weeds will not be controlled. These products may be tank-mixed with other labeled herbicides, but keep in mind that nothing is labeled for control of emerged grasses in grass forage.
• Split applications of pendimethalin are better than a single, high-rate early-season application. Make the first application in early spring (mid-March to early April) but before weed germination (2-3 pt/A); and then the second application right after first (or second) cutting (3-4 pt/A).
• These herbicides may be applied to mixed stands of established cool-season forage grasses and alfalfa (established alfalfa is defined as alfalfa planted in fall or spring, which has gone through a first cutting/mowing). Do not apply to mixed stands of cool-season forage grasses with other forage legumes (e.g., clover) besides alfalfa.
• There is no preharvest or pre-grazing interval for pendimethalin-treated grass forage, green chop, silage, hay, or pasture.
• Mixed stand alfalfa/cool-season forage grasses may be grazed or harvested for forage or hay 14 or more days after application.
• One thing to note about pendimethalin is that it requires about an inch of rainfall within a week or so after application for it to be activated. In some cases, there might be enough moisture from dew or light showers to prompt weed seed germination, but not enough to properly incorporate the herbicide so poor weed control might result. In this case, other control tactics will need to be initiated at some point during the growing season.

Keep in mind that pendimethalin products do not control established perennials like roughstalk bluegrass (Poa trivialis), which is becoming more of a problem in small grain and forage fields across the state. Fall and early spring are the best times to apply effective herbicides to control this weed; unfortunately, there are no effective products that control roughstalk bluegrass in grass forages. However, in alfalfa settings, the use of Gramoxone/paraquat, Raptor, or Select/clethodim after the first cutting generally provides 85-90% control.

Also, now is the time to scout grass pastures and hayfields in search of winter annual and biennial weeds.  Winter annuals, including the mustard species, common chickweed, horseweed/marestail, deadnettle, henbit, fleabane, etc., are growing rapidly and have already or will begin to flower and set seed very soon. Biennials, including bull, musk, and plumeless thistle, burdock, poison hemlock, wild carrot, etc., should be treated before they begin to bolt and the smaller the better. (Late fall or early spring is really the best time to treat them.) Both of these types of weeds are potentially susceptible to control right now and an effective herbicide application will prevent flowering and seed production.

Management of perennial broadleaf weeds such as dandelion, Canada thistle, horsenettle, hemp dogbane, milkweed, smooth bedstraw, and the woody perennials such as multiflora rose and autumn olive is best performed a bit later in early summer after plants reach the bud-to-bloom stage.  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 (Clarity, etc.), triclopyr products (Crossbow, Remedy Ultra, 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. Another new herbicide to consider for grass stands that include white clover is NovaGraz from Corteva. It preserves white clover but controls many common broadleaf weeds. However, it tends to be weak on many of the problem perennials mentioned above. (Be cautious, if forage grasses were recently seeded and are not yet established, as many of these herbicides can cause severe crop injury.) Refer to Table 2.6-11 in the 2025 Penn State Agronomy Guide for ratings of these herbicides on many different weed species.