By Dwight Lingenfelter
Only a limited number of herbicides can be used in seedling forages. See below for additional details.
Herbicide selection for new forage grass and/or legume seedings are limited. Most herbicide labels (e.g., 2,4-D, dicamba, triclopyr) for grasses like orchardgrass, timothy, smooth bromegrass, etc. state that the grasses should be well established with at least 4-5 inches of growth and have a well-developed root system before application.
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 to ensure that they can 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, although we want the crop to have sufficient growth, the targets are generally "small" annual weeds which are easier to kill – a bit of a paradox. Products are even more limited with mixed seedings. Pursuit is labeled for established alfalfa-grass mixtures, as is metribuzin (Dimetric, MetriCor, etc.); MCPA (0.5 pints/A) has a label for use in seedling mixed stands of alfalfa, clover, trefoil with small grain or forage grass companion seedings. Butyrac (2,4-DB) and Maestro are technically not labeled for use in legume-grass seedling mixes. Maestro is labeled on legume-small grain companion seedings, and several other products have labels for CREP. Prowl H2O and Satellite HydroCap have supplemental labels for use in cool- and warm-season forage grasses to control certain annual grasses and broadleaf weeds.
Keep in mind, active ingredient only controls weeds before they emerge. If weeds have already emerged an effective foliar herbicide will need to be tank-mixed with Prowl H2O or Satellite. There are no herbicides labeled to control existing weedy grasses in grass hay or alfalfa/grass mixes, let alone in the forage seedling stage. However, Facet L (quinclorac) can be used in established cool-season grass systems (pastures or hay) to control/suppress certain grassy weeds (large crabgrass, foxtails, barnyardgrass, but not Japanese stiltgrass) that are less than 2 inches tall. Acclaim Extra, which can be applied in turf settings, is not labeled for use in forage grasses.
Source : psu.edu