Wet soils in alfalfa fields right after cutting can lead to quick growth of weedy grasses like foxtail and crabgrass, particularly in a thin alfalfa stand.

Management of these weeds begins early, with a thick stand with good soil fertility so the field will compete aggressively with invading grasses. In addition, harvest alfalfa only after it begins to bloom or when new shoots appear at the base of the plants.  Then alfalfa should regrow rapidly so grasses don't have much time to become established.

While this sounds obvious, it's not easily done and can have a cost. Harvesting alfalfa after bloom begins may sacrifice some forage quality.

Herbicides are another option.  Roundup works great, but only for Roundup Ready varieties.  In conventional alfalfa, two post-emerge herbicides that control annual grasses well are Select Max and Poast Plus.  These herbicides work well on seedling grasses that are less than 4 inches tall, and alfalfa tolerates both herbicides very well.  If you see these grass seedlings after you cut your alfalfa, apply the herbicides soon after harvest and before alfalfa regrowth forms a canopy.  None of these herbicides has any soil residual activity, so good plant coverage of the weedy grasses is necessary for control. You may need repeat spray after the next harvest if new weeds emerge.