Wet soils in alfalfa fields right after cutting provide a good opportunity for foxtail and other weedy grasses to grow competitively with the alfalfa regrowth.
The best way to reduce this threat is to keep your alfalfa thick and thrifty so it will compete aggressively with invading foxtail. Thick initial stands and good soil fertility are needed. In addition, harvest alfalfa only after it begins to bloom or when new shoots appear at the base of the plants. Alfalfa should regrow rapidly so foxtail doesn't get much time to become a problem.
Unfortunately, this method often is easier said than done as delaying harvest until after bloom begins sacrifices some forage quality.
Herbicides are another option. Roundup® works great, but only for Roundup Ready® varieties. In regular alfalfa, Select Max and Poast Plus often are used to control annual grasses. These herbicides work well on seedling grasses less than 4 inches tall and are well tolerated by alfalfa. A riskier herbicide option is Gramoxone Extra. Gramoxone burns back top growth of all green plant material. This will kill most annuals like foxtail, but it also can injure alfalfa regrowth. Therefore, only use Gramoxone immediately after harvest and before any new regrowth shoots appear.
None of these herbicides has any soil residual activity, so good plant coverage is necessary and repeat sprayings may be necessary if new weeds emerge.
Source: unl.edu