When soil moisture is available after a cutting of alfalfa, the grasses and weeds will grow just like their name suggests: like weeds. How can these plants be held at bay?
Thick alfalfa stands will be the first defense against foxtail, crabgrass, pigweed, etc. A good fertility plan to keep stands thick will go a long to reducing weeds. Sandy soils with pH levels above 7.2 may benefit from sulfur application. Conversely, if your alfalfa field pH falls below 6.7 then liming may be needed to prevent sharp yield drops.
Delaying alfalfa harvest until the next shoots are beginning to appear at the crown, may be another option to suppress weed growth. This allows the alfalfa to grow back a bit more quickly after harvest and be more competitive. Unfortunately, this means that the quality of the cutting will be lower since the plants will be more mature.
One more option is the herbicide route. With Roundup Ready varieties, Roundup can be used. In conventional varieties, grass control can be achieved using Select Max, Poast or Arrow if the grasses are less than 4 inches tall. If broadleaf control is needed, Pursuit or Raptor can be used on established alfalfa before the regrowth has 3 inches of new growth. These will set back alfalfa and the Pursuit label says it cannot be fed, grazed, or harvested for 30 days after application.
Weed problems in summer can be taken care of with herbicides, but shooting for a thick, robust stand will contribute to yield and weed control.