The best practice, when using solid or liquid manure on alfalfa, is to apply when the crop is dormant, in the winter or early spring prior to green up, (abiding by local application regulations) or immediately after cutting the alfalfa.  Most manure is broadcast applied on forage crops, however, if local regulations require knifing, you may have to risk stand damage.  Some additional cautions, the high amount of nitrogen (N) in some manure may cause the Rhizobia bacteria that fix N from the air to be less effective.  Remember, dry feedlot manure, beef or dairy, may contain weed seed.  Don’t be surprised if you see weeds and grass appearing in the alfalfa after a manure application.  Monitor soil accumulation of phosphorus (P) and potassium (K) since those can build-up to excessive levels with very heavy rates of application.  Also, check for soluble salts in the soil as those can increase to the point that they are detrimental to alfalfa growth.