Dave,

You've got potato leafhoppers. And if it is turning yellow, you are already too late. If it has "just a tinge of yellow", you could spray it and save the cutting. However, if it is really yellow, the best course of action is to mow it. Look for leafhoppers in the regrowth and spray the regrowth if needed.

Information on pest management in alfalfa

Another good source- Your local Extension office has a good publication on pest management in alfalfa called:  A Pest Management Program for Alfalfa in PA.  Stop in and pick up a copy.

There is a professional crop management service in your area. I encourage you to check it out. They can really help you with decisions such as this. Questions? e-mail me.

Martin