Mowing Speed for Alfalfa

Hi, I'm new here, and kind of new to agriculture as well. What kind of ground speed do you travel at with a discbine? Seems as though it cuts great no matter how fast I go as long as my body can handle it. Our mowing tractor is a JD 4430 cab tractor, mow at WOT. Learning the machine I started out in B 1-2, and gradually increased speed as I found comfortable with making the corners at the end of the row. 

 

For those not familiar with these tractors, the gearing is A 1-2, A 3-4, B 1-2, C 1-2, B 3-4, D 1-2, C 3-4, and D 3-4 in progressive order of ground speed. I got up to C 3-4 whereas my boss only ever tried as fast as B 3-4. Beats me up quite a bit even with a quite level field, but it sure makes quick work. Cut is clean once I put new blades on all the heads, but it skips when it hits the gopher mounds as would be expected no matter the speed. C 4 I am guessing is probably 12 mph maybe a hair more. 

 

Seems to me as though the cut isn't quite as clean if I slow the ground speed down. Is it better to beat myself up a bit in the cab and go as fast as I can for the cleanest cut or is that hurting the machine to push it that far? 

Mowing Speed for Alfalfa