We bought one of the first disc mowers sold in the area, a Vicon, and still have it though it s showing signs of age. If you refuse to mow at more than 5 mph, a sickle bar may best. Heavy crops of grass and disc mowers were made for each other. A field we used to mow at 2 mph with a sicklebar we now mow at 6 mph. Disc mowers are simpler in design and easier to maintain.
Rocks do bad things to all mowers, though.
Sicklebar mowers take less horsepower per foot of width, but this isn t usually a problem on most dairy farms. Since disc mowers are much faster, a 10 wide one will mow as much or more hay per hour as a 12 sicklebar. Some people worry that disc mowers damage alfalfa because it whacks off the stems rather than shearing them as a sicklebar does. However, since alfalfa regrows from the crown, not the cut stem, there should be no difference in plant recovery. Others fret that the rapidly spinning blades on a disc mower recut the bottom inch or so of the alfalfa stem, which drops to the ground thus reducing yield.