Calf manna is used for a cheap energy and protein boost. It has cheap ingredients like lots of corn which encourage fat rather than muscle and coat development. Rabbits pack fat around their reproductive organs rather than the outer part of the body making them not breed well even when they appear to be in good condition. After having issues with mastitis I found other people saying it had a higher chance of causing mastitis than other energy sources.
Oats have the highest fat of any grain at up to 7%, pellets are around 3%, and BOSS blows it out of the water for kcals and omega fatty acids. One or both will give you a better result than calf manna for barely any more cost considering how much calf manna you'll end up throwing away because of the 50lb bags that you feed so little out of. A small handful of boss or oats and a sprinkling of flaxseed can make a very balanced, healthy supplement for improved flesh condition, coat, and milk production. We replaced grain for our horses with BOSS to make them healthier and lower cost. 1 cup of boss can replace 2 large scoops (4-6lbs) of grain mix.