First, horses do not have the health issues with high fat diets that humans do. Horses can be fed up to 15% of their diet as fat without problems. Also the body takes a couple of weeks to adjust to a higher fat content in the diet or a higher protein content in the diet, so constantly switching your diet from high fat to not high fat doesn't really allow you to get all the benefit of a high fat diet. Calf Manna is not really that high in fat but it is high in protein which is a less effective source of digestible energy for weight gain. You would be better off adding straight corn or vegetable oil to his diet and cutting out the Calf-Manna as far as weight gain is concerned.
Ok, you say he's 16h and has never gone over 1200lbs--this may or may not be too little weight for your horse. What breed? Can you describe the way he looks? Pictures would be extremely helpful. (If you've already posted pictures, point me to the thread.)
How old is he? EXACTLY what are you feeding him--hay by weight, is he on good pasture, concentrate feed other than beet pulp and calf-manna? Has he had his teeth checked recently? How often do you deworm and with what?
Registered Veterinary Technician