I have been a faithful Calf Manna user for big horses and minis for 30+ years. It was around a LONG time before there were such things as "ration balancers" which also list protein at about 30% or more. The trick is that you are adding it to the regular diet in such small amounts (generally just ounces per day), that it does little to alter the overall protein percentages across the diet as a whole. In other words, if you are feeding an average grass hay at about 8% protein, you probably NEED to add a supplement that is high in protein to bring the overall dietary protein percentage up to the 10-12% range. Broodmares in late gestation and early nursing phase will need even more protein in the diet (14-16%).
I routinely use Calf Manna for all of my broodmares starting their last 30 days and the first 60 days of nursing. It is also my foals' first feed, wetted into a mash. It is also my first line of treatment for any underweight rescue horses. Most equines find the taste irresistable (probably the ferungeek which gives it a bit of a licorice smell).
Do be careful, however, when offering it to overweight minis, previously foundered horses or horses with recurrent laminitis or other metabolic issues. The main difference between Calf Manna and ration balancers is the starch values. Calf Manna previously used animal protein as its protein source, but thankfully has changed their formula to utilize soy instead. However, CM is HIGH in starch as the pellets contain a good amount of ground corn. This is probably why is it so good at building and maintaining weight, but a high starch diet may be detrimental to horses with metabolic issues. One way to boost the protein percentage and nutrient levels for these types of horses is to add a small amount of ration balancer to their regular mare/foal feed or add additional flax seed (2-3 oz per day). The ration balancer will pump up the protein a bit, plus add some additional vitamins/minerals but WITHOUT the added starch. The flax seed will provide extra calories and protein.
Robin C