Corn gluten feed can be variable from batch to batch in energy content and quality, so a big part of the answer deals with the consistency of supply. If energy value varies by 15% from batch to batch, which isn’t out of the question, then you have the potential of over- or under-feeding, which would be like altering the amount of a consistent grain source without changing how much you feed. That would increase incidence of colic if you are feeding horses more than 3 or 4% of corn gluten feed per body weight per meal (3 or 4 lb for a 1,000-lb horse). So, keep amounts per meal low by mixing with something else (commercial companies do this), or be very careful about feeding large amounts in a meal or in group feeding situations that allow some to overeat. It will have a lot of phosphorus and not much calcium, so don’t feed a lot of it to growing horses in particular as mineral needs are not balanced. There is also concern with cattle about its potentially high sulfur content, but horses tend to tolerate higher levels of sulfur. So feeding an alfalfa-based forage (hay, cubes, etc.) would be beneficial to increase the calcium content of the overall diet and keep the calcium-to-phosphorus ratio in balance (about 2:1).