It's not the amount in a single feedstuff but the amount in the overall diet that you need to look at. A 28% protein feedstuff is probably a ration balancer and designed to be fed at a rate of 1-3 lbs per day. 1 lb of 28% protein feed fed along with 14 lbs of a 9% protein forage is going to give you a protein content of 10.26% for the overall diet.
To calculate protein content of the diet, use this formula:
[(wt of concentrate X %protein)+(wt of forage X % protein)]/ total weight of diet= % protein
So, you really need to know the protein content of the forage that is being used to know how much of any concentrate feed of a given protein content is going to be providing sufficient protein in your horse's diet. That being said, protein is not the biggest factor in weight loss or weight gain. In fact, there is only a slight change in protein requirements for heavy work or pregnancy. So if the diet is providing 10-12% protein then it's likely that there is simply not enough digestible energy being provided. This occurs when there is simply not enough feed stuff being given---generally a lack of forage is the main culprit. The first thing you should look at is just how much forage your horse is getting. Horses need a minimum of 1.5% of their body weight in good quality forage per day. For a 1000 lb horse that is 15 lbs of hay or grass a day. Some horses need up to 3% of their body weight in forage per day to meet their energy requirements. The Horse | Excerpt from Revised <i>Understanding Equine Nutrition</i>: Protein