Christine King BVSc, MANZCVS (equine), MVetClinStud
Crude Protein (CP) Content
This test is really just an analysis of the nitrogen content of the forage, so it's well-named, as it's a very crude measure of protein content. Not all of the nitrogen in the forage is in the form of protein, particularly if a chemical fertiliser, containing an inorganic form of nitrogen, was used to enhance plant growth.
But for simplicity's sake, let's assume that the CP content equals the protein content. We won't get into how much of that protein is available for digestion and absorption by the horse. Instead, we'll stick with the facts and figures that are used most often.
Good quality grass hays have a CP content of between 8% and 12%; most are in the 10–11% range.
Good quality legume hays (e.g., lucerne) have a CP content of between 16% and 24%; most are in the 18–20% range.
Why is CP content important?
Dietary protein is essential for all body functions, and even more is needed for growth, recovery from illness or injury, and regular exercise. However, most horses don't need as much CP as the heavily-promoted, highly-processed feeds provide.
Generally, healthy adult horses who are not pregnant or lactating, and who are in only light work, do well with diets that have a CP content of between 8% and 10%. So, a good quality grass hay ably meets the CP needs of this group of horses.
Growing horses as well as pregnant or lactating mares need greater amounts of dietary protein, but generally not much more than about 14% CP from all sources (i.e., the total diet is about 14% CP).
Adult horses recovering from serious illness, injury, or surgery may also benefit from a higher CP intake, although the 10–12% range is generally enough in these circumstances. Senior horses, too, may need this higher CP intake because they become less efficient at using and retaining protein with age. Compounding the problem in seniors with equine Cushing's disease (pituitary pars intermedia dysfunction or PPID) is that this condition causes muscle wasting if untreated.
There is yet another group of adult horses for whom I advise increasing the CP content of the diet a little: those who are overweight or obese, particularly if they already have laminitis. These horses need a strictly calorie-controlled and 'low-carb' diet, but too often this goal is met by feeding a poor quality hay. Such a diet is lacking in the protein needed for maintenance, let alone repair, and unless supplemented it is generally lacking in vitamins and minerals as well — not to mention interest, enjoyment, and satiety! While the horse may lose weight on this diet, such a diet is not conducive to good health, so 'inmates' on this 'prison' diet never fully recover. And when the horse inevitably breaks out for a good feed, the result is all too often disastrous.
In performance horses of all types and disciplines, the more demanding the exercise program, the higher the daily CP needs. Both the bone and muscle development that occurs with regular training and the demands of recovery post-exercise increase the body's protein needs. However, the horse's CP needs generally don't exceed amounts that can be provided by good quality grass hay until the horse is in the 'heavy' or 'very heavy' exercise category.
While on the subject of performance, it is worth noting that an excess of dietary protein may interfere with the horse's fluid and acid-base balance, potentially increasing the horse's water requirements and making the blood more acidic. If water intake is limited for any reason, or water losses through sweat and respiration are greatly increased, excessive dietary protein may limit performance or contribute to early onset of fatigue during exercise, which is when mistakes happen that can lead to injury. This dynamic is most likely to have an impact on horses exercising in hot weather.
There is also some indication that an excess of dietary protein may interfere with calcium balance in growing horses, resulting in calcium loss. Enough protein really is enough; and more than that may be too much.
Grass hays with a CP content much lower than 8% usually are either too mature for horses (stemmy, coarse, with relatively low palatability, digestibility, and nutrient content) or they were grown in soil too lacking in nitrogen and potentially other nutrients to make good hay for horses.
By the same token, grass hays with a CP content much higher than 12% either have some legume mixed in (which is generally not a problem) or the hay field was 'goosed' with a lot of inorganic nitrogen fertiliser to make the grass grow quickly and the resulting hay look good. However, unless the haygrower has been tending the soil well, this sort of rapid, nitrogen-fueled plant growth may occur at the expense of nutrient (particularly mineral) content.
Legume hays such as lucerne naturally have a much higher CP content than grass hays, so legume hays make good protein supplements for situations in which the CP (and/or calcium) content of the horse's diet needs to be a little higher.
Otherwise, legumes (as hay, chaff, cubes, or pellets) are optional. That said, I do like to include a little legume in the horse's diet to satisfy the 'variety quotient' that is so often lacking in modern horse diets.