Q. I recently ended up with a ton of nice-quality alfalfa. I have a couple of ulcer-prone horses and started supplementing their forage ration with it. Within a few days, my oldest gelding, 21, started peeing in his stall, which is a new behavior for him. And it’s a lot of pee! Is the alfalfa making my horse urinate more and changing in his “bathroom behavior”?

A. Alfalfa is nearly always higher in protein and calcium than grass hays, and research has shown that it is also a better stomach acid buffer. Adding some amount of alfalfa to the rations of horses prone to ulcers or who live a lifestyle that puts them at greater risk of ulcers might help reduce ulcer development. Alfalfa has other benefits over grass hay, such as providing more lysine, which might help some horses build and support topline, hoof quality, and the myriad of other bodily functions that require quality protein.

However, as with many things, more is not always better. In the Western United States, high-alfalfa diets have been linked to a greater risk of developing enteroliths (intestinal stones). Alfalfa is also higher in calories per pound than most grass hays. I’m a big believer in keeping as much forage in front of horses as possible. When you feed a higher-calorie hay, you will need less of it to maintain condition. When feeding easy keepers that already seem to get fat on air, feeding alfalfa can further limit forage intake in an attempt to control calories. For these reasons I rarely recommend feeding more than 25-30% of the daily forage intake as alfalfa in super easy keepers, and in other individual situations I might skip alfalfa all together.

It is easy to overfeed protein, even when feeding controlled amounts of alfalfa. Alfalfa is a legume, meaning it’s able to fix nitrogen from the atmosphere, resulting in its higher protein content. According to a leading forage testing lab, of 247,456 legume samples tested, the average crude protein content was 21.3% with a normal range of 18.7 to 23.9%. A 500-kilogram (1,100-pound) horse in light work requires 699 grams of crude protein each day. If the horse is eating 2% of its body weight per day (10 kilos or 22 pounds) in dry matter, the diet would need a crude protein concentration of 6.99%. This crude protein percentage is far lower than most people realize. I rarely ever see an equine diet deficient in crude protein.

If eating an alfalfa with a crude protein content of 18.7%, this horse would only need to consume just more than 8 pounds (3.7 kilos) of alfalfa to meet its daily crude protein requirement. If also feeding a 10% crude protein grass hay to make up the remaining 14 pounds of hay necessary to get to 2% of body weight as forage, that would provide an additional 636 grams of protein for a total crude protein intake of 1,335 grams. This is almost twice the horse’s crude protein requirement. Note that I am assuming the hays in question have a high dry matter value, so they have little difference between as-fed and dry matter consumed.

What does the horse do with protein in excess of his requirements? Protein is broken into its constituent amino acids, which are necessary for numerous bodily functions. However, amino acids superfluous to this requirement need to be broken down and the nitrogen they contain eliminated from the body. This is done by excreting it as urea in urine. Therefore, horses consuming diets containing far more protein than required or large amounts of low-quality protein will urinate more.

For most horses this is not an issue; however, it does lead to a greater need for water consumption and results in higher levels of ammonia in stalls, which is then breathed in and can irritate sensitive lung tissue. You might also find yourself going through more bedding in your stalls.

While it does sound like the increased urination coincides with starting to feed alfalfa, suggesting a likely cause, it is important to understand other potential causes of polyuria (abnormally high volumes of urine). One of these is pituitary pars intermedia dysfunction (PPID). Given the age of the horse in question, which puts him in an age bracket considered to be at greater risk of having PPID, I would observe him closely for other possible clinical signs and discuss with your vet. Meanwhile, double-check how much alfalfa you are feeding to make sure you’re not getting carried away. Try to keep it to no more than about 25-30% of the total forage intake each day.