How do horses process excess protein? Does protein digestion differ in horses of different ages? Does excess protein lead to thyroid problems?
As you know, protein plays an integral role in growth, muscle development, and performance. Too little protein can stunt growth, affect muscle function, alter hormone function, and reduce nutrient transport.
On the other hand, too much dietary protein increases ammonia output and adversely affects respiratory health, disrupts acid-base balance, and can potentially decrease bone mineralization. Excess nitrogen resulting from protein metabolism is excreted as urea in urine. The higher protein content causes an increase in water consumption, leading to more frequent urination. This is evidence that protein is not stored in the body for later use like fats and carbohydrates.
Many factors are involved in the calculation of protein requirements, including digestibility of protein, amino acid content of protein, protein to energy ratio of the diet, and age and life stage of the horse. Mature horses, both idle and working, have relatively low protein requirements compared to other life stages.
A horse in light work, for example, requires between 8-10% protein in the total diet (about 769 g). This level can easily be achieved by a diet of good-quality forage. The NRC provides the following equation for calculating crude protein (CP) requirements for mature, idle horses: body weight (BW) x 1.26 g CP/kg BW/d.
Research shows that the true digestibility of protein in a variety of mature horse diets averaged 71%. In senior horses, decreased digestibility of protein can occur for several reasons. The effectiveness of the intestinal lining decreases with age, which makes it difficult for nutrients to pass the mucosal surface in order to reach the bloodstream. Research has documented a decreased absorption of phosphorus, vitamins, and protein in aged horses. Additionally, horses with parasite damage have compromised digestive tracts that negatively affect nutrient absorption. The common occurrence of muscle loss in geriatric horses can be expedited without proper protein levels. These are all reasons why feeds labeled for senior horses tend to have higher protein content.
The quality of the protein source is a key factor in meeting the horse’s amino acid requirements; high-quality protein sources are those that contain an assortment of amino acids that approximate the needs of the horse. Soybean meal is included in most feeds and considered the gold standard.
When basing the nutrient values of hay on average values collected from Equi-Analytical, a forage-testing laboratory, it is important to remember that the actual nutrient content of your hay will differ. For instance, a stemmy, mature hay may contain less protein than expected, which is why commercial feeds are formulated to make up for some nutrient deficits.
Ration balancers provide good-quality protein, macrominerals, trace minerals, and vitamins at the appropriate levels to complement forages of unknown quality when fed at 1-2 lb (0.45-0.9 kg) per day.
Lastly, you asked about the effect of excess protein on the thyroid. Levels of certain nutrients—protein, zinc, copper, and iodine, for example—can influence thyroid function. Working with a veterinarian to determine if your horse has thyroid malfunction is your best bet, especially if you suspect hypothyroidism.
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