Evaluating the technique of using nitrogen retention as a response criterion for amino acid studies in the horse

Six Quarter Horse yearling fillies were used in a duplicated 3 × 3 Latin square experiment to evaluate the technique of using nitrogen retention as a response criterion for amino acid studies in the horse. Yearlings were paired by age and randomly assigned to one of three concentrates fed with medium-quality Coastal Bermudagrass hay. Diet A, a soybean meal–based concentrate, was amino acid sufficient. Diet B, a cottonseed hull–based concentrate containing 2% urea, was amino acid deficient. Horses fed diet C, identical to diet B, were orally dosed with synthetic essential amino acids and cysteine, immediately before eating, to match amino acid levels in diet A. Nitrogen retention was not different between diets A and B. There was a trend for horses fed diet C to have higher nitrogen retention, as a result of higher nitrogen intake. Nitrogen retained as a percent absorbed was lower (P < .05) for diet B than for diet A, whereas values for diet C were not different from diets A or B. Nitrogen retention was not an effective response criterion in this study. Differences in nitrogen retained as a percentage absorbed were observed between amino acid–sufficient and deficient diets. However, horses fed amino acid–deficient diets and dosed with synthetic amino acids likely require some modified dosage level to achieve the same or higher values as those for amino acid–sufficient diets.