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 x 3 Latin square designed experiment to evaluate the technique of nitrogen retention as a response criterion for amino acid studies in the horse. The yearlings were paired by age and randomly assigned to one of three concentrates fed with a medium quality Coastal Bermudagrass hay throughout the study. Diets were fed at approximately 1.9% of horse body weight per day, divided into twice daily feedings with a 60:40 concentrate: hay ratio. With the exception of lysine and threonine, proposed amino acid requirements for yearling horses were calculated using nutrient to calorie ratios of gilts weighing 80-120 kg and gaining 325 g/d. Diet A was amino acid sufficient, as provided by a soybean meal-based concentrate. Diet B was amino acid deficient, with a cottonseed hull-based concentrate. Diet A and Diet B were isonitrogenous, containing approximately 12% crude protein. Diet C used the identical concentrate as Diet B, with synthetic essential amino acids and cysteine orally dosed to match the amino acid levels in Diet A. Nitrogen retention was not different between Diet A and Diet B. Diet C resulted in differences from Diets A and B in nitrogen retention; however, differences were a consequence of nitrogen intake. Nitrogen retained as a percent of nitrogen absorbed was lower (P < 0.05) for Diet B than for Diet A, for data not accounting for endogenous fecal and urinary losses. There were no differences in nitrogen retained as a percent of nitrogen absorbed for horses fed Diet C, when compared to either Diet A or Diet B, for data not accounting for endogenous losses. It was concluded that differences in nitrogen retained as a percent of nitrogen absorbed were observed between amino acid sufficient diets and amino acid deficient diets. However, horses fed amino acid deficient diets and orally dosed with synthetic amino acids, likely require some modified dosage level to achieve the same or higher values in nitrogen retained as a percent of nitrogen absorbed as those values for amino acid sufficient diets.