CASE STUDY: Relationships Between Dietary, Plasma, and Muscle Amino Acids in Maintenance Horses, Exercising Horses, and Growing Weanling Horses1

The objective of this study was to evaluate relationships between diet, plasma, and muscle amino acids for horses in maintenance, exercise, and growth. Eight horses in each maintenance (MH) and exercise (EH) group were fed diets for 8 wk, whereas 6 weanlings in the growth (GH) group were fed diets for 4 wk. Blood samples and muscle biopsies were taken at the beginning of the study from the MH and EH groups and at the end of the study for all groups. Plasma urea-N and lysine concentrations were higher for the EH group than for the MH group, whereas plasma urea-N concentrations were higher for the GH group than for the MH group. Plasma histidine and 3-methyl-histidine concentrations for the GH group were higher than for the MH group at the conclusion of the study. Poststudy muscle methionine concentrations for the EH group were lower than for the MH group. Correlations were found between plasma and muscle concentrations of urea-N for the MH and EH groups. Plasma and muscle correlations also existed with serine for the EH group and with threonine, leucine, valine, histidine, phenylalanine, and serine for the GH group. Fewer correlations for the MH and EH groups may indicate an excess of amino acids in the diet in relation to needs, whereas the large number of correlations for the GH group may indicate a closer relationship between supply of amino acids in the diet and need by the body. More exploration of this area is needed for horses in all functions.