CASE STUDY: Relationships Between Dietary, Plasma, and Muscle Amino Acids in Maintenance, Pregnant, and Lactating Mares1

Amino acid requirements have not been extensively studied in pregnant or lactating mares. The objective of this study was to evaluate relationships between dietary, plasma, and muscle amino acids for horses in maintenance, gestation, and lactation. Twenty mares were divided into 3 groups: maintenance (MM), pregnancy (PM), and lactation (LM). Mares were fed their respective diets for 8 wk. Initial and final blood samples and muscle biopsies were obtained. Poststudy plasma urea-N and 3-methyl-histidine concentrations were higher for the PM group compared with the MM group. Poststudy plasma concentrations of arginine and urea-N were higher, whereas 3-methyl-histidine was lower for the LM group compared with the MM group. Poststudy muscle urea-N concentrations were lower in the PM group but were higher in the LM group compared with the MM group. There was a correlation between poststudy plasma and muscle urea-N concentrations for the MM group. Correlations between poststudy plasma and muscle concentrations of lysine, threonine, isoleucine, leucine, phenylalanine, valine, urea-N, and 3-methyl- histidine were observed in the PM group. Correlations between poststudy plasma and muscle concentrations of lysine and urea-N were found for the LM group. Lysine and phenylalanine intakes were both correlated with their respective poststudy plasma and milk concentrations in the LM group. The many plasma and muscle amino acid correlations between the PM and LM groups may indicate a close relationship between amino acid demand and supply from the amino acid pool. The amino acid needs of the lactat- ing and pregnant mare warrant further investigation.