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Varied effects of multifunctional amino acids on reproduction in lactating dairy cows
In addition to being the basic building blocks for proteins, many amino acids have recently been classified as “functional amino acids” due to their effects on physiologic, metabolic, epigenetic, and nutritional properties. Functional actions of amino acids are being actively researched in many species and have important implications for health, productivity, cellular function, and reproduction in lactating dairy cows. Two examples of functional amino acids, methionine (Met) and arginine (Arg), will be discussed. Met, along with lysine, and histidine are considered the first rate-limiting amino acids for milk production in high-producing dairy cows fed diets based on corn silage or alfalfa haylage. Early bovine embryo development until the blastocyst stage appears morphologically normal during in vitro culture with physiological concentrations of Met. However, deficiencies of Met caused by incubation with the Met antagonist, ethionine, inhibited the transition from morula to blastocyst during the first 7 d of gestation. In addition, recent data from our laboratory demonstrated that supplementation of lactating dairy cows with rumen-protected Met increased milk protein yield and altered gene expression in the early embryo, although gross morphology of the early embryo was unchanged. Gene expression was mostly inhibiting by Met supplementation, as might be expected due to Met-facilitated DNA methylation in the early embryo. Later embryonic development may also be inhibited by insufficient circulating Met in lactating dairy cows, based on studies using cultures of rat embryos in bovine serum. The effects of Arg supplementation on reproduction, particularly in gestating sows, ewes, and rats, have been extensively researched. Dietary requirements for Arg increase during gestation due to placental development during early gestation (days 30-60 in pigs) and fetal growth during later gestation (>60 d of gestation). In addition, Arg increases production of polyamines (essential for implantation) and nitric oxide (increases utero-placental blood flow due to vasodilation and angiogenesis), and activates the mTOR cell signaling pathway (increasing protein synthesis and cell proliferation). Under practical conditions, supplementation with ~0.8% arginine between d 14-25 or 30 to 114 increased litter size or reduced embryonic death in sows. Nevertheless, Arg supplementation during the first few d after breeding reduced litter size, highlighting the importance of targeting Arg supplementation to the proper window of pregnancy. In addition, stressful conditions, such as virus infection, dramatically increased pregnancy loss, and supplementation with Arg or glutamine prevented these pregnancy losses, possibly due to enhanced immune function. Intravenous infusion of late pregnant ewes with Arg (last 21 d of pregnancy) increased percentage of lambs born alive and enhanced birth weights.
Keywords: amino acids, dairy cows