Influences of dietary guanidinoacetic acid supplementation on performance and proteins involved in milk fat and protein synthesis in dairy cows

Guanidinoacetic acid (GAA) increases milk production in dairy cows by promoting milk fatty acid (FA) and protein synthesis; however, its mechanism is unclear. It was hypothesized that provision of rumen-protected GAA (RPGAA) would increase milk production, and milk fat and protein synthesis, while revealing the underlying mechanisms. Forty-four multiparous Holstein dairy cows (33.5 ± 1.31 kg/d of milk yield, 667 ± 11.8 kg of body weight, and 38.5 ± 2.14 d in milk [DIM]) were blocked by daily milk yield and DIM, and allocated to different treatments in a randomized-block design, namely, the control (without RPGAA), low-RPGAA (0.18 g/kg DM of GAA), medium-RPGAA (0.36 g/kg DM of GAA), and high-RPGAA (0.54 g/kg DM of GAA) groups. The experiment lasted for 95 d, including a 10-d covariate period, a 15-d adaptation period, and a 70-d sampling period. Although the DM intake and BW were not impacted (<i>P</i> > 0.05), the yields of actual milk, 4% fat-corrected milk, and energy-corrected milk increased linearly and quadratically (<i>P</i> < 0.05) with RPGAA supplementation; milk fat and protein percentage also showed quadratic increases (<i>P</i> < 0.05). The de novo and mixed production of FA in milk increased linearly and quadratically (<i>P</i> < 0.05), whereas that of preformed FA was increased quadratically (<i>P</i> < 0.01). Administration of RPGAA linearly increased the daily secretion yields of Arg, Cys, His, Ser, Thr, Trp, and Tyr (<i>P</i> < 0.05), and quadratically increased those of Ile, Lys, Met, Phe, Asp, and Glu in milk (<i>P</i> < 0.05). Furthermore, the apparent digestibilities of DM, OM, CP, and EE increased quadratically (<i>P</i> < 0.05), and those of NDF and ADF increased linearly (<i>P</i> < 0.05), following GAA supplementation. Serum concentrations of glucose, total protein, albumin, Arg, and creatine increased linearly (<i>P</i> < 0.05); concentrations of insulin-like growth factor 1, estradiol, and prolactin increased quadratically (<i>P</i> < 0.05), and that of blood urea nitrogen decreased quadratically (<i>P</i> = 0.018). Furthermore, addition of 0.36 g/kg DM of GAA from RPGAA promoted the expression of proteins concerned with mammary gland proliferation, FA synthesis, and milk protein synthesis, which were done in tissue biopsies. These results indicate that RPGAA provision enhanced milk production, FA synthesis, and milk protein synthesis by promoting the expression of proteins involved in mammary gland development, FA synthesis, and milk protein synthesis.