Effects of supplementing rumen-protected methionine on lactational performance of Holstein dairy cows during early and mid-lactation

Abstract Text: Supplementing rumen-protected methionine (RPMet) has been shown to maintain milk and milk protein yields when dietary metabolizable protein is decreased by 5% due to its direct impacts on milk protein synthesis in the mammary gland. The present study investigated production responses of lactating dairy cows to RPMet supplementation in sub-optimal protein [SOPD, 15.5% crude protein (CP)] and normal protein diet (NPD, 16.5% CP). Eight lactating dairy cows (53 days-in-milk on average) were blocked by parity and days-in-milk, and the experiment was performed in a duplicate 4 × 4 Latin square design. Within each square, cows were randomly assigned to a sequence of 4 diets during each of the four 21-d periods (14 d of treatment adaptation and 7 d of data collection and sampling). A 2 × 2 factorial arrangement was used; SOPD or NPD was combined without or with RPMet: SOPD without RPMet, SOPD with RPMet (S+Met), NPD without RPMet, and NPD with RPMet (N+Met). An experimental RPMet product from CJ CheilJedang (Suwon, South Korea) were supplemented in the S+Met and the N+Met at 30 g/cow/d. Supplementation of RPMet did not affect dry matter intake (DMI; 25.4 kg/d) and milk yield (40.6 kg/d). Supplementing RPMet resulted in a similar milk true protein concentration (2.80%) with a numerical increase in milk protein yield at 3.6%. In contrast, supplementing RPMet increased milk fat concentration (P = 0.02) and yield (P = 0.03), 3.5% fat-corrected milk (FCM) yield (P = 0.05), and tended to increase energy-corrected milk (ECM) yield (P = 0.06) regardless of CP level. In addition, trends were observed for increased 3.5% FCM yield/DMI (P = 0.09) and ECM yield/DMI (P = 0.10), and the positive effects were greater under NPD than SOPD, resulting in trends toward interaction between CP and RPMet (P= 0.06). Overall results in the current study suggest that supplementing RPMet in SOPD and NPD improved milk fat concentration possible due to increases in apolipoprotein and phospholipid syntheses in the liver, leading to an increase in fatty acid supply to the mammary gland via very low density lipoproteins.

Keywords: feed efficiency, lactational performance, rumen-protected methionine