1597 Determination of relative methionine bioavailability in lactating cows fed Smartamine M, Mepron, and AminoShure M using the plasma-free AA dose–response method

In vivo measurements of bioavailability of Met in rumen-protected Met (RP-Met) products are critical to determine their contribution to metabolizable Met supply. The objective of this experiment was to use the plasma free AA dose–response method to compare the bioavailability of Met in Smartamine M (Adisseo, Antony, France) from the new (SM1) and original (SM2) production plants along with 2 additional commercially available RP-Met products, Mepron (MPN; Evonik Ind., Kennesaw, GA) and AminoShure-M (ASM; Balchem Corp., New Hampton, NY). Ten multiparous lactating Holstein cows, fed Met-deficient diets, were used in a replicated 5 × 5 Latin square design with 7-d periods. Treatments included a negative control with no added RP-Met (CON) or 30 g of Met supplied by the 4 RP-Met products. Milk samples were collected and DMI measurements were made on d 5, 6, and 7. Blood samples were collected 2, 4, 6, and 8 h after the morning feeding (0500 h) on d 5, 6, and 7. Plasma was pooled by cow per day for AA analysis by HPLC. Data were analyzed using the MIXED procedure in SAS. Milk yield (46.0 kg/d; SEM = 1.0; P = 0.60), DMI (27.4 kg/d; SEM = 0.6; P = 0.93), and milk fat content (3.60%; SEM = 0.11; P = 0.95) were unaffected by treatments. Milk protein content was greatest (P = 0.002; SEM = 0.04) for cows fed SM1 (2.98%) and SM2 (3.00%), intermediate for those fed MPN (2.93%), and least for ASM (2.89%) and CON (2.87%). Plasma total sulfur AA (TSAA) concentrations (μM) were greatest for cows fed SM1 and SM2 and intermediate for cows fed MPN and ASM compared with cows fed CON (Table 1). Plasma TSAA concentrations were expressed as a percent of total AA; TSAA were regressed on the 0- and 30-g Met treatments. Slopes for SM1, SM2, MPN, and ASM were 0.070 (SE = 0.004), 0.070 (SE = 0.004), 0.020 (SE = 0.004), and 0.015 (SE = 0.005), respectively. The bioavailability of Met (% of total Met), calculated as the slope ratio relative to SM1, were 100, 28, and 22% for SM2, MPN, and ASM, respectively. Based on the published bioavailability of Met in SM2 (80%), the calculated bioavailabilities of Met in SM1, MPN, and ASM were 80, 23, and 17%. There was no difference in Met bioavailability between SM1 and SM2. Bioavailability of Met in MPN and ASM are less than previous estimates compared with Smartamine M.