The plasma free AA dose-response technique is used to determine AA bioavailability of commercially available rumen-protected (RP-AA) products. The technique assesses AA bioavailability compared with a positive control AA infusion, but ruminally cannulated cows are needed. Our objective was to measure the Met bioavailability of a RP-Met supplement relative to a reference RP-Met supplement already documented in terms of its bioavailability using the plasma free AA dose-response technique. Six multiparous Holstein cows fed a Met deficient diet were used in a replicated 3 × 3 Latin square with 7-d experimental periods. The 3 treatments were: 1) a negative control (CON); 2) 24 g/d Met from Smartamine M (SMM; Adisseo; Met bioavailability = 80%); and 3) 24 g/d Met from EB-Met (EBM; Milk Specialties Global). Intake, milk yield and milk components were collected the last 3 d per period. Blood was collected from the tail vein the last 3 d per period 2, 4, 6 and 8 h after the morning feeding (0500 h). Data were analyzed using the PROC MIXED and REG procedures of SAS. Significance was declared at P ≤ 0.05. Milk yield and DMI were unaffected by treatment. Milk protein content was increased for cows fed SMM relative to CON and EBM. Plasma Met and total sulfur AA (TSAA) concentrations (µM) were greatest for cows fed SMM compared with CON or EBM. Slopes for RP-Met supplements expressed as %TAA−TSAA was greater for SMM compared with EBM. The resulting bioavailability of EBM was 20.8% [0.01436/0.06908] relative to SMM. Evaluating bioavailability of RP-AA supplements can be done relative to a reference RP-AA supplement with a known bioavailability to eliminate the need for ruminally-cannulated cows. Table 1 (Abstr. T176). Intake, lactation performance and plasma AA concentrations for cows fed RP-Met supplements

Item	Control	SMM	EBM	SEM	P-value
DMI, kg/d	25.3	25.7	25.0	0.80	0.78
Milk yield, kg/d	38.1	39.0	37.8	1.37	0.78
Milk protein content, %	3.02b	3.12a	3.01b	0.03	0.04
Methionine, µM	19.5b	46.4a	21.3b	1.84	<0.001
TSAA, µM	81.3b	120.1a	85.0b	3.75	<0.001
Slope for TSAA, %TAA – TSAA	—	0.06908a	0.01436b	0.0015	<0.001

^a,bMeans within rows differ at P < 0.05.