Estimating metabolizable Met (MP-Met) in diets and supplements is difficult and expensive; however, the seleno-Met technique can estimate relative supply of MP-Met easily and inexpensively. The technique is based on the proportionality of a change in supply of dietary MP-Met to the change in milk Se-Met concentration (Weiss and St-Pierre, 2009). Our objective was to further evaluate the Se-Met procedure and determine the MP-Met supplied by a RP Met prototype (BRPM, 72% Met, Balchem Corp., New Hampton, NY) and a commercially available RP product (Mepron, 85% Met, Evonik, Germany). Holstein cows (15) averaging 31 kg/d milk were fed a basal diet (0.3 mg/kg Se from Se-yeast) for 10 d. Then in 5 replicated 3 × 3 Latin squares with 10 d periods, cows were fed the basal diet plus 1 of 3 treatments: 1) Mepron fed at 19 g of Met/d; 2) BRPM fed at 19 g of Met/d (BRPM-1X) and 3) BRPM fed at 38 g of Met/d (BRPM-2X). After the last treatment period, cows were fed the basal diet for 9 d. Milk was sampled from the final 4 milkings of each period (basal and treatment), composited by cow-period, and assayed for N and Se. The relative dilution of milk Met when treatments were fed was calculated as the ratio of basal Se/N to treatment Se/N (Table 1). Data were analyzed using Proc Mixed (SAS) with the random effects of square and cow within square and the fixed effects of treatment and period. Feeding Mepron or BRPM-1X increased MP-Met by 13% compared with basal. BRPM-2X increased MP-Met by 25%, which was significantly greater than Mepron or BRPM-1X. On a gram of Met basis, results were identical for the 1X and 2X BRPM treatments. The identical results for the 2 BRPM treatments lend validity to a method that is a practical and useful tool to evaluate met supplements. Table 1 (Abstr. T172). Relative change in MP-Met with different supplements

Treatment	Milk Se/N, μg/g	Ratio1
Basal	1.12	—
Mepron	0.99a	1.129a
BRPM-1X	0.99a	1.128a
BRPM-2X	0.89b	1.254b
SEM	0.020	0.008

^a,bValues with differing superscripts differ (P < 0.05). ¹Calculated as [(Milk Se/Milk N)_Basal]/[(Milk Se/Milk N)_Treatment] and equals the change in Met entry rate relative to the basal diet.