Methionine (Met) and lysine (Lys) are often limiting amino acids in lactating cow diets. The objective of this work was to assess a lipid encapsulated Lys (RP-Lys) and 3 lipid encapsulated Met (RP-Met) prototypes (P1, P2, and P3) to determine animal performance responses. Twenty Holstein cows were randomly assigned to 2 trials (n = 10 each) in a replicated Latin square design with 14 d periods. Both trials were analyzed using a linear mixed effect model, however, the Lys trial was analyzed using a dose response technique. The base diet was predicted to be deficient in metabolizable Met (−14.8 g/d) and Lys (−16.1 g/d). In the Met trial, the base diet was supplemented with RP-Lys to meet the lysine requirement. The treatments included no added RP-Met (NC), Smartamine (SM), and P1, P2, or P3 at 148% of the Met content of SM. In the Lys trial, the base diet was supplemented with RP-Met to meet the methionine requirement. Treatments included no added RP-Lys (NC), AjiProL (AL), or the RP-Lys prototype at 55%, 78%, or 102% of the Lys in AL. Performance results are listed in Table 1. Milk protein percent significantly increased when diets were supplemented with P2 or P3 compared with NC, but none were different from SM. Overall, P2 had the greatest numerical production response among the 3 Met prototypes suggesting it had the greatest efficacy when supplemented into these rations. There was a significant linear increase for milk protein percent for the RP-Lys prototype compared with AL when fed at a range of 55–102% of lysine content, indicating that it could support comparable performance. Table 1. Performance results when supplementing RP-Met or RP-Lys Prototypes

	DMI,kg/d	Milk,kg/d	Milk fat,%	Milk protein,%	Milk protein,kg/d
Met Trial
SM	28.1	45.8	3.50	3.11b	1.42
NC	28.3	45.2	3.67	3.02a	1.35
NC+P1	28.2	45.1	3.77	3.07a,b	1.38
NC+P2	27.7	45.5	3.64	3.12b	1.42
NC+P3	27.6	45.3	3.51	3.12b	1.41
Lys Trial
AL	29.2	50.4	3.38	3.05	1.53
NC	29.0	50.1	3.52	3.04	1.50
55%1	28.9	50.2	3.44	3.07	1.57
78%	28.9	50.2	3.41	3.09	1.55
102%	28.8	50.2	3.37	3.11	1.54

^a-bvalues with differing subscripts differ (P<0.05). ¹Significant linear effect of Lys dose on milk protein %.