Methionine is often the first limiting amino acid (AA) for milk production. Supplementation of rumen-protected methionine (RPM) is a strategy to improve metabolizable protein (MP) utilization and animal performance. However, current literature shows inconsistent results regarding size of responses to RPM. The objective of this study was to better understand how other factors affect performance responses to RPM and therefore speculate about the feeding situations with greater potential of better responses. Thirty-two published studies yielding 39 mean comparisons between a control diet (CD) and CD + RPM were analyzed using a meta-analytical approach. Studies were classified according to the level of dietary crude protein (CPdiet), days in milk (DIM) of the cows, and duration of the treatment. Study was included as random effect and models were weight by the number of observations in each study. Results (Table 1) indicate that the response of milk protein yield (Mprot) to RPM is influenced by CPdiet and that this relationship changes according to stage of lactation. Furthermore, the interaction between CPdiet and DIM may also affect responses of dry matter intake and milk yield to RPM. The duration of the treatment also affected Mprot response (P = 0.07). When cows received the RPM for longer than 60 d, the average increase in Mprot was 54 g/d, while when shorter than 60 d it was only 19 g/d. Overall, the results suggest that better responses to RPM can be expected when cows are past peak of lactation, receiving diets higher than 16% of CP for a period longer than 60 d. Greater responses in Mprot when diet CP is high may challenge the assumption that better AA profile of MP allows for diet CP reduction. Table 1. Size of responses to rumen-protected methionine supplementation (n = 39) according to diet crude protein (CPdiet; > or <16% CP) and DIM

	0-59 DIM			60-109 DIM			110-140 DIM		SEM	P-value
	>16%	<16%		>16%	<16%		>16%	<16%		CPdiet	DIM	CPdiet × DIM
DMI, kg/d	−0.73	0.15		0.59	−0.60		0.36	0.17	0.45	0.50	0.48	0.01
Milk, kg/d	−0.50	1.26		1.17	−0.67		0.92	−0.26	0.60	0.20	0.97	<0.001
Protein, g/d	21	31		73	−1		77	19	24	<0.001	0.64	0.04
Fat, g/d	4	73		50	48		133	72	51	0.94	0.26	0.18