The responses in milk yield (MY) and composition to a supplementation in Met, Lys and His at 2 levels of NE_L were investigated. Four Holstein cows received 2 dietary levels of NE_L (LE: 30.9 vs. HE: 36.0 Mcal/d) and duodenal infusion of 2 AA mixtures, arranged in factorial and distributed according to a 4 × 4 Latin square, with 3-wk periods. Diets were composed of corn silage and concentrates (60:40). The 2 AA mixtures, both providing 92 ± 2 g/d of PDI (MP in INRA 2018), contained (g/d) Lys (28), Met (9), His (9) and NEAA in AA+ or only NEAA in AA−. Diets plus infusions provided 2.4 vs. 2.0, 7.2 vs. 5.8, 2.4 vs. 2.0% of PDI of Met, Lys and His respectively in AA+ vs. AA- compared with INRA (2018) requirements at 2.4, 7.0 and 2.4% PDI, for Met, Lys and His, respectively. Data were analyzed using the MIXED procedure of SAS with treatment and period as fixed effects and cow as random. Treatments did not affect DMI. The CP content of LE and HE diets were 147 vs. 145 g/kg DM, but current estimated MP supply increased by 101 g/d between LE and HE diets. Analyses of rumen N degradability will confirm the MP supply from LE and HE diets and associated MP efficiency (MPef) variations in response to NE_L. Increasing NE_L supply increased milk protein yield (MPY: 73 g/d) through an increased MY (1.8 kg/d) and increased milk fat yield (MFY: 84 g/d). Better balancing AA increased MPY (61 g/d) through a tendency to increased MY at the 2 NE_L levels and increased true milk protein content (MPC) at LE only (AA × E interaction), leading to an overall increase in MPef from 73.8 to 78.5%. Milk fat yield (38 g/d) tended to increase in response to AA+ vs. AA-. Milk lactose content tended to be lower in LEAA- than in the other treatments (AA × E). In conclusion, better balancing Met, Lys and His improved MPef at both levels of NE_L and has other effects on MY and composition. Table 1 (Abstr. T169).

	Treatment				SEM	P-value
LEAA−	LEAA+	HEAA−	HEAA+	AA	E	AA × E
DMI, kg/d	21.1	21.1	21.4	21.0	0.4	0.12	0.71	0.18
MPY, kg/d	828	916	927	962	39	0.02	0.01	0.22
MY, g/d	28.2	29.6	30.1	31.2	1.3	0.07	0.02	0.82
MPC, g/kg	29.3	31.1	30.9	30.9	0.9	0.08	0.13	0.08
MFY, g/d	1,289	1,317	1,363	1,410	55	0.07	<0.01	0.59
Lactose, g/kg	52.6	49.2	50.1	50.8	1.4	0.21	0.66	0.08
MPef, %	75.0	80.6	72.6	76.3	2.0	<0.01	0.03	0.43