Amino acids and glucose have been shown to regulate protein synthesis in the mammary gland through cell signaling. Acetate might also have an effect through AKT pathway, since it is the main energy source for the mammary secretory cell. Six Holstein multiparous cows, averaging 55 kg milk/d, were used in a 6x6 Latin square with 14-d periods. In each period, cows were fed the same diet for 10 d, after which they were feed-restricted for 4 d to 85% of ad libitum intake and abomasally infused for 1 of 6 treatments: acetate (A), glucose (G), each at 5% of ad libitum metabolizable energy intake (Mcal/d), casein (C) at 15% of ad libitum metabolizable protein intake (g/d), A+C (AC), G+C (GC), or a saline solution (S, negative control). Data from d 13 and 14 of each period were analyzed with Proc Mixed of SAS, using data from the d 9 and 10 as covariate. LS means are reported in the table. Casein by itself had no effect on any of the variables, compared with the negative control. This could be a consequence of lack of energy due to feed restriction, since glucose infused with casein numerically increased milk protein yield by 121 g/d. Glucose increased milk yield and nitrogen use efficiency, as indicated by the lowest MUN. Acetate had no effect on any of the variables, rejecting our hypothesis that it would enhance milk protein synthesis by improving energy status of the cell. Instead, acetate was used for fat synthesis as shown by the numerically greater fat yield for the acetate treatments. Interestingly, the infusion of casein along with acetate increased milk fat secretion more than acetate alone, which might be related to the effect of specific AA on fat synthesizing enzymes recently reported in the literature. Table 1.

Item	A	AC	C	G	GC	S	SEM	P-value
Milk, kg/d	37.2B	39.2AB	38.4AB	42.2A	41.3AB	37.1B	1.8	<0.10
Lactose, kg/d	1.67AB	1.79AB	1.71AB	1.89A	1.84AB	1.65B	0.08	<0.10
Fat, kg/d	1.59	1.7	1.43	1.54	1.44	1.44	0.12	NS
Protein, kg/d	1.13B	1.19AB	1.19AB	1.27AB	1.31A	1.15AB	0.07	0.11
MUN, mg/dL	11.4bc	15.1a	12.2bc	10.4c	11.2bc	12.4b	0.8	<0.01

Means followed by different lowercase (uppercase) letters differ at P < 0.10 (P < 0.15).