This study examined the interaction of dietary energy source and metabolizable protein (MP) supply on cow performance. Eight Holstein cows were used in a double 4 × 4 Latin square design with 21-d periods, with dietary energy source and MP supply tested in a factorial arrangement: high fiber, high protein (HFHP); high fiber, low protein (HFLP); high starch, high protein (HSHP); and high starch, low protein (HSLP). Diets were formulated to supply 100% of the NE_L requirement, and 100% (HP) or 70% (LP) of MP requirement. There were no interactions between energy source and MP supply on the parameters measured. Intakes of NE_L (Mcal/d) and MP (g/d) were: HFHP: 31.5, 2148; HFLP: 29.5, 1492; HSHP: 37.8, 2699; HSLP: 36.2, 2007. Milk yield (34.4 vs. 29.9 kg/d) and DMI (22.6 vs. 18.4 kg/d) were higher in cows fed HS compared with those fed HF, whereas DMI tended to be lower in cows fed LP vs. HP (19.9 vs. 21.1). Milk protein (3.04 vs. 2.85%) and lactose (4.62 vs. 4.56%) concentrations were higher in cows fed HS than those fed HF, but milk fat concentration was higher with HF compared with HS (4.22 vs. 3.49%). Milk protein (1.04 vs. 0.85 kg/d) and lactose (1.59 vs. 1.36 kg/d) yields were higher in cows fed HS than those fed HF. Milk protein yield tended to be higher with HP than with LP (962 vs. 925 g/d), and milk urea-N was higher with HP than LP (14.5 vs. 7.9 mg/dL). Dietary energy affected milk yield and composition more than protein supply did. Although diets were formulated to be iso-energetic, the large decrease in DMI in cows fed HF mainly drove the effect of HS vs. HF. The smaller effects of protein supply were not affected by energy source. Table 1.

Parameter	Treatments				SEM	P-value
HFHP	HFLP	HSHP	HSLP	HF vs. HS	HP vs. LP
DMI, kg/d	19.0	17.8	23.1	22.0	0.85	<0.01	0.06
Yield, kg/d
Milk	30.4	29.4	34.9	33.9	1.28	<0.01	0.20
Protein	0.87	0.83	1.06	1.02	0.04	<0.01	0.09
Fat	1.31	1.22	1.19	1.16	0.07	0.16	0.34
Lactose	1.38	1.34	1.62	1.57	0.06	<0.01	0.24
Milk composition, %
Protein	2.87	2.85	3.04	3.04	0.06	<0.01	0.84
Fat	4.22	4.21	3.52	3.46	0.16	<0.01	0.80
Lactose	4.56	4.55	4.62	4.63	0.04	0.04	0.91
MUN, mg/dL	15.7	10.4	13.2	5.4	0.80	<0.01	<0.01