This study explored the effect of feeding different starch concentrations and conventional or brown midrib corn silage on the performance of lactating dairy cows. Forty-eight Holstein cows were assigned to 1 of 4 diets using a randomized complete block design with a 2-wk covariate period followed by 8-wk experimental period. Experimental diets were arranged as a 2 × 2 factorial with 2 types of corn silages [conventional (CS) and brown midrib (BMR) corn silage] and 2 dietary starch concentrations (19 and 25% of DM). Diets were formulated to contain 60.7% forage and 39.3% concentrate on DM basis. Dried corn grain was replaced with soyhulls and beet pulp to decrease dietary starch concentration. Cow was the experimental unit. Silage × starch interactions were detected (P ≤ 0.05) for milk yield, energy-corrected milk (ECM), and feed efficiency (ECM/DMI). Milk yield was similar between cows fed BMR-25% starch and CS-19% starch, but was greater than for cows fed other diets. ECM was greatest for cows fed BMR-25% starch compared with the other 3 diets. Feed efficiency was greatest for cows fed CS-19% starch and BMR-25% starch and least for cows fed BMR-19% starch. Milk protein percentage was affected by starch concentration, resulting in greater protein concentration for cows fed 25% starch compared with cows fed the 19% starch diet. There was no effect of diet on DMI, milk fat percentage, milk fat and protein yield, and MUN. Overall, the milk and ECM of lactating dairy cows was superior when feeding BMR-25% starch, but cows fed BMR-19% starch responded similarly to cows fed CS diets at either 19 or 25% starch. Table 1.

Item	CS			BMR		SEM	P>F1
	19%	25%		19%	25%
DMI, kg/d	25.9	26.8		27.0	26.8	0.54	NS
Milk, kg/d	44.1ab	43.4b		43.4b	45.7a	0.69	I
ECM, kg/d	45.1b	44.9b		44.2b	46.8a	0.71	I
ECM/DMI	1.76a	1.69ab		1.66b	1.76a	0.04	I
Fat, %	3.83	3.89		3.87	3.88	0.09	NS
Fat, kg/d	1.66	1.68		1.63	1.73	0.05	NS
Protein, %	2.91	3.00		2.90	2.98	0.04	S
Protein, kg/d	1.27	1.27		1.23	1.32	0.02	NS
MUN, mg/dL	11.1	10.5		11.4	11.0	0.35	NS

^abMeans with different superscripts differ (P ≤ 0.05). ¹S = starch effect; I = silage by starch effect; NS = not significant.