The objective of this study was to evaluate methane (CH₄) and ammonia (NH₃) emissions of lactating dairy cows fed different starch level and corn silage type. After the completion of an 8-wk production study, 48 Holstein cows were allocated to 1 of 4 air-flow controlled chambers (2 cows/chamber) for 6 d in a randomized complete block design. Chamber was the experimental unit. Cows were fed 1 of 4 diets arranged as a 2 × 2 factorial with 2 corn silage hybrids [conventional (CS) and brown midrib (BMR) corn silage] and 2 dietary starch concentrations (19 and 25% of DM). Performance data from the last 6 d and emission measurements last 3 d were recorded and used for analysis. Soyhulls and beet pulp replaced corn grain in the diet to decrease starch concentration. There was no effect of dietary starch concentration and corn silage on DMI, ECM, ECM/DMI, and milk protein percentage, however milk fat percentage was greater (P < 0.03) for cows fed diets formulated at 25% starch rather than diets with 19% starch. An interaction of silage × starch (P < 0.03) was observed for CH₄ expressed as per unit of DMI and for MUN. Cows fed CS-25% starch had the lowest MUN. Cows fed BMR-25% starch produced 1.3 g CH₄ less per unit of DMI than cows fed CS-25% starch, but were similar to cows fed 21% starch for any silage type. Emissions of CH₄ and NH₃ (g/d), and CH₄/ECM did not differ among treatments. It was concluded that cows fed the BMR-25% starch have the potential to reduce CH₄ emissions per unit of DMI even though productive performance was not improved. Table 1.

Item	CS			BMR		SEM	P > F1
	19%	25%		19%	25%
DMI, kg/d	25.4	25.1		24.9	25.9	0.64	NS
ECM, kg/d	44.0	44.7		40.8	44.6	1.42	NS
ECM/DMI	1.74	1.79		1.65	1.74	0.06	NS
Fat, %	3.91	4.07		3.68	4.02	0.11	S
Protein, %	3.04	3.10		3.05	3.08	0.07	NS
MUN, mg/dL	11.9a	10.8b		11.2ab	11.8ab	0.37	I
CH4, g/d	503.0	515.9		494.0	497.4	13.0	NS
NH3, g/d	22.7	27.6		23.4	23.1	2.65	NS
CH4/DMI	19.8ab	20.5a		19.9ab	19.2b	0.56	I
CH4/ECM	11.5	11.6		12.1	11.2	0.34	NS

^abMean differences (P < 0.05). ¹S = starch effect; I = silage by starch effect; NS = not significant.