Several dietary strategies have been evaluated for their efficacy to mitigate methane (CH₄) emissions, but their effects at the farm level had been scarcely examined. The aim of this study was to simulate the effect of corn dried distiller grains with soluble (DDGS), linseed oil (LO) and brown midrib corn silage (BMR) on net income (NI) and environmental output using the whole-farm model N-CyCLES. Virtual farms, representative of 3 contrasting Canadian dairy regions were built: Maritimes, Central, and Prairies. Enteric CH₄ computation was adapted to fit results of experiments using these feeds. The DDGS and LO were tested in the 3 regions while BMR was evaluated in Central region, which has the appropriate climate for this plant. A constraint on lactating cow diets was added for each feed (DM basis) tested: DDGS at 10%; LO at 4% and BMR, replacing conventional corn silage (CS), at 40% of the diet. Results (Table 1), compared with CS-based diets, show a slight decrease in enteric CH₄ (averages of 2% for DDGS; 4% for LO and 0.1% for BMR) with typical on-farm diets. The whole-farm evaluation shows that most strategies, except DDGS, had a negative impact on NI. Also, LO and BMR, but not DDGS, resulted in a slight reduction of total greenhouse gas emissions. These results suggest that farm NI and environmental cost from feed importation should be accounted for before a modification in the diet is made to decrease greenhouse gas emission. Table 1. Economic and environmental impacts of dietary strategies to reduce CH₄ production¹

Item	Maritimes				Central					Prairies
CS	DDGS	LO	CS	DDGS	LO	BMR	CS	DDGS	LO
Net income,$/100 kg of FPCM	10	11	3		21	22	16	20		23	24	16
CH4 production2
Early lactation	34	33	31		33	32	30	33		34	33	31
Mid-late lactation	48	49	54		56	57	56	57		58	55	56
GHG emissions2
Total	153	160	150		154	162	152	152		147	151	137
Milk allocation	122	127	118		116	119	112	115		115	118	107
Cash crop allocation	6	7	8		15	19	18	13		3	2	2

¹Maritimes = 63 cows and 513,584 kg/yr; Central = 71 cows and 613,841 kg/yr; Prairies = 144 cows and 1,212,875 kg/yr; FPCM = 4% fat- and 3.4% protein-corrected milk. ²In kg of CO₂eq/100 kg of FPCM