Replacement of cottonseed with an extruded flaxseed-based supplement alters milk fatty acid profile without affecting yield or methane emissions in Holstein dairy cows

Fats are considered effective in reducing enteric methane (CH₄) emissions in ruminants, but limited research exists on the use of flaxseed fatty acids for this purpose in Holstein dairy cows. We evaluated the effects of an extruded flaxseed-based supplement that included a mix of flaxseed, peas, and alfalfa hay (LinPRO-R) on enteric CH₄ emissions and milk quality. Holstein cows (n = 24, 113 ± 31 DIM) were arranged in a 3 × 3 Latin square design and assigned to one of the 3 treatments: control (CON), ration containing 4.5% LinPRO (4.5% LinPRO-R), or ration containing 9% LinPRO (9% LinPRO-R) on a DM basis. In the 9% LinPRO-R group, cottonseed was replaced with the flaxseed supplement; therefore, the fatty acid profile was altered, and the forage NDF increased with higher inclusion. However, the total fat content of different diets remained similar. After 2 wk of adaptation, each of the experimental periods lasted for 28 d. Milk yield and enteric emissions were captured daily. Weekly milk samples were analyzed for milk fat, protein, lactose, SNF, MUN, and SCC. All data were analyzed using a mixed model ANOVA in the GLIMMIX procedure in SAS (SAS Institute Inc.). Body weight change, DMI, and milk yield did not differ among the different LinPRO-R treatments. Milk fat percentage was lower in LinPRO-R supplemented cows compared with the CON group (CON: 4.67 ± 0.14%, 4.5% LinPRO-R: 4.47 ± 0.14%, and 9% LinPRO-R: 4.51 ± 0.14%), but milk protein, lactose, SNF, MUN, and SCC were unaffected. Methane and CO₂ emissions did not differ among treatments, whereas H₂ production was lower in CON than in 9% LinPRO-R (CON: 4.42 ± 0.14 g/d, 4.5% LinPRO-R: 4.66 ± 0.14 g/d, and 9% LinPRO-R: 4.88 ± 0.14 g/d). The percentage of cis-9 C18:1 in milk fat was higher in the 9% LinPRO-R group compared with the CON and 4.5% LinPRO-R groups. The percentage of C18:3n-3 was greater in LinPRO-R-supplemented cows (CON: 0.52 ± 0.03%, 4.5% LinPRO-R: 1.05 ± 0.03%, and 9% LinPRO-R: 1.46 ± 0.03%). Therefore, supplementation of Holstein cow diets with 4.5% and 9% LinPRO-R did not affect milk yield or enteric CH₄ emissions, likely due to alterations in forage NDF content and the dietary fatty acid profile.