Short communication: Effect of fatty acid supplements on apparent ruminal synthesis of B vitamins in lactating dairy cows

The effect of fat supplements (FS) providing different proportions of saturated (SFA) and unsaturated (UFA) fatty acids on supply, apparent ruminal synthesis (ARS), and duodenal flow (DF) of some B vitamins (thiamine, riboflavin, niacin, vitamin B₆, folates, and vitamin B₁₂) were evaluated in an experiment using 8 ruminally and duodenally cannulated lactating Holstein cows. The experiment was a replicated 4 × 4 Latin square design with 21-d treatment periods. The 4 treatments were a control diet without fatty acid supplement and 3 diets with 2.5% additional fatty acids from supplements containing (1) SFA, (2) an intermediate mixture of SFA and UFA, or (3) UFA. All diets were served as a total mixed ration once daily at 115% of the expected intake. B-vitamin concentrations were analyzed in feed and duodenal digesta. Apparent ruminal synthesis of each B vitamin was calculated as the DF minus the intake. B-vitamin concentrations were similar among the 4 treatments; consequently, daily intake of the vitamins followed the same pattern as dry matter intake. Adding FS decreased B-vitamin intakes (except vitamin B₁₂), as did increasing the proportion of UFA. Riboflavin and niacin DF and ARS, expressed as total daily amount or per unit of dry matter intake, were not affected by FS, but increasing the proportion of UFA decreased riboflavin and niacin DF and ARS. Fat supplements decreased DF of vitamin B₆, expressed either as total daily amount or per unit of dry matter intake. No treatment effects were observed on total daily folate DF and ARS. However, when expressed per unit of dry matter intake, folate DF and ARS were greater when cows were fed FS and they increased linearly with the proportion of UFA in the supplement. Inclusion of fat supplements into the dairy cow diet had a limited effect on the fate of most B vitamins in the rumen although increasing the proportion of UFA in the FS linearly decreased apparent synthesis of riboflavin and niacin in the rumen and the amounts of these vitamins reaching the small intestine.