between
infusion periods. Administration of AA had no effect on
concentration or yield of any milk components. Addition
of molasses increased milk fat concentration (2.71 vs.
2.94 ± 0.21%), but had no effect on yields of milk fat or
protein. Dietary molasses decreased total volatile fatty
acid concentration (141 vs. 133 ± 4.6 mM), decreased
the molar proportion of propionate, and increased the
molar proportion of butyrate in ruminal fluid. Molasses
also increased ruminal pH (5.73 vs. 5.87 ± 0.06),decreased the yield of trans-10 C18:1, and increased the
yield of trans-11 C18:1 in milk fat. These data provide
evidence that molasses may promote mammary de novo
fatty acid synthesis in cows fed high-energy rations by
moderating ruminal pH and altering ruminal fatty acid
biohydrogenation pathways.