fatty acid concentrations but decreased milk fat from
3.22 to 2.72% in corn silage diets but less in alfalfa hay
diets. Medium-chain milk fatty acids (% of total fat)
were lower for alfalfa hay compared with corn silage diets,
and short-chain milk fatty acids tended to decrease
when Rumensin was added. In whole rumen contents,
concentrations of trans-10, cis-12 C18:2 were increased
when cows were fed corn silage diets. Rumensin had
no effect on conjugated linoleic acid isomers in either
milk or rumen contents but tended to increase the concentration
of trans-10 C18:1 in rumen samples. Molasses
with urea increased ruminal NH3-N and milk urea N
when cows were fed corn silage diets (6.8 vs. 11.3 and
7.6 vs. 12.0 mg/dL for M vs. MU, respectively). Based
on ruminal fermentation characteristics and fatty acid
isomers in milk, molasses did not appear to promote
ruminal acidosis or milk fat depression. However, combinations
of Rumensin with corn silage-based diets
already containing molasses and with a relatively high
nonfiber carbohydrate:forage neutral detergent fiber
ratio influenced biohydrogenation characteristics that
are indicators of increased risk for milk fat depression.