Effects of the forage-to-concentrate ratio on apparent
ruminal synthesis of thiamine, riboflavin, niacin,
vitamin B6, folates, and vitamin B12 were evaluated in
an experiment using 14 ruminally and duodenally cannulated
Holstein cows. The experiment was a crossover
design with two 15-d treatment periods and a 14-d
preliminary period in which cows were fed a diet intermediate
in composition between the treatment diets.
Treatments were diets containing low-forage (44.8%
forage, 32.8% starch, 24.4% neutral detergent fiber) or
high-forage (61.4% forage, 22.5% starch, 30.7% neutral
detergent fiber) concentrations. Both diets were formulated
with different proportions of the same ingredients.
Concentrations of B vitamins were analyzed in feed and
duodenal digesta. Apparent ruminal synthesis of each B
vitamin was calculated as the duodenal flow minus the
intake. The high-forage diet had the highest concentrations
of riboflavin, niacin, vitamin B6, and folates,
whereas the low-forage diet had the highest thiamine
concentration. Vitamin B12 in the diets was under the
level of detection. Consequently, despite a reduction in
dry matter intake when the cows were fed the high-forage
diet, increasing dietary forage concentration increased
or tended to increase intakes of riboflavin, niacin, and
vitamin B6 but reduced thiamine and folate intakes. Increasing
dietary forage concentration reduced apparent
ruminal degradation of thiamine and apparent ruminal
synthesis of riboflavin, niacin, and folates and increased
ruminal degradation of vitamin B6, but had no effect
on ruminal synthesis of vitamin B12. As a consequence,
increasing the forage-to-concentrate ratio had no effect
on the amounts of thiamine, riboflavin, and vitamin B12
reaching the small intestine but decreased the amounts
of niacin, vitamin B6, and folates available for absorption.
Apparent ruminal syntheses of riboflavin, niacin,
folates, and vitamin B12 were correlated positively with
the amount of starch digested in the rumen and duodenal
flow of microbial N, whereas these correlations were
negative for thiamine. Apparent ruminal syntheses of
thiamine and vitamin B6 were negatively correlated
with their respective intakes, whereas folate intake was
positively correlated with its synthesis in the rumen.