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Apparent ruminal synthesis of B vitamins in lactating dairy cows fed diets with different forage to concentrate ratios.

M. Seck, J.A. Voelker Linton, M.S. Allen, D.S. Castagnino, P.Y. Chouinard, C.L. Girard
journalOfDairyScience, Journal of Dairy Science Vol. 100, Issue 3, p1914–1922
Published: January 1, 2017

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.