Particle size and endosperm type of dry corn grain altered duodenal flow of B vitamins in lactating dairy cows.

The objective of the experiment was to determine if factors such as endosperm type (floury vs. vitreous) and particle size (fine vs. medium) of dry corn grain, known to affect starch digestibility in the rumen, modify apparent ruminal synthesis and duodenal flow of B vitamins in lactating dairy cows. Eight lactating multiparous Holstein cows equipped with rumen and duodenal cannulas were assigned randomly to a treatment sequence according to a 2 × 2 factorial arrangement in duplicate 4 × 4 Latin square design experiment. Duration of each experimental period was 21 d. When expressed per unit of dry matter intake (DMI), floury treatments increased duodenal flow and apparent ruminal synthesis of niacin and folates but tended to increase apparent degradation of thiamin in the rumen. Duodenal flow of thiamin, riboflavin, niacin, folates, and vitamin B12, expressed per unit of DMI, decreased with an increase in particle size. Similarly, apparent degradation of thiamin and riboflavin was greater and apparent synthesis of niacin, folates, and vitamin B12 was reduced when cows were fed coarser dry corn grain particles. Neither endosperm type nor particle size had an effect on duodenal flow and apparent ruminal synthesis of vitamin B6. Apparent ruminal syntheses, expressed per unit of DMI, of all studied B vitamins but thiamin were negatively correlated with apparent ruminal digestibility of neutral detergent fiber. Duodenal flow of microbial N was positively correlated with apparent ruminal synthesis of riboflavin, niacin, vitamin B6, and folates. Under the conditions of the present experiment, except for thiamin, the effects of factors increasing starch digestibility of dry corn grain in the rumen on the amounts of B vitamins available for absorption by the dairy cow seem to be mediated through differences on ruminal digestibility of neutral detergent fiber and, to a lesser extent, on duodenal microbial N flow.

Yeast culture supplementation prevented milk fat depression by a short-term dietary challenge with fermentable starch.

Effects of yeast culture on responses to a fermentable starch challenge were evaluated in an experiment with a crossover arrangement of treatments for yeast culture supplementation with 28-d periods and a fermentable starch challenge on the last 2 d of each 28-d period as a split plot within period. Eight ruminally cannulated, midlactation, multiparous Holstein cows (96 +/- 14 d in milk) were randomly assigned to treatment sequence. Treatments were yeast culture or control (mix of dry ground corn and soybean meal), top-dressed at 56 g per head per day throughout each period. Diets containing dry ground corn grain were fed from d 1 through 26 of each period. On the last 2 d of each period, the dry ground corn was replaced by finely ground high-moisture corn grain on an equivalent dry matter basis to abruptly increase ruminal fermentability of dietary starch. Response variables were averaged for d 25 and 26 for the dry corn treatment and for d 27 and 28 for the high-moisture corn treatment each period. The fermentable starch challenge decreased dry matter intake by 1.9 kg/d and tended to increase milk yield compared with the dry corn diet. However, effects of the fermentable starch challenge on yield of milk fat varied for the yeast culture and control diets; yield of milk fat decreased from 1.42 to 1.30 kg/d for the control treatment but increased from 1.40 to 1.47 kg/d for the yeast culture treatment. Milk fat concentration tended to decrease from 3.34 to 3.03% during the dietary challenge compared with the base diet for the control treatment but was not affected (mean = 3.32%) by the dietary challenge for the yeast culture treatment. An interaction of treatments was also detected for fat-corrected milk, which increased from 41.0 to 43.0 kg/d for the yeast culture treatment but decreased from 41.6 to 39.8 kg/d for the control diet with the fermentable starch challenge. Frequency of ruminating bouts was decreased by yeast culture compared with control (12.8 vs. 15.7 bouts/d) but not the fermentable starch challenge. No treatment interactions were observed for any measure of ruminal pH, total or individual volatile fatty acid concentration in ruminal fluid, acetate:propionate ratio, or individual fatty acid isomers in milk fat. Yeast culture supplementation may help prevent depression in milk fat during transition to a diet with highly fermentable starch, but the mechanism responsible remains to be elucidated.