Introduction: This study was done to evaluate the effects of dietary addition of yeast extract
and autolysed yeast on feed intake, ruminal fermentation parameters, IL-6 gene expression and
antibody titers in early lactating dairy cows.
Materials&Methods: Twenty-five lactating cows were divided into five experimental groups
and five replicates in a completely random design. Experimental diets include basal diet
(control group), basal diet+20 g/d yeast extract, basal diet+40 g/d yeast extract, basal diet+20
g/d autolyzed yeast, and basal diet+40 g/d autolyzed yeast. At the end of the experiment period,
ruminal content samples were taken and fermentation products were measured. In the fifth
week of the experiment, the cows were injected with a vaccine of foot-and-mouth disease. On
weeks 7, blood sampling was taken and chemical components, antibody titer against foot-andmouth
disease virus and interleukin-6 gene expression were analyzed.
Results: Cows receiving yeast extract and autolyzed yeast at both levels had more dry matter
intake than the control group (P<0.05). Autolyzed yeast and yeast extract had no effect on the
daily milk production (P>0.05). Cows receiving autolyzed yeast had higher antibody titer
against foot-and mouth-virus than the control group (P<0.05), but cows receiving yeast extract
had no significant difference in antibody titer with the control group (P<0.05). Autolyzed yeast
had no effect on concentration of volatile fatty acids (P>0.05), but yeast extract increased
concentration of volatile fatty acids. Yeast extract decreased the molar percentage of acetate
and increased the molar percentage of propionate (P<0.05). The highest increase in the relative
expression of the interleukin-6 gene was observed in cows receiving autolyzed yeast.
Conclusion: Based on the results of this study, autolyzed yeast at 40 g/d can improve the
immune system, and yeast extract at 40 g/d can improve fermentation conditions in the rumen
of dairy cows.