Effects of diet fermentability and supplementation of 2-hydroxy-(4-methylthio) butanoic acid and isoacids on milk fat depression: 2. Ruminal fermentation, fatty acid, and bacterial community structure

The experiment was conducted to understand ruminal effects of diet
modification during moderate milk fat depression (MFD) and ruminal effects
of HMTBa and isoacids on alleviating MFD. Five ruminally cannulated cows
were used in a 5 × 5 Latin square design with the following 5 dietary
treatments (dry matter basis): HF-C, a high forage and low starch control
diet with 1.5% safflower oil; LF-C, a low forage and high starch control
diet with 1.5% safflower oil; LF-HMTBa, the LF-C diet supplemented with
HMTBa (0.11%; 28 g/d); LF-IA, the LF-C diet supplemented with isoacids
(0.24%; 60 g/d; IA); and LF-COMB, the LF-C diet supplemented with HMTBa
and IA. The experiment consisted of 5 periods with 21 d per period (14-d
diet adaptation and 7-d sampling). Ruminal samples were collected to
determine fermentation characteristics (0, 1, 3, and 6 h after feeding),
long chain fatty acids (FA) profile (6 h after feeding), and bacterial
community structure by analyzing 16S gene amplicon sequences (3 h after
feeding). Data were analyzed using the MIXED procedure of SAS in a Latin
square design. Preplanned comparison between HF-C and LF-C were conducted
and the main effects of HMTBa and IA and their interaction within the LF
diets were examined. The LF-C diet decreased ruminal pH and the ratio of
acetate to propionate, with no major changes detected in ruminal FA
profile compared with HF-C. The alpha diversity for LF-C was lower
compared with HF-C, and beta diversity also differed between LF-C and
HF-C. The relative abundance of bacterial phyla and genera associated
indirectly with fiber degradation was influenced by LF-C versus HF-C. As
the main effect of HMTBa within the LF diets, HMTBa increased the ratio of
acetate to propionate and butyrate molar proportion. Ruminal saturated FA
were increased and unsaturated FA concentration were decreased by HMTBa,
with minimal changes detected in ruminal bacterial diversity and
community. As the main effect of IA, IA supplementation increased ruminal
concentration of all branched chain volatile FA and valerate and increased
the percentage of trans-10 C18 isomers in total FA. In addition, alpha
diversity and the number of functional features were increased for IA.
Changes in the abundances of bacterial phyla and genera were minimal for
IA. Interactions between HMTBa and IA were observed for ruminal variables
and some bacterial taxa abundances. In conclusion, increasing trans-10 C18
isomers did not decrease milk fat yield for LF-C versus HF-C when dietary
PUFA level was similar. Supplementation of HMTBa increased
biohydrogenation capacity, and supplemental IA increased bacterial
diversity, possibly alleviating MFD. The combination of HMTBa and IA had
no associative effects in the rumen and need further studies to understand
the interactive mechanism.