| Four experiments and one data mining study were conducted to identify factors that alter odd and branched-chain fatty acids (OBCFA) in rumen, milk, or a rumen in vitro system. In the first experiment, ruminal infusions of volatile fatty acids (VFA) were expected to increase concentrations of their homologous OBCFA in rumen contents and milk. Four cows were assigned to a 4 x 4 Latin square and infused for 18 h with acetate, propionate (PR), isovalerate (IV), and anteisovalerate (AIV). Rumen and milk OBCFA responses were minimal following infusions of IV and AIV, suggesting limited use of IV, and AIV for de novo OBCFA synthesis. Minor increases in milk odd chain fatty acids (FA) after PR infusions support the presence of post-absorptive synthesis of these FA. The next experiment used between-animal variation to observe the effect of exchanging rumen contents between pairs of cows on rumen and milk branched-chain fatty acid (BCFA). Based on principal component analysis, animals on the same diet had greater cow by day effects on rumen BCFA than cow effects. Milk BCFA were influenced more than rumen by individual cow. An in vitro experiment compared microbial BCFA production between branched-chain amino acids, Ile and Leu, versus their branched-chain VFA products, AIV and IV. Inclusion of Leu resulted in greater FA accumulation than Ile. Aside from their BCFA products, AIV and IV had similar FA yields. To gauge the effect of anteiso fatty acids to support microbial growth in an acidic environment, two carbohydrate sources, isolated alfalfa silage neutral detergent fiber (FIB) and corn starch (STA) were fermented in media at pH 5.6 or 6.3, with or without AIV. Fermenting FIB increased FA the greatest in pH 6.3 media. As expected, anteiso C15:0 and anteiso C17:0 increased with AIV for FIB, with greater anteiso FA in pH 5.6 media. No differences were observed in anteiso FA for STA treatments. The final study used data from three lactation studies and observed that milk C17:0 to milk C15:0 was a better predictor of plasma non-esterified fatty acids, an indicator of energy balance and adipose mobilization, than C18:0 to C14:0. |
