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This experiment attempted to lower rumen lipolytic activity, biohydrogenating activity, or both using antimicrobial compounds. In vitro incubations were carried out with rumen fluid, 80 mg of soybean oil, and .5 g of commercial concentrates as substrate. Unless stated otherwise, the final concentrations of the additives in the incubation was 20 ppm. Lipolysis and biohydrogenation were determined by separation of triacylglycerols and FFA by TLC; the fatty acid composition of each was determined by GLC before and after incubation and with or without additive. With some of the antibiotics, lipolysis was inhibited 10 to 20%, and the most potent inhibitors were ionophores and amoxicillin. Biohydrogenation (including C18:1) decreased only for lasalocid, but no additive could prevent hydrogenation of linolenic acid liberated from triacylglycerols. Some additives decreased hydrogenation of linoleic acid, but only slightly. Lipolytic activity decreased VFA production more than the other potent additives (amoxicillin, avoparcin, lasalocid sodium, monensin, and salinomycin sodium). This result could indicate a more specific toxic effect on lipolytic microbes. Finally, different antimicrobials influenced fermentation patterns differently (VFA proportions and CH4 production), but shifts were always in accordance with stoichiometric principles.

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