Halogenated hydrocarbons such as polychlorinated biphenyls (PCB's), heptachlor (HEP), 1, 1, 1,-trichloro-2, 2-bis(p-chlorophenyl)-ethane (DDT), and pentachlorophenol (PCP) are environmental contaminants and, at times, can bioaccumulate in the food chain. Cattle have been contaminated in a variety of ways, but generally it is believed that they are only affected by high concentrations of the chemicals. Rumen microorganisms, however, may be affected at lower doses, thus possibly affecting the cow's growth and milk production. Polychlorinated biphenyls, HEP, DDT, and PCP were tested by a l-stage in vitro fermentation procedure. Substrate utilization was determined by measuring percent dry matter disappearance. Four concentrations (0, 10, 50, and 100 ppm) were studied, and in vitro incubations were conducted for 24 and 48 hr. Samples were removed from 48-hr incubations to determine if the chlorinated hydrocarbons were metabolized during fermentation. Dry matter disappearance proved to be a reliable method to determine microbial activity in the presence of chemicals. Substrate dry matter disappearance for controls and all concentrations of PCB's, HEP, and DDT was approximately 50 and 80% at 24 and 48 hr, respectively. The PCP significantly (P < 0.05) depressed the percent dry matter disappearance in 50- and 100-ppm cultures to 45 and 30% at 24 hr and 70 and 50% at 48 hr, respectively. Metabolic changes in the test chemicals were not detected by gas chromatographic analysis.
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