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The bovine gastrointestinal tract is a reservoir for E. coli O157:H7, Salmonella, and to a lesser extent, Campylobacter. Cross-contamination from the intestinal track to the carcass can occur during harvesting and fabrication. Postharvest interventions developed to kill pathogens on the carcass have been very effective, but can be overwhelmed if cattle entering the slaughter facility have too much existing contamination. Practical preharvest interventions are needed to compliment the industry’s multi-hurdle approach to pathogen reduction and elimination. Quantitative risk assessments indicate such interventions would reduce human exposure to pathogens.
U.S. Department of Agriculture (USDA) researchers have developed a chlorate product that exploits the ability of E. coli and Salmonella to utilize nitrate. This strategy selectively targets bacteria possessing the enzyme nitrate reductase so that these pathogens are killed but beneficial anaerobes lacking the enzyme are unharmed. The selective bactericidal activity has been demonstrated both in vitro and in vivo. A nitrate preconditioning period has been shown to enhance the bactericidal effect of chlorate in broilers and pigs; however, because of the rapid reduction of nitrate in the rumen, this strategy is not practical in cattle.
The researchers proposed that using nitrocompounds, rather than nitrate, might still be effective in enhancing the bactericidal effect of chlorate. Nitroethane is a relatively innocuous compound that can also improve gross energy utilization, thus giving producers a cost-effective preharvest intervention.
The objectives of this project were to obtain fundamental information on the absorption, passage and ruminal and gastrointestinal metabolism of nitroethane. This information will help determine if this can be developed into a practical and cost-effective preharvest intervention technology.
Two experiments were conducted. In the first, two ruminally cannulated Holstein cows were administered 0.12 grams of nitroethane per kilogram of body weight per day. This was done twice a day for a period of eight days. Rumen fluid and feces collected before and during the treatment period were analyzed for methane-producing activity.
In the second experiment, 18 head of feedlot cattle were administered 0, 0.08 or 0.16 grams of nitroethane per kilogram of body weight per day. The supplement was fed twice a day as part of a 50 percent concentrate ration. In order to adequately evaluate a nitroethane and chlorate supplement’s effect on reducing pathogen incidence, the steers in this study were inoculated four days prior to the initiation of treatment with Campylobacter and Salmonella. Ruminal samples and fresh fecal samples were collected and cultured for E. coli, Salmonella and Campylobacter. After 14 days of nitroethane treatment, the cattle were fed an experimental chlorate product and were again sampled 24 and 48 hours later to determine the effects on ruminal and fecal bacteria.
The administration of nitroethane reduced ruminal methane-producing activity from pretreatment measures by as much as 75 percent. Fecal methane-producing potential appeared to increase however, which indicated to the researchers that ecological changes were occurring within the rumen and lower gastrointestinal tract. Results from an affiliated study revealed that whole animal methane production was still reduced by as much as 30 percent compared to nontreated control animals. Rates of ruminal and fecal nitroethane degradation were also increased during nitroethane administration thus indicating the in vivo enrichment of nitroethane-respiring bacteria. This suggests that while the nitroethane may initially cause a direct inhibition of ruminal methanogenesis, the rumen appears to adapt to nitroethane over time. The adaptation appears to be beneficial because increased populations of nitro-respiring bacteria may effectively outcompete methanogens making nitroethane a viable methane inhibiting strategy for cattle.
Unlike the results observed in Experiment 1, the nitroethane treatment reduced fecal methane- producing activity by more than 60 percent compared to the rates seen in the steers administered no nitroethane. The researchers hypothesized that the differences in fecal methane-producing activity between the two experiments might be due to the higher dose that the steers in Experiment 2 received or that the animals were on significantly different diets. The cows in Experiment 1 were on a primarily rye grass diet, while the steers in Experiment 2 were on a 50 percent concentrate feedlot ration.
Analysis of whole animal methane emissions using a sophisticated tracer gas technique revealed that nitroethane-treated steers lost 15 to 25 percent less of their gross energy intake as methane than did untreated control steers. The limited number of steers in the study did not allow the researchers to demonstrate that the differences were statistically significant.
Nitroethane administration had no effect on ruminal or fecal concentrations of generic E. coli or Campylobacter. Salmonella were recovered only from enriched specimens, thus indicating that concentrations were quite low (<10 colony forming units/gram of contents). Salmonella were recovered from pretreatment specimens, either ruminal or fecal contents, from 12 of the 18 steers. Two of these steers had been randomly allocated to be controls, five to receive 0.16 gram nitroethane (per kilogram of body weight per day). Salmonella was recovered from only one steer (a steer that had been treated with 0.16 grams of nitroethane per kilogram of body weight per day) at the end of the 14 days of nitroethane treatment. The chlorate treatment given at the end of the trial (day 15) reduced generic E. coli concentrations more than 1,000-fold within 24 hours after treatment. By 48 hours after treatment, E. coli concentrations were only 10-fold lower than the pretreatment levels.
Contrary to findings from numerous laboratory studies, results of this study did not support the hypothesis that nitroethane combined with chlorate would reduce Campylobacter and Salmonella in feedlot cattle. The researchers indicated this could be due to a rapid absorption and rumen degradation that prevented the accumulation of nitroethane to levels needed to be effective against enteropathogens. Feeding chlorate in the last day’s ration reduced fecal E. coli concentrations by more than 1,000-fold, but this was not enhanced by prior nitroethan treatment. The nitroethane treatments did reduce methane-producing activity (an indirect measure of numbers of methane-producing bacteria) in the rumen contents of range and feedlot cattle.