Jan 30, 2008 (CIDRAP News) Recent studies suggest that an ethanol production byproduct that is widely fed to cattle may make cattle more likely to shed deadly Escherichia coli O157, possibly contributing to the surge in beef contamination cases in 2007.
The feed ingredient, called distillers' grain (DG), is what's left of corn or other grain after the starch in it is fermented into ethanol for fuel. Ethanol plants sell DG to cattle producers for feed. The material is cheaper than corn and other livestock feeds, helping cattle producers cope with high feed costs fueled by the ethanol boom, and yields valuable revenue for the ethanol plants, according to a recent report in the Des Moines Register.
But the growing use of DG as cattle feed may be contributing to E coli contamination in beef, recent studies suggest. In the latest study, published this month in Applied and Environmental Microbiology, Kansas State University researchers found that cattle that were fed DG were more than twice as likely to shed E coli O157 in their feces, compared with cattle that didn't eat DG.
The latest findings about the effects of DG follow a spike in beef recalls due to E coli contamination. In 2007 there were 21 recalls, totaling a record 33.4 million pounds of beef, according to Laura Reiser, a US Department of Agriculture (USDA) spokeswoman in Washington, DC. In 2006 there were just eight recalls involving about 182,000 pounds, and 2005 saw five recalls totaling about 1.25 million pounds.
Concern bout the DG research findings has prompted the USDA to launch its own study. The agency is checking the E coli prevalence in manure from 300 cattle being fed a diet that includes DG and in manure from another 300 cattle whose feed is free of the material.
The study was launched last fall at the USDA's Roman L. Hruska Meat Animal Research Center in Clay Center, Neb., and is to be finished by the end of this year. Feed given to the treatment herd in the study will be 40% distillers' grains, according to the Register report.
Study: Up to 9% of samples had E coli
In the Kansas State study, 379 yearling heifers were assigned to one of three diets, each of which consisted mainly of steam-flaked corn. The rest of the feed consisted of (1) 15% corn silage, (2) 15% corn silage and 25% DG, or (3) 5% corn silage and 25% DG. Fecal samples were collected and analyzed weekly for 12 weeks.
E coli O157 was found in 9.0% of the samples from the cattle that ate DG with 15% corn silage; the pathogen was found in 7.3% of samples from the group that ate DG with 5% corn silage. Both of these were significantly higher than the 3.6% prevalence in samples from the cattle that ate only corn and corn silage.
The findings agreed with results of a previous study from the same research team, according to the report. The earlier study also showed a higher E coli prevalence in heifers that were fed DG, but samples were collected and tested only twice during a 150-day feeding period, rather than weekly.
In the new study, the scientists also assessed the effect of DG on the growth of E coli in cultures of cattle stomach fluid and feces. Using a dozen steers that were fed diets with or without DG, the scientists took samples of stomach (ruminal) fluid and feces, added E coli O157, and incubated them for 24 hours. The ruminal samples from the DG-fed steers were found to have higher levels of E coli after 24 hours than the samples from the non-DG-fed steers, but the fecal samples showed no difference.
In a further culture experiment, the researchers took ruminal and fecal samples from two steers, added varying amounts of DG to the samples, incubated them, and then measured the levels of E coli O157 after 24 hours. Results from the ruminal samples were inconsistent, as the smallest amounts of DG led to growth of the pathogen, whereas larger amounts did not. But in the fecal preparations, greater amounts of DG stimulated more growth of E coli O157. The authors concluded that the fermentation experiments "provided some evidence that DG may actually stimulate the growth of E. coli O157."
The authors said their findings have "potentially serious ramifications" because of the growing use of ethanol as a biofuel and of DG as a feed ingredient. "The utilization of distillers' by-products as components of feedlot diets may depend in part on our ability to devise feeding strategies that do not compromise the perceived safety of beef products," the report states.
USDA study could weigh heavily
Francisco Diez-Gonzalez, PhD, associate professor of food science and nutrition at the University of Minnesota in St. Paul, said the methods and conclusions of the Kansas State researchers appear sound, but he cautioned that the findings should be seen as preliminary. It would be premature to conclude that the use of DG has contributed to recent E coli outbreaks and meat recalls, he told CIDRAP News.
"While the use of distiller's grains has become a very common practice in the Midwest where the bulk of the ethanol plants are located, we do not really know what type of diet the cattle that was the source of beef were fed for most of the [recent E coli] outbreaks," Diez-Gonzalez said. In the case of Topps Meat Co., which recalled 21 million pounds of beef last year, the beef trims used in ground beef were imported from a company in Alberta, he pointed out.
In addition, he said, "There is a possibility that the combination of distiller's grains with steam-flaked corn may have caused the observed increase in prevalence and counts." This would limit the scope of the findings, because steam-flaked corn is not commonly used as cattle feed in the Upper Midwest, where cracked corn is more common, he commented.
Mentioning the USDA study in Nebraska, Diez-Gonzaleza added, "Let's wait for their findings."
Implications for ethanol industry
The possible link between DG and E coli levels may also have implications for the future of the ethanol industry. A 2006 study in Minnesota suggested that without the use of DG for animal feed, the benefits of ethanol production would be seriously undermined. The study was reported in the Proceedings of the National Academy of Sciences.
The researchers conducted a comprehensive analysis of the costs and benefits of ethanol production from corn and biodiesel production from soybeans. They determined that ethanol from corn yields 25% more energy than is required to produce it. However, this positive energy balance was attributed almost entirely to an energy credit for the DG yielded by ethanol production. The energy credit was given because the DG offsets the production of other commodities such as corn and soybean meal, the report said.
Even if further research confirms a connection between DG and E coli in cattle, the USDA is unlikely to halt the use of DG in feed. According to the Des Moines Register story, Richard Raymond, the USDA's under secretary for food safety, said the agency had no intention of restricting the use of DG. He said it would be up to the industry to decide how to deal with the problem.
Jacob ME, Fox JT, Drouillard JS, et al. Effects of dried distillers' grain on fecal prevalence and growth of Escherichia coli O157 in batch culture fementations from cattle. Appl Environ Microbiol 2008 Jan;74(1):38-43 [Full text]
Brief description of USDA research project on the effects of DG on E coli O157 in feedlot cattle
July 2006 PNAS study of the costs and benefits of ethanol and biodiesel production