Transport of fecal bacteria to fields unaffected by corn, WDGS diets
Manure applied to fields from cows fed corn or dried distillers grains with solubles did not affect the transport of fecal indicator bacteria, according to research conducted by Agricultural Research Service scientists at the Agroecosystems Management Research Unit in Lincoln, NE.
Studies in recent years have found a higher prevalence of Escherichia coli 0157:H7 (E. coli) bacteria in the manure and on the hides of cattle fed high amounts of wet distillers grains with solubles (WDGS). Research has also looked at how E. coli pathogens travel through the beef production process to improve the safety of the food supply.
The recent study by the Agricultural Research Service’s (ARS) Lincoln lab was not focused on food safety, which is typically the focus of E. coli studies, but instead looking at bacteria from a manure management standpoint, according to Lisa Durso, research microbiologist at the ARS lab at the University of Nebraska-Lincoln.
Durso collaborated with other ARS scientists to determine if animal diet (corn vs. 40 percent WDGS) affects how fecal bacteria and viruses are transported in runoff when cattle manure is applied to agricultural fields as a nutrient source.
“We were concerned because of the difference in E.coli prevalence in corn and distillers grains manures,” she said. “We wondered if there were any differences in how it persisted in the environment.”
The researchers used manure from both corn and WDGS-based diets to con ventional till and no-till fields and applied both at one, two and four-year application rates. They then conducted a series of simulated rain events and analyzed samples of the field runoff.
The team found neither diet nor tillage method significantly affected fecal bacteria transport.
“We were expecting to find differences and we didn’t see many at all,” Durso said. “In general, the differences in members of bad E. coli associated with manure in animals did not persist in the environment.”
Durso explained that E.coli numbers are low to start with compared to all bacteria in manure. That number is even smaller in a soil-tomanure ratio.
“There are so many microbes in the soil that compete with E. coli and break down,” she said. “E. coli is much happier inside the animal than outside in the environment. While there is some bacterial reproduction, out in the environment, E. coli is more in survival mode.”
While there were no differences in bacterial transport from either diet, the team did find that diet affected the transport of bacteriophages viruses that invaded bacteria in runoff from fields.
Viruses are another water quality fecal indicator. Durso explained that viruses are smaller in size than bacteria, so that size difference may be a possible cause of the difference in transport size.
The higher transport of viruses came from the WDGS diets, she said. The team was able to recover significantly more phage in runoff from plots amended with manure from animals fed WDGS diets compared to the corn-fed and control diets.
Durso said she believes there is an explanation of those results.
“I think that one of the main factors that might be influencing the results we observed is the difference in pH between the two manures,” she said. “The phage particles are just that—particles. They are small lone fragments that can potentially be influenced by pH.”
Durso said cattle manure from animals fed WDGS has different physical, nutrient and microbial qualities compared to manure from animals fed traditional corn finishing diets, and sometimes includes greater concentrations of pathogenic bacteria like E. coli 0157:H7.
But unlike phage, Durso and the other scientists found no differences in how bacteria are transported in run-off.
“This indicates there is no additional concern over the microbiological quality of surface and ground waters in agricultural watersheds when distillers grains are fed to animals instead of corn,” Durso said.
“Unlike bacteria, we did observe differences in how some non-human viruses were transported,” she said. “These differences were attributed to particle size and pH of the manure.”
Durso said there is still a lot to be learned about how to best use manure as a fertilizer while minimizing adverse environmental impacts, but said that additional studies will likely depend on funding. — Cheryl Anderson, DTN