With the new VFD (Veterinary Feed Directive), farmers are bound by new restrictions regarding antibiotics they can administer. The VFD is the result of increasing concern about antibiotic resistance, although it only applies to fed antibiotics administered to food-producing animals.
Although meat and milk are routinely tested for the presence of antibiotics, what about the presence of antibiotics in manure, and does that potential residue affect soil?
Dr. Stephanie Lansing, Department of Environmental Science and Technology at the University of Maryland, has been studying the persistence of antibiotics in manure and what happens to antibiotics in manure even though the cow may have passed the number of days to withhold milk.
“Even though the milk from treated cows is not going into the milk supply, the manure is still in the manure supply,” said Lansing. “We know how long an antibiotic stays in the milk, but we don’t know how long it stays in the manure. How much is being excreted, and if it is, what do we do with that manure?”
Part of the research is focused on trying to determine whether the treatment technologies for manure (composted, digested or directly field applied) are contributing to antibiotic resistance, or do the treatment technologies deal with the degradation of antibiotics.
Lansing says her research group secured 11 dairy herds to work with, and they’re tracking antibiotic treatments for various health issues such as hoof problems, pinkeye and mastitis. One of the findings is that some farms gave the same antibiotic for the same issue, but in some cases, different antibiotics were used for the same problem. “It’s based on the vet the farmer has and what the vet prescribes,” she said, “so we’re not seeing consistency.”
The work Lansing is doing also includes manure from the University of Maryland dairy barn, which is equipped with a flush system that flushes several times each day using water from the lagoon. “They rake out bedding, and the water flushes it by gravity into the first pit,” she said, comparing it to an alley scraping system used in other barns. “What we’re trying to understand is how manure handling affects the degradation of antibiotics. Some drug classes break down more easily than others. Penicillin, for example, breaks down really easily. We know penicillin is being administered, we take the sample and an hour later it’s gone. But other drugs are much more persistent.”
After material is flushed from the barns, it enters the first holding pit. “About once a day, that material is sent to a bar press system,” Lansing explained. “What happens is that the solids come out in a big cone, and there’s a volcano-shaped pile of solids. The liquids go underneath into a pit.”
On an average farm, the liquid portion would be pumped to a nearby lagoon, but at the university research farm, it’s pumped across the road, up a hill to a lagoon system.
The university dairy farm composts the solid portion of manure, and Lansing says there are different concentrations of nutrients in the solids versus the liquids. “The compost has been processed and doesn’t smell,” she said. “We can put that on fields at any time of year without people complaining about the smell.”
An additional benefit of solid separation is that in states with strict cut-off dates for manure application, manure solids can be stored in composted rows until application is allowed. Additional research includes looking at how much antibiotic degradation there is after solids are separated from liquids, and how much residue there is in the composted portion.
Dairy manure collection and processing in relation to testing for antibiotic residue is challenging due to the variety of cow housing and manure handling systems in use. Some barns are scraped, while others are flushed or use a bedded pack; especially for sick animals. Manure treatment also varies widely from farm to farm, and includes solids separation, composting, anaerobic digestion and long-term storage on a pad. Antibiotic usage varies widely on farms depending on what conditions are being treated, the frequency of treatments and the veterinarian’s preference. Further complicating the issue is the fact that in tests conducted so far antibiotic residues have not always corresponded with treatment events.
After a cow is treated, the antibiotic gets into the manure, is spread on the field and potentially enters the soil. “What we’re concerned about is the bacteria in the soil, and how that soil is affected by antibiotics,” said Lansing. “Soil bacteria exposed to antibiotics over time could develop resistance. It isn’t a manure-to-food issue. We’re more worried about manure-to-food-to-bacteria, and the potential for developing resistance.”
Lansing says the farmers she has been working with on the project have been very helpful, and have provided daily of treatment records for individual animals. “We know exactly the concentration of antibiotics on each farm,” she said. “Each of the 11 dairy farms has a different manure management system. We know how much antibiotic has been administered, and we can track it.”
So far, research has shown that antibiotics tend to adhere to the solids portion of manure. Nutrient concentrations increase in the solids portion as moisture content is reduced and nutrients are conserved. Compost appears to be an effective method of positively contributing to antibiotic degradation on most dairy farms. Continued testing of compost piles over time will help determine antibiotic degradation in relation to composting temperatures.
“One of the things we’re trying to do is understand that process better,” said Lansing. “We’re looking at sick cow bedding, and what temperature do we have to reach during composting? We’re looking at both pathogen management and antibiotics — what does a composter or digester do?”
Research has also shown that farm antibiotic administration varies throughout the year and depends on illnesses in the herd and time of year. Antibiotic spikes (in manure samples) can correlate to a specific treatment event. Because these spikes may be seen in only one part of the manure treatment system, testing is being conducted to track antibiotic spikes through the entire manure trail.
“We want to understand the variability throughout the whole process,” said Lansing. “We don’t have good answers yet, but we want to develop a platform so that farmers, veterinarians and policy makers can communicate. We don’t want to pit environmentalists against farmers. It’s a complicated issue.”