The science of manure, including its management and handling, options for treatment, use as fertilizer, the impact it can have on the environment, and new technologies being developed to improve its use. In this blog I strive to provide a scientific perspective and really dig into the issues. This blog is brought to you by Iowa State University Extension and Outreach. Follow me on Twitter @DrManure or find me on Facebook at Iowa State Manure and Nutrient Management Lab.
Wednesday, November 18, 2015
Emerging Issues in Manure Management: Are manures increasing antibiotic resistant bacteria?
Did you know it is the World Health Organizations Antibiotic
Awareness Week? In light of this, I decided it was a good week to look at antibiotic
resistance bacteria in manures. Now this isn’t a topic I consider myself and
expert on so I’m going to be borrowing heavily from my college at Iowa State, Dr.
Antimicrobials, such as antibiotics, are used in the
livestock industry at therapeutic levels for disease treatment. Once the
antibiotic has been administered, the animal will begin to metabolize it (break
it down), but not all of it is metabolized, some is excreted with the feces and
urine, ending up mixed with the manure. For example, in the case of tylosin about ¾ of
the mass administered to an animal ends up in the manure.
Why does this matter? Well once these compounds are in the manure
they put a selective pressure on the microbes in it to become resistant to that
compound. It is sort of like this, if its cold outside you can stand outside
and shiver or you can put a jacket on. For microbes it’s sort of the same thing;
the presence of that compound might negatively impact most of the microbes, but
perhaps a few of them will figure out to put a jacket on (become resistant to
that antibiotic). Then when the manure is land applied as a fertilizer, these resistant
bacteria enter the environment. If it happens to be a microbe that makes people
sick and they come into contact with it there is a chance that our normal
antibiotics might not be as effective for this guy, because it already has some
built up tolerance to it. Sure, there are lots of ifs in this case – are they
in the manure to start with, how long do they last, does the animal and human
antibiotic even work in the same way (similar mechanisms) – but that’s what
people are trying to find out.
Picture of Enterococcus
So in a recent study Dr. Soupir performed she tested how swine
manure application (fall applied swine manure at 168 kg N/ha, or about 150 lb
N/acre compared to spring injected UAN) and tillage practices (chisel plowing
versus no-till) impacted the persistence and transport of enterococci; both
total enterococci and those resistant to tylosin (a type of bacteriosat that
was used at the farm the manure was obtained from).
So they measured lots of things in this study but what we
are going to focus on is enterococci (and tylosin resistant bacteria) in the manure
(at the time of application), in the soil (both in the fall after manure was
applied and in the spring), and in tile water over the following growing
season. They found that the manure had between 90,000 and 570,000 colony
forming units per gram of manure and that between 70-100% of these bacteria
were resistant to tylosin. When it came to the soil samples, they tested both
in the manure application band, outside the manure application band, and in the
control plots that didn’t receive manure. Not surprisingly, enterococci
concentrations were the greatest in the manure injection band, and lowest in
the control soils that didn’t receive manure. Concentrations of enterococci between
the manure bands were similar to the non-manured soils. Over the winter,
enterococci concentrations decreased by about 70 and by the time the manure had
been in the soil for a year had returned to levels equivalent to soils not receiving
manure. No difference in enterococci concentrations in the tile drainage water
So where does this leave use? At least during this study,
when weather conditions were drier than normal for Iowa, it doesn’t appear that
manure injection changed the risk of to water quality. However, different
weather conditions where it is wetter during and after manure application, may
impact these results.
A publication detailing this study is available at:
Check it out if you are interested in learning more.