Swimmers beware, land application of manure can increase antibiotic resistance downstream

A summary of Catchment-scale export of antibiotic resistance genes and bacteria from an agricultural watershed in central Iowa by Neher et al. 2020

Key points

  • With some year-to-year variation, manure application increased antibiotic resistance surface water downstream of application site.
  • The CAMRADES team, led out of Iowa State University, will be expanding efforts to monitor and model AMR in agricultural watersheds in the region.


While the livestock industry produces high-quality beef and pork products, it also generates manure. The accumulating manure is usually applied to row crops to fertilize the soil and sustainably manage the manure. The unfortunate consequence of manure application is the release of unwanted substances like undigested antibiotics. Antibiotics are a critical part of animal production that keeps animals healthy and increases their quality of life. Frustratingly, the use of antibiotics is a double-edged sword, where elevated concentrations of antibiotics can cause surrounding bacteria to develop immunity, known as antibiotic resistance. A global effort is being made to reduce the chances of antibiotic resistance from spreading. In animal production, efforts toward this goal include targeted antibiotic treatment, limited use as growth promoters, and evaluation of alternative manure utilization projects. This study aims to understand if manure application raises the levels of antibiotic resistance in downstream surface water.

The monitoring study developed by a research team in the Water Quality Research Lab at Iowa State University took place over two years from 2017 to 2018 in western Iowa’s Black Hawk Lake. A watershed is the area of land that drains water into a downstream area. There were ten swine confinements and six cattle lots within the watershed, and 74.6% of the land was in row crop production.

Monitoring location and gauge installation.

The study was designed to compare antibiotic resistance in downstream water from areas with manure application against areas without manure application. The researchers obtained water samples every two weeks and measured indicators of antibiotic resistance. The indicators measured in the water samples were bacteria species known to originate from the inside of warm-blooded animals. These indicator bacteria were grown in the lab in the presence of two different antibiotics. Any bacteria that grew was identified as resistant to the antibiotic. The researchers counted the resistant bacteria from each location and tested differences between areas with manure application and those without manure application.

Sampling locations of intermittent flow with manual collection.

The comparison of two smaller areas (600 acres) showed that the bacteria resistant to the antibiotics were higher in the manure applied area than in the area without manure application in 2017. However, there were no differences between the two in 2018. Additionally, the study measured the antibiotic-resistant bacteria from two slightly bigger areas (2000 acres) within the Black Hawk Lake Watershed. One area had manure application, and an upstream urban area influenced the other. The researchers found similar results where the antibiotic-resistant bacteria from the animal-manured area was higher than the urban-influenced area in both years of monitoring.

This study implies that areas with manure application increased antibiotic resistance in downstream surface water. These findings are important when considering the impact of manure application on antibiotic resistance in surface water. Many of the streams and rivers downstream of animal production eventually lead to recreational areas like lakes and public beaches with a greater chance of human exposure.

Thus, beginning in 2022 a new multi-state effort (called CAMRADES) to expand monitoring will kick-off in the region, combined with work to model effective mitigation strategies for managing AMR in the environment. Research efforts like this will continue to be essential because understanding the role of livestock production on the spread of antibiotic resistance will help keep antibiotics effective in human and animal medicine for generations to come.

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Learn more about mitigation and monitoring of AMR in freshwater bodies:


Written by Timothy Neher, graduate research assistant, Iowa State University.

The scientific research summarized in this article was published as:

Neher TP, Ma L, Moorman TB, Howe AC, Soupir ML (2020) Catchment-scale export of antibiotic resistance genes and bacteria from an agricultural watershed in central Iowa. PLOS ONE 15(1): e0227136. https://doi.org/10.1371/journal.pone.0227136