Report on research conducted at the University of Nebraska, originally printed in the 2021 Nebraska Beef Cattle Report.
Summary
There is a growing public concern regarding antibiotic resistance and the use of antibiotics, including in livestock management. Understanding the ecology of antibiotic resistance among microbes, identifying resistance gene reservoirs, and implementing antibiotic resistance mitigation practices in livestock production is critical to protecting animal and human health while meeting increasing food demands. This research is one of several studies seeking to assess risk for livestock-to-human transfer of antibiotic resistance and to identify mechanisms for reducing that risk where possible. This study evaluated the impact of forage concentration and supplemental essential oil in beef cattle finishing diets on antibiotic resistance in freshly excreted and consolidated beef feedlot manure. Results indicate that antibiotic resistance in manure was not impacted by either of the two dietary treatments considered.
Introduction
Antibiotics are widely used in agricultural livestock production and human medicine for the treatment of infectious diseases. However, the use of antibiotics applies selective pressure to the gut microbiome of animals and humans, resulting in the excretion of antibiotic-resistant (AR) bacteria in animal and human feces. The widespread use of animal manures as fertilizer in agricultural production has resulted in growing concerns about the potential risks of antibiotics, AR bacteria, and AR genes present in animal manures and their impact on the public, animal, and environmental health.
Forage is included in feedlot diets to improve microbial protein synthesis in the gut but inclusion is minimized because the economic gains from improved ruminal health do not generally outweigh the losses due to a lower average daily weight gain. However, the documented benefits of forage on the ruminal microbiome suggest that increasing forage in finishing diets could reduce AR development in the animals, thereby influencing potential AR-related food safety and environmental exposure risks to people. Essential oils are believed to possess strong antimicrobial effects, suggesting that the addition of essential oils to animal feed may be a viable alternative to antibiotics in animal feed and a means to prevent the development of AR in the animal gut.
The objectives of this study were to quantify the effect of essential oil and forage concentration in beef finishing diets on the concentrations of four AR bacterial populations important to human and animal health – azithromycin (AZ)- and tetracycline (TET)-resistant Escherichia coli and tylosin (TY)- and TET-resistant Enterococci – in freshly excreted manure and consolidated pen surface material from a beef feedlot operation.
Procedure
This study was conducted at the Eastern Nebraska Research and Extension Center (ENREC), near Mead, NE. Four hundred, twenty beef cattle were assigned to 42 pens with each pen assigned randomly to one of six treatments: feed containing 14%, 47%, or 80% corn silage with or without essential oil supplement. The remainder of the diet consisted of dry-rolled corn, 16% wet distillers’ grains, monensin (30 g/ton), and tylosin (Tylan®) (90 mg/steer/day). Samples of freshly excreted cattle manure and consolidated feedlot surface material from two areas of each pen – near waterers and at the backs of pens – were retrieved from each pen four times (February through June) during the finishing period. Samples were spiral-plated in duplicate on agar to select for four types of antibiotic-resistant bacteria: azithromycin (AZR)- and tetracycline (TETR)-resistant Escherichia coli and tylosin (TYR)- and TETR-resistant Enterococci. Colony-forming units per gram of sample were enumerated by manual plate counting and log transformation was performed on all values.
Results and Discussion
Examination of the ratio of AR bacteria to total bacterial concentration (Table 1) reveals that the concentration of TYR Enterococci and AZR E.coli were quite high relative to the measured total concentration of each bacteria in samples throughout this study. These high concentrations are not surprising given that the animals were fed tylosin, which suggests that bacteria with resistance to tylosin would have had an advantage over other bacteria. Perhaps more surprising is that AR bacteria were present in all the manure samples collected in this study, including bacteria that were resistant to antibiotics not administered to the animals (tetracycline) indicating either a certain degree of baseline resistance must be expected or an environmental selection for tetracycline resistance not directly related to antibiotic use.
When the impact of dietary forage concentration was averaged for both presence and absence of essential oils TETR E.coli showed significant (a=0.05) differences due to forage concentration in both the freshly excreted manure and pen surface material (Table 1). In freshly excreted manure the mean ratio of TETR E.coli was lower in manure samples from pens where cattle received a 14% forage diet and the highest bacterial concentrations in manure from cattle receiving an 80% forage diet. However in consolidated pen surface material, the mean ratio of TETR E.coli was lowest in samples from pens where cattle received an 80% forage diet and highest in samples from cattle receiving a 47% forage diet, the 14% diet was not significantly different from either of the two higher concentration diets. The results of this study indicate that a beef cattle finishing diet low in dietary forage concentration produces the same effect on AR bacteria concentrations in manure as high forage, and in one population (TETR E.coli in pen fresh manure) a low dietary forage concentration was the most effective for reducing AR in manure.
The inclusion of a proprietary blend of essential oils to the finishing diets of cattle in this study did not impact any of the AR bacterial concentrations in freshly excreted manure or consolidated feedlot pen surface material (Table 1).
| Treatment | Fresh Manure | Pen Surface Material |
||||||
|---|---|---|---|---|---|---|---|---|
| AZR E. coli/ Total E. coli | TETR E. coli/ Total E. coli | TETR Enterococci/ Total Enterococci | TYR Enterococci/ Total Enterococci | AZR E. coli/ Total E. coli | TETR E. coli/ Total E. coli | TETR Enterococci/ Total Enterococci | TYR Enterococci/ Total Enterococci |
|
| Essential Oil | P = 0.087 | P = 0.148 | P = 0.622 | P = 0.133 | P = 0.579 | P = 0.723 | P = 0.450 | P = 0.185 |
| Yes | 0.68 | 0.25 | 0.52 | 0.94 | 0.74 | 0.21 | 0.73 | 0.89 |
| No | 0.72 | 0.20 | 0.52 | 0.94 | 0.75 | 0.19 | 0.72 | 0.87 |
| Forage Concentration | P = 0.459 | P = 0.003 | P = 0.073 | P = 0.519 | P = 0.743 | P = 0.041 | P = 0.686 | P = 0.357 |
| 80% | 0.72 | 0.21 b | 0.08 | 0.75 | 0.76 | 0.15 a | 0.23 | 0.55 |
| 47% | 0.69 | 0.18 a | 0.11 | 0.74 | 0.74 | 0.25 b | 0.23 | 0.58 |
| 14% | 0.69 | 0.17 a | 0.22 | 0.68 | 0.73 | 0.19 ab | 0.27 | 0.53 |
Conclusions
The results of this research indicate that beef finishing diets with low silage concentrations (14%) are equally or more effective than diets with higher silage concentrations for reducing AR bacteria concentrations in manure. The presence of bacteria resistant to antibiotics not given to the animals during the study also indicates that co-selection for multiple resistances inside the animal’s digestive tract or environmental factors at the feedlot may have more impact on AR in manure than dietary treatments. Furthermore, because there was little impact by dietary changes on AR bacteria in manure, it will be important to continue to examine manure treatment, storage, and application strategies that may mitigate potential human health risks from manure-borne AR bacteria.
Acknowledgments
Funding for this research was provided by USDA-NIFA Award No. 2017-68003-26497.
Authors
- Mara Zelt, graduate student, Biological Systems Engineering, University of Nebraska–Lincoln
- Amy Schmidt, associate professor, University of Nebraska– Lincoln
- Noelle Mware, graduate student, University of Nebraska–Lincoln
- Xu Li, associate professor, University of Nebraska – Lincoln
- Galen Erickson, professor, University of Nebraska – Lincoln