When it comes to the land application of livestock manure, a major environmental concern is the loss of manure-borne nutrients to surface runoff. Management practices and regulations focus on how nutrients travel in runoff as functions of the timing and method of land application as well as proximity to water sources. When manure is applied to soil, manure constituents other than nutrients are also introduced to soil, such as heavy metals, antibiotics, and antibiotic resistance genes. Knowledge about the behaviors of these manure constituents in the environment as a function of manure application strategies is limited.
In this article we will discuss the effects of various land application strategies on the fate and transport of manure borne antibiotics and antibiotic resistance genes in soil and runoff. This will be broken down into three sections: manure storage; land application methods; and vegetative barrier. Although our studies were conducted using swine manure slurry, it is expected that that the general conclusion would also apply to other types of manure. Prior to a detailed description of our findings, we will first present some background information about manure-borne antibiotics and antibiotic resistance genes.
Why Care About Antibiotics?
Antibiotics are often used in concentrated animal feeding operations (CAFOs) to prevent and treat diseases in livestock animals, increasing the density at which livestock can be kept. A substantial portion of these antibiotics can move thorough the digestive system of livestock, and end up in livestock urine and feces. These antibiotics can persist in the livestock manure and go on to alter the microbiome of soil and water. Bacteria exposed to these antibiotics may gain resistance to the antibiotics. This can be a major public health concern, because even the antibiotic resistant bacteria are harmless they may spread the resistance genes to pathogens. This, in turn, could impact human and animal health, as the antibiotics which we rely upon to treat infection will no longer be effective on treating resistant pathogens.
Prior to land application, manure is usually stored in livestock waste management structures. In one study, the effects of anaerobic storage of manure on the fate of antibiotics and antibiotic resistance genes in manure were investigated. In this study, the levels of chlortetracycline and tylosin in manure slurry were monitored. The two antibiotics in swine manure degraded substantially over time under the anaerobic condition. The antibiotic resistance genes corresponding to chlortetracycline was also reduced substantially during manure storage. In contrast, the resistance genes corresponding to tylosin did not decrease significantly.
Manure and manure slurry may be applied to fields using application methods such as broadcast, injection, and incorporation. These land application methods have varying effects on the spread of antibiotics and antibiotic resistance genes in croplands. A study was conducted to investigate how these land application methods may affect the concentrations of antibiotics and antibiotic resistance genes in runoff and soil following the land application of swine slurry. Results show that land application methods had no statistically significant effect on the aqueous concentrations of antibiotics in the runoff. However, among the three land application methods tested, broadcast resulted in the highest total mass load of antibiotics in runoff from the three simulated rainfall events. Similarly, broadcast resulted in higher concentrations of antibiotic resistance genes in runoff than did injection and incorporation. In manure amended soils, the effects of land application on the concentration of antibiotics were compound specific. No clear trend was observed in the antibiotic resistance gene levels in soil.
Vegetative barriers are strips of densely growing perennial plants seeded downslope on cropland adjacent to surface water. Vegetative barriers can stabilize the soil in local areas and reduce dissolved and sediment bound compounds in runoff, such as nutrients and particulates. The barriers are often used as an erosion control measure and some states regulate the use of vegetative barriers next to bodies of water and in areas with high slopes. One study tested whether vegetative barriers are effective in reducing antibiotics and antibiotic resistance genes. Results show that stripes of switchgrass (panicum virgatum L.) can effectively reduce antibiotic tylosin and its corresponding resistance gene erm(B) in runoff. Hence, vegetative barriers can be used as a low-cost option to reduce the spread of antibiotic and antibiotic resistance gene through runoff.
The control of manure-borne antibiotics and antibiotic resistance genes in the environment is complicated. Different antibiotic compounds have different properties, such as vulnerability to photo degradation and tendency to adsorb to soil particles. Hence, it is hard to use one land application strategy to effectively manage multiple antibiotics in both runoff and soil. Hence, knowing the dominant antibiotic compounds in the manure can be important. Similarly, different antibiotic resistance genes may be hosted in different bacterial species. These bacteria differ in their metabolisms and consequently respond differently to various land application strategies. Like the case for antibiotics, it is difficult to develop one land application strategy that would be effective to all classes of antibiotic resistance genes in manure. So, in addition to land application strategies, attention should also be given to develop manure storage strategies to reduce antibiotics and antibiotic resistance genes prior to land application.
For more information about this article, contact Xu Li.