Quantification and persistence of ionophore antimicrobials associated with poultry litter

Quantification and persistence of ionophore antimicrobials associated with poultry litter

Summary

Veterinary anticoccidials, biochemically known as ionophores are used in therapeutic and sub-therapeutic levels in animal feed, for prophylaxis and growth promotion. A significant amount of ionophores have been detected in animal manure like poultry, and surrounding soil and waters as reported in various literature. In our previous study on poultry manure, we quantified four ionophores, namely monensin, salinomycin, narasin and lasalocid at the range of 20-100 microg kg-1 .
Ionophores have been found to be toxic to different species of flora and fauna including humans. Case studies on poultry farm worker, report, ionophore toxicity at the range of 0.5-5 mg kg-1. Ionophores can also contribute to clinical antibiotic resistance in pathogens by different mechanisms including horizontal gene transfer.
It is important to quantify the concentration and study the mobility and persistence of these ionophores in the agricultural soils so as to better assess the environmental risks associated with them. This will enable the farmers to improve the feed management practices, so that these compounds are not allowed to accumulate and persist in the agricultural environment. It will not only lead to reduction of environmental and health risks for farm workers exposed to ionophores, but also improve soil and water quality and conserve natural resources which in turn will improve the lives of farm communities. Hence this project deals with determination of sorption and desorption characteristics of the most commonly found ionophore in coastal plain soils that will give mechanistic information about its mobility in the agricultural soil systems, where poultry manure is typically land applied.

Objectives/Performance Targets

The main objective of the proposed project is to determine sorption and desorption characteristics of the most commonly found ionophore in coastal plain soils that will give mechanistic information about its mobility in the agricultural soil systems, where poultry manure is typically land applied. This project will determine the effect of soil texture and pH in both the A and B horizons of different soils that represent common textures on the Delmarva Peninsula.
As of now, we have sampled soils from the Delmarva Peninsula, representing a range of different soil types. We have also finished major physico-chemical characterization of the soils including pH, % organic matter, texture (% sand, silt, clay) and cation exchange capacity. We have conducted a preliminary batch equilibrium study to optimize our method (as described in the next section).

Accomplishments/Milestones

So far, soil samples have been collected from 5 different field sites on the Delmarva Peninsula. Web soil survey and GPS were used to locate the sampling points. In our proposal, we proposed analyzing only three soil types, but we decided to increase the sample size to more than thirty in order to make our results statistically more powerful.
Both A and B horizon of each map unit (representing a soil type) were sampled using a Giddings Probe. Each map unit was sampled in triplicate, and a composite for each was made for better representation. After bringing the soils to the lab, they were processed before analyses as per standard protocol.
The soil samples have been characterized for pH, cation exchange capacity, percentage organic matter and soil texture (% sand, silt, clay).
Preliminary sorption study was done on five of the above soil samples. We looked at monensin ionophore, since that was found in the highest concentration in our previous study among four different ionophores in poultry manure. A known amount of soil was spiked with the analyte and shaken for a given time. After which the samples were centrifuged and supernatants analyzed using a liquid chromatograph triple quadrupole mass spectrometer to determine the concentration of ionophore.
Both A and B horizons of the five soil samples were analyzed. So far, our batch equilibrium method has been working well for our samples. We are planning a bigger batch equilibrium study including all the other soil samples for monensin sorption. This will be followed by desorption study as proposed. We also plan to run a few more batch equilibrium study by varying the pH of the soils, and also sterilizing the soil to minimize microbial degradation of ionophore. We will then be in a position to co-relate our sorption and desorption data with the soil physico-chemical properties like texture, pH, % organic matter, etc. most of which we have already analyzed.
In our preliminary study, we found certain trends. Monensin sorption was found to vary with soil texture and type. Sorption tended to decrease with increasing sand content and decreasing silt, clay, or organic matter content.
As pKa of Monensin is ~ 6.5 and our soil samples had pH below that, Monensin is expected to be in protonated form, sorbing more to negatively charged clay and organic matter. B horizons in general had more sorption capacity than A horizons.
We plan to study these trends in details, in our bigger batch study that we are planning now.

Impacts and Contributions/Outcomes

Apart from the above trends, we don’t want to be conclusive on any results as yet. But we do see that monensin have a potential to be sorbed to the soil and the extent of sorption is dependent on different soil characteristics. Hence we see variable monensin sorption in the different soil types and also in between horizons.
Our preliminary results have been presented in form of a poster at the International Annual ASA-CSSA-SSSA held in Cincinnati, OH from 21st-24th October 2012. We received positive feedback and recommendations from soil scientists, farmers, field technicians and general public interested in this area.
As proposed before, studying all these trends will give us important mechanistic information on the mobility of monensin, the most abundant ionophore, in soil system in the area under study, and its variation as a function of soil texture, compounded with organic matter content and pH, at different soil depths, representing the A and B soil horizons. The information obtained will aid in designing future studies on ionophore mobility, fate and transport. Ultimately, this project will provide information, critical to reducing environmental and human health risks associated with poultry litter use in agriculture.

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