A step towards the environmental prioritisation of veterinary medicines from animal manure

A step towards the environmental prioritisation of veterinary medicines from animal manure

Animal manure from intensive livestock farming is spread on arable fields and grassland on a large scale in the Netherlands. This manure can contain residues of veterinary medicines that have been given to livestock. Some of these substances are increasingly found in groundwater and surface water. Policymakers and soil and water managers would therefore like to have more insight into the likelihood that veterinary medicines will end up in the environment.

Article source: Water Matters

We have therefore carried out an exploratory study into two sectors in Dutch intensive livestock farming, pig farming and veal farming. In both sectors the manure from animals kept in stalls is collected in slurry pits and spread on arable fields and grassland from early spring onwards. The study focused only on the environmental behaviour of veterinary medicines, and not on any risks to human health and the ecosystem.
How many active substances from the administered veterinary medicines end up in the environment depends on – among other things - their use, the persistence during the manure storage and the behaviour in the soil (particularly persistence and mobility). Persistence is the degree to which a substance remains present without breaking down or disappearing otherwise. Mobility relates to the likelihood that substances leach out into groundwater and surface water.

Methodology

The study focused on the 20 most commonly used antibiotic and antiparasitic medicines in intensive pig and veal farming. We collected national consumption data for the years 2012 - 2014 for pigs and for 2015 for veal from the WUR Farm Accountancy Data Network (Bedrijveninformatienet) and reports from the Netherlands Veterinary Medicines Institute (Autoriteit Diergeneesmiddelen - SDa). We used the following usage classes to categorise the active substances from the products:

  • more than 10,000 kilograms of active substance per year
  • 5000-10,000 kg a.s./yr
  • 1000-5000 kg a.s./yr
  • Less than 1000 kg a.s./yr

The decomposition of a number of important antibiotics and antiparasitics in slurry was determined experimentally in test tubes in which the conditions in slurry pits were simulated (research by the RIKILT). We used the remaining percentage of active substance at the end of these experiments as an indication for the persistence in manure (Table 1).
Information about the rate of decomposition and mobility in the soil was found in the public literature. The rate of decomposition is specified by the half-life (DT50): the time required for 50 percent of the active substance in the soil to disappear. The sorption coefficient on soil organic carbon (Koc) was used for the mobility. This is calculated by determining in an equilibrium situation how much of the substance is bound to organic carbon in soil, and dividing this fraction by the fraction dissolved in pore water. The DT50 and the sorption coefficient were classified using an existing classification from the University of Hertfordshire (Great Britain), see Table 1.

Table 1: Criteria for characterising the environmental behaviour of veterinary medicines

1) Fraction of the active substance still present in the manure after 24 days
1) Fraction of the active substance still present in the manure after 24 days

Results

The information about the 20 active substances is summarised in Table 2. The first notable aspect is that the information is incomplete. One or more parameters are missing for approximately half of the 20 active substances.

Active substances which are somewhat stable in manure and in soil and which show high mobility have a greater probability of leaching into groundwater and surface water. This particularly applies to the sulfonamides (sulfamethoxazole and sulfadiazine) and trimethoprim. Based on persistence in the soil and high mobility, florfenicol could also end up in groundwater and surface water, but the persistence of this substance in manure is not known.

A recent overview of the occurrence of veterinary medicines in various types of water gives an indication as to how accurate our predictions are (table 2, click on the table for a better view). This shows that sulfamethoxazole, sulfadiazine and trimethoprim do indeed occur in the water chain, but for trimethoprim and sulfamethoxazole this is probably partly due to human use. In addition, and contrary to expectations, oxytetracycline and flumequine from animals were found in surface water in low concentrations, and amoxicillin has been found in drinking water made from groundwater. Out of the antiparasitics, levamisole has been found in waste water, but this is probably also related to human use.
There are also substances which are expected to occur in the soil and not in the groundwater of fertilised land. These substances are persistent and have little mobility in the soil. Examples are the antibiotics oxytetracycline, tilmicosin and flumequine and the antiparasitic ivermectin (Table 2). The fluoroquinolones enrofloxacin and marbofloxacin are immobile, but there is no information about their persistence in the soil. The penicillins are very rarely encountered. They hydrolyse very rapidly in manure, and are also broken down well in the soil.

Table 2: Classification of us, persistence in manure, persistence in soil and mobility in soil for 20 veterinary medicines widely used in the Netherlands in intensive pig and veal farming. The intensity of the colours used indicate high values for the relevant characteristics. (click on the table for a better view)

Table2.jpg
  1. see summary in the report by ter Laak e.a. (2017)
  2. estimated from literature, no own measurement
  3. these different substances are presented together here because only combined consumption data is available (the persistence in fertiliser is virtually identical for both substances)
  4. use is considerable but not quantified

Meaning of the results

The approach in our study is relatively simple and semi-qualitative. The results may therefore give rise to comments. Firstly we have taken no account within the chain from animal to environment of the conversion in the animals’ body. However, we do know that metabolites play virtually no role for many antibiotics. Either they are hardly formed, or they are not active. Another point is that soils differ: leaching occurs more quickly in sandy soils than in clay, where surface run-off to ditches or drainage via drainpipes is more likely to occur. Veterinary medicine use in both sectors will also change over time.

The prioritisation outlined here is therefore indicative. Where sufficient substance data is available, the behaviour can be simulated with environmental fate models in order to predict concentrations of veterinary medicines in the soil, groundwater and surface water. However, our study shows that for many veterinary medicines there is no public data about their environmental behaviour. This crucial gap in knowledge is also highlighted in other publications. One important recommendation is therefore particularly to generate and/or publish more data about the environment characteristics of veterinary medicines.
Manure is increasingly being processed nowadays, as a result of which less slurry is ending up directly on the land. More pig manure than calf manure is currently being processed. However, a larger proportion of the calf manure produced is being processed (approximately a quarter) than the proportion of pig manure treated (approximately one tenth). The study did not examine manure processing and the use of the processed products. This will certainly have to be an area for attention in the future.

Finally we would like to stress that our study only examined the likelihood of encountering veterinary medicines in the environment. Evaluation of the potential effects on human health and the ecological risks will probably lead to an adjustment to the prioritisation. For example, ivermectin is known to be very toxic to water organisms. As a result, this substance could have effects in surface water despite the small chance of leaching. Ivermectin is therefore possibly still relevant for water management.

Despite the limitations, we believe that our overview is a useful first step towards prioritisation. Substances such as sulfamethoxazole, sulfadiazine, trimethoprim and possibly also florfenicol are relevant for protecting drinking water and surface water quality. These are therefore important substances for drinking water companies and district water boards to monitor and identify the risks. In addition, the antibiotics oxytetracycline (and probably also doxycycline), tilmicosin, flumequine, enrofloxacin and marbofloxacin and the antiparasitic ivermectin may end up in the soil via slurry. The most persistent substances in this group might even accumulate in the soil and reach terrestrial food chains (e.g. through soil fauna). We therefore recommend further investigation by the soil sector and the agricultural sector into the spread and risks of these substances as well.

Sources

Bedrijveninformatienet (Farm Accountancy Data Network - FADN) http://www.wur.nl/nl/Expertises-Dienstverlening/Onderzoeksinstituten/Economic-Research/Data-Insights/Bedrijveninformatienet.htm

SDa Netherlands Veterinary Medicines Institute. Various reports. http://www.autoriteitdiergeneesmiddelen.nl/nl/sda-rapportages

Ter Laak, T., R. Sjerps & S. Kools, 2017. Quick-scan diergeneesmiddelen in de waterketen (Quick scan of veterinary medicines in the water chain). Report 2017.037, KWR Water Cycle Research Institute, Nieuwengein, 47p.

University of Hertfordshire, 2017. The University of Hertfordshire Agricultural Substances Database Background and Support Information, version: September 2017, The University of Hertfordshire, Great Britain.