Review articleChemical contaminants in feedlot wastes: Concentrations, effects and attenuation
Introduction
The Australian meat and livestock industry has adopted a proactive stance towards the implementation of best practices for the management of chemical contaminants that may be present in manure and effluent from commercial feedlot operations. While numerous activities will be necessary to achieve this, the first requirement is the identification of chemicals likely to warrant closest scrutiny. This review of primarily international data and research was undertaken to identify key chemicals for which local analytical efforts appear most justified.
Commercial feedlots are a major method of finishing beef cattle in preparation for slaughter in Australia. Cattle entering feedlots are typically 12–24 months of age. Depending on the intended market for the cattle, they may be fed for 60 days to up to 400 days while gaining about 100–350 kg in weight.
The main by-products from cattle feedlots are the manure harvested from the surface of the pens and liquid effluent collected during rainfall runoff events. A typical animal entering a feedlot (e.g. 340 kg for heavier markets) produces approximately 20 kg of manure per day, increasing to up to 36 kg manure per day for a heavy finished animal (600 kg). Fresh manure, which comprises of faeces and urine, is normally composed of around 90% water and 10% solids.
Good feedlot pad management requires a balance between environmental and animal health considerations and the economic cost of pen cleaning. However, the period of time between pen cleanings generally means that there is some decomposition of pad manure before it is removed from the pen.
Depending on variations in management and weather, manure harvesting rates have been reported to vary between 0.41–1.05 t dry weight per head per year (Lott et al., 1994).
While manure harvesting from pens occurs at regular intervals, manure spreading or dispatch offsite depends upon management methods, weather conditions and cropping cycles which influence when manure can be spread or sold. Accordingly, manure is often stockpiled at feedlots for periods of months or occasionally years. During stockpiling, manure undergoes partial decomposition and drying which can lead to significant mass decrease. If stockpiles are carefully managed ensuring aeration as well as optimal water content and carbon to nitrogen ratios, manure may be composted in the process. Much of the organic matter can be mineralised or volatilised during composting leading to mass reductions of a further 50% of the original bulk.
From the point of manure deposition on the pad, through stockpiling, to eventual removal from the feedlot site, opportunities exist for the transport or transformation of chemical components or contaminants. The major transport routes may include runoff from impermeable surfaces and leaching through soil to groundwater. Additional routes may involve human intervention such as manure stockpiling or application to agricultural fields. Likely chemical transformations include oxidative or reductive degradations, which may be photochemically or biochemically mediated. The degree of such transformations and/or transport is important when assessing the probability of human exposure. This in turn is necessary for the task of identifying best practices for the management of feedlot manure and effluent to ensure the full protection of human health.
Chemicals of concern in cattle feedlot manure and effluent may include endogenous chemicals such as hormones, as well as non-endogenous natural and synthetic chemicals used to maintain the health and optimum growing conditions for the animals. Furthermore, there is potential for animals (and hence manure and effluent) to be unintentionally exposed to chemicals in the environment or via contaminated feed products. To reduce the likelihood of this occurring, all feedstuff purchases in Australia require a Commodity Vendor Declaration stating the fodder type and any chemicals such as pesticides applied during production.
Veterinary chemicals may be administered to animals while at the feedlot or, alternatively residues of such chemicals applied at a previous location may be (wittingly or otherwise) transported to the feedlot with incoming animals. Again, management practices to minimise this in Australia include the requirement that all feedlot animals are subject to a National Vendor Declaration, which serves as a travel document and describes all chemicals used in the production of that animal.
After consideration of the international scientific literature on this subject, steroidal hormones, antibiotics, ectoparasiticides, mycotoxins, heavy metals and dioxins were identified as being most worthwhile for further analysis in the Australian context. Each of these categories of chemicals is addressed in the following sections.
Section snippets
Steroidal hormones
Steroidal hormones potentially present in feedlot manure and effluent include endogenous (naturally occurring) hormones and some synthetic hormones applied in agriculture. Endogenous hormones are commonly identified in animal excretions including manure and urine (Hoffmann et al., 1997). The levels of these residues vary considerably with sex, age, breed, castration, and pregnancy.
Both natural and synthetic steroidal hormones are used in many countries as hormonal growth promotants (HGPs) in
Antibiotics
Antibiotics include naturally-occurring, semi-synthetic and synthetic chemical compounds with antimicrobial activity. They are used in veterinary medicine to treat and prevent disease, and for other purposes including growth promotion in food animals (Prescott et al., 2000). Depending on their chemical nature, they can be administered orally, parenterally or topically. Antibiotics are commonly administered to livestock in the USA and Australia via feed or water or via a slow-release implant (
Ectoparasiticides
Externally used parasiticides, known as ‘ectoparasitides’ are chemical formulations used to control external parasites (ectoparasites) such as ticks, flies and lice on livestock including cattle (Waltisbuhl et al., 2005). A number of application methods are used including dips, sprays, pour-on solutions, ear tags and back rubbers (Reeves, 2005, Spence, 1994). Back rubbers are provided for self-treatment by cattle.
Many ectoparasitic infections are seasonal and therefore predictable. Accordingly,
Mycotoxins
Mycotoxins are toxic chemicals produced by fungi such as mould (Newsome, 2006). Thousands of mycotoxins exist and infestation of cereal grains is common (Murphy et al., 2006). It is generally assumed that mycotoxins are of greatest concern in developing countries where climatic conditions, and agricultural and storage practices, are considered conductive to fungal growth and toxin production (Aziz et al., 2006, Singhal and Kaur, 2005). However, mycotoxin contamination of forages and feeds is
Heavy metals
The accumulation of heavy metals such as Cd, Pb and Hg in soils is a potential concern for Australian agriculture due to the capacity for these elements to adversely affect food quality, crop growth and environmental health (McLaughlin et al., 2000). Livestock manure is a possible source of low concentrations of some heavy metals. For example, Pb concentrations of 1.6–8.6 mg/kg and Cd concentrations of 0.1–0.7 have been reported in cattle manure in some countries (Bolan et al., 2004).
Some
Dioxins
Dioxins are halogenated organic compounds derived from industrial processes. Many of this group of 210 chemicals are persistent and have become ubiquitous in the environment (Berry et al., 1993, Jones and Sewart, 1997, Vandenheuvel and Lucier, 1993). Dioxins include chlorinated dibenzo-p-dioxins (CDDs), chlorinated dibenzofurans (CDFs) and certain polychlorinated biphenyls (PCBs). The term ‘dioxin’ is commonly used to refer to the most studied and one of the most toxic dioxins,
Conclusions
While a large number of natural and synthetic chemicals are present in feedlot cattle manure and effluent, if any animal welfare, environmental or public health risks exist, they are expected to be associated with trace concentrations of some key biologically significant compounds.
Steroidal hormones (both natural and synthetic) have the potential to be significant environmental pollutants that may affect a diverse range of organisms. In particular, it has been shown that androgenic hormones
Acknowledgements
The authors are grateful for funding for this research provided by Meat and Livestock Australia (MLA) and the matching funds provided by the Australian Government via MLA. Furthermore, the authors appreciate the helpful comments provided by Prof. David Sedlak and Dr. Pat Blackall.
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