2. Introduction
The quality of drinking water is assumed to be a key factor in ensuring an efficient animal production system. While we all 'know' that this is true, what we actually mean by quality, and specifically good quality, is not at all well-defined. This review provides an overview of the international scientific, technical and regulatory literature on water quality for livestock, and in particular for ruminants in New Zealand.
Livestock production in New Zealand relies on both surface water and groundwater supplies. Water quality in streams and dams (surface waters) is influenced by catchment geology, topography, soil type, weather and climate, and by the influence of livestock themselves in terms of contamination of the supply. Groundwater, which is used as a source of drinking water for livestock in parts of New Zealand, may contain large quantities of dissolved salts, depending on the soil and parent rock of the surrounding area, and many other factors including rainfall, evaporation, vegetation and topography. The quality of both ground waters and surface waters may be affected by catchment land use practices including agriculture, mining and other industries, with the potential for increased concentrations of salt, nutrients and other contaminants, such as pesticide residues and heavy metals.
A large variety of microbial pathogens can be transmitted to stock from drinking water supplies contaminated by animals and their faeces. The risk of contamination is greatest in surface waters (dams, watercourses, etc) which are directly accessible by stock or which receive runoff or drainage from intensive livestock operations or from sites containing human wastes (ANZECC 2000). The incidence of ground water contamination by pathogens is generally low, particularly for deep bores and wells, although the situation will be different with shallow ground water. The actual impact of microbial contamination of water supply for ruminants is an open question, although there is considerable evidence for problems associated with such quality factors in monogastric species and in young ruminants (such as calves and lambs), albeit at the pre-or early-ruminant stage.
The focus of this review is on ruminants with the major interest in factors such as mineral and chemical components (but excluding pesticides), microbial contamination of water and on factors that may influence the productivity of livestock in the New Zealand environment.
2.1 What is Water Quality?
There are no consistent definitions of water quality and those that are reported depend on the context in which quality is considered. In fact there is a dearth of information on the impact of water quality on productivity of ruminant livestock, and hence the factors that define quality have not been well-characterised. The above notwithstanding, several aspects of water quality have been discussed by various researchers and, in no particular order, include:
- organoleptic properties (odour and taste);
- physiochemical properties (pH, total dissolved solids, total dissolved oxygen and hardness);
- the presence of toxic compounds (heavy metals, toxic minerals, organophosphates and hydrocarbons);
- excess minerals or compounds (nitrates, sodium sulfates and iron);
- bacteria and algae.
However the extent to which the various components of water influence aspects of water quality and to what extent they impact on productivity are not well-defined. The most extensive studies of water composition and its influence on productivity were carried out by Willms and co-workers in Canada who conducted trials comparing different water sources (Willms et al. 1996, 2000, 2002). They investigated water quality in dams and measured a vast array of water components. However, they were not able to identify any individual components of water that had a particular influence except that, in specific experiments, they showed that faecal contamination influenced the palatability of water for stock and hence water consumption and live weight gains.
There are several published guidelines for water quality but these are often based on few data, with little information as to how the guidelines were formulated and how "acceptable" levels of water components and contaminants were defined. In fact, many of the guidelines do not appear to be based on experimental data, but rather on the anecdotal experience of people in the field, or alternatively on standards of drinking water for human consumption.
Locally, the Australia and New Zealand Environment and Conservation Council (ANZECC) has developed "trigger values" (the level at which there is minimal risk to animal health, but if levels exceed the trigger value, the risk to stock should be investigated). In their 2000 review, ANZECC updated the information previously used in determining the guidelines for livestock drinking water (ANZECC 1992). With several notable exceptions, few examples of new studies were found, with most information coming from the 1960s and 1970s.
Two differing approaches have been used in developing guidelines in other countries. A toxicological approach, as proposed by the Canadian Council of Ministries of the Environment (CCME 1993), is based on the following principles:
- the method of developing guideline values is transparent and consistent;
- selection criteria and appraisal protocols ensure only valid sound scientific data are used;
- data can be obtained through feeding trials with animals.
Some disadvantages of this approach include the fact that:
- the need to make many assumptions on factors such as, the value of a 'safety factor' for inter- and intra-species differences, long-term effects, and the contribution of water consumption to total intake of a chemical;
- no account is taken for the risk of animals consuming the contaminants;
- differing animal ages and condition, climatic conditions and feed types are not usually addressed;
- interactions with other elements in the metabolism of animals are not considered;
- users of the guidelines have to interpret the suitability of the water in specific cases.
An alternative is a more 'holistic' approach, as taken by the Department of Water Affairs and Forestry (DWAF 1996b) in developing the South African guidelines. This approach includes the use of in situ observations and studies to identify the level of a constituent at which no adverse effect would be expected, taking into consideration the major synergistic and antagonistic factors affecting the onset of adverse effects. Guidelines are given in the context of a risk-based approach, with an indication given of contaminant levels that might be tolerated for short periods of exposure, or following adaptation to the water source. Where possible, differences among animal species and physiological state are considered.
In summary any consideration of the potential impact of water quality on ruminant productivity is likely to be complex, as the important contributors to water quality are not well-defined. This review provides a summary of the published scientific and technical literature on the subject and proposes some further studies that are likely to be particularly relevant to the New Zealand farming situation.
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