Water salinity is the sum of all the salt ions dissolved in water, including sodium, calcium, magnesium, chloride, sulphate and carbonate. Water salinity is generally the most important water quality limitation for livestock as they can refuse to drink excessively saline water leading to loss of production. Excessively saline water may cause salt poisoning in livestock.
Salinity is usually determined by measuring the electrical conductivity of the water. The commonly used units of electrical conductivity are milliSiemens per metre (mS/m). Less commonly, conductivity may be expressed as milligrams/litre (mg/L), which is approximately the same as parts per million (ppm).
Total dissolved solids (TDS) is the sum of all the ions dissolved in water plus dissolved organic acid molecules. Because salts usually dominate, salinity and TDS are usually highly correlated, however organic acids may contribute to the conductivity reading in water with a high organic content. These days it is more common and practical to measure conductivity, rather than TDS.
Tolerance to saline water varies between livestock species (see Table 1). Pregnant, lactating and younger classes of livestock are less tolerant than mature dry stock. For optimum production in these classes of livestock, water supplies should not exceed the salinity levels towards the lower limits of category B in Table 1. Livestock grazing green feed can better tolerate salinity levels at the upper limit of each category compared with those grazing dry feed or salt bush or on a high salt diet, as the high water content of green feed will dilute the salinity levels of the water supply.
|Livestock||A: No adverse effects on animals expected (mS/m)||B: Livestock may have Initial reluctance to drink or may scour, but should adapt without production loss (mS/m)||C: Loss of production and decline in animal health/condition expected. Livestock may tolerate these levels for short periods if introduced gradually (mS/m)|
|Dairy cattle (milking)||640|
|Sheep (weaners, lactating and pregnant)||1100|
Note: To convert conductivity in mS/m to TDS in mg/L or ppm, multiply by 5.5. The factor 5.5 provides a better approximation in water in Western Australia.
Sheep on lush green feed may tolerate up to 2360mS/m (13 000mg/L TDS) without loss of condition or production.
Salinity of a water supply can change over time and between seasons. Salinity levels of dams, creeks, soaks and waterholes should be checked during summer as salinity often increases during summer due to evaporation. Salinity levels can be kept as low as possible by cleaning tanks before each summer and scrubbing and flushing water troughs frequently, even up to twice a week.
Stream salinity can increase after the first rains as salts left on dry creek beds over summer are flushed downstream. Long-term increases in water salinity are usually associated with land clearing and rising watertables leading to more salt seeping into water supplies. Groundwater is usually more saline than surface water.
While TDS and salinity provide a guide to water quality, the concentration of calcium, magnesium, nitrate/nitrite and sulphate ions may also need monitoring especially if using groundwater as they can cause purgative or toxic effects. For example, while magnesium is essential in animal nutrition, it may cause scours and decreased production at high levels, so when TDS exceeds 3000mg/L, it may be advantageous to know the concentration of magnesium.
Water testing for salinity is available in most regional towns, chemistry laboratories and from the department's Diagnostic Laboratory Services (DDLS) at South Perth. See water testing section below.
Blue-green algae are a group of algae including Nodularia spumigena, Microcystis aeruginosa and Anabaena circinalis. They can produce spectacular blooms appearing like iridescent green paint or curdled greenish milk on water surfaces.
Algae multiply rapidly (‘bloom’) in shallow, still warm water when the water is contaminated by plant nutrients including organic and faecal matter and phosphorus.
Livestock can be poisoned by drinking water contaminated with blue-green algae and their toxins. For more informatino refer to How to avoid poisoning of livestock by blue-green algae.
Contamination with debris or animals
Water can become contaminated by birds, animal droppings, animal carcasses or run-off from bare paddocks, intensive livestock industries or sewerage waste. This can result in low production, disease or deaths in livestock. Botulism and salmonellosis are two livestock diseases that may result from contamination of water with organic matter. For more information refer to Botulism in cattle and Salmonellosis of sheep.
Mats of organic matter should be removed from water by skimming the surface within 48 hours to prevent sinking. Water polluted with organic material can be treated with chlorine, although chlorination does not kill all disease-causing bacteria and does not affect toxins already present in the water. Chlorination is not suitable for farm dams.
Contamination with heavy metals or chemicals
Water can contain elements and compounds which, at elevated concentrations, can cause toxic effects or residue issues. Heavy metals, such as arsenic, lead, mercury, selenium, zinc and the fluorides, and pesticides are of particular concern. While high concentrations of some heavy metals can be found in groundwater, sources of these metals and pesticides are generally run-off, seepage or spillage from arable land and industrial or sewerage waste. If there is a high contamination risk or where productivity losses are suspected, a detailed water analysis should be conducted.
Never use containers that have been used for herbicides, pesticides or fertilisers for carting water for stock. Deaths have occurred in livestock due to water contaminated with nitrogenous compounds from fertiliser containers. There is also a risk that livestock could develop chemical residues in their tissues which, if detected, could result in serious trade consequences.