The effects of Na and Cl are difficult to separate since neither exists in its pure state in nature, and the elements usually occur together in water. It is also difficult to separate the chronic effects of NaCl from those attributed to TDS in the literature, as the Na+ and Cl- ions are major constituents of salinity under natural conditions. Nevertheless, it is the Na+ ion that seems to be responsible for most of the recognized effects of "salt" poisoning.583 At present, there is not sufficient data to make any specific recommendations regarding Cl- in drinking water for livestock or wildlife.
The toxic effects of Na are very dependent upon the availability of fresh water. If abundant, good quality drinking water is available, animals can tolerate large doses of Na. This fact notwithstanding, at some point excess dietary Na exceeds the ability of the organism to excrete Na+, and acute poisoning results, regardless of water intake. The threshold for acute toxicity seems to be approximately 1g Na/kg BW for swine, with cattle equal or slightly less sensitive and sheep considerably less sensitive (roughly 2.5 g Na/kg BW). Most animals will limit their consumption of NaCl to approximately 400 mg Na/kg BW/day, if possible. Beyond this dose, feed intake, water consumption, and productivity decline. We couldn't find sufficient equine-specific data to nominate a toxic dose, but most reviewers indicate horses are roughly similar in sensitivity to cattle and swine. Similarly, there is virtually no quantitative data on wild ungulates, but there is no reason to suspect wild ruminants are significantly more sensitive than cattle.
If the only water available is also the major source of dietary Na, long-term impacts will occur at lower dosages. Chronic health effects, mainly decreased production, have been reported at water concentrations as low as 1,000 mg Na+/L in dairy cows; however, other studies with beef heifers in cooler climates reported only minimal effects at 1,600-2,000 mg Na+/L. Interestingly, the actual doses of Na consumed by the cattle in all of these studies (250-400 mg Na+/kg BW) were similar. Dosages greater than 800 mg Na/kg BW resulted in effects ranging from weight loss and diarrhea to death.
It is theoretically possible animals maintained on high-Na+ water for prolonged periods will, if suddenly exposed to low-Na+ water, develop acute Na+ ion intoxication, and anecdotal reports suggest that such has happened under field conditions. We were unable, however, to discover sufficient quantitative information to make any recommendations other than animals should be transitioned from high to low Na+ water sources gradually.
Assuming water consumption typical of a rapidly growing steer (see Introduction) and only background feed Na concentrations, the no-effect level would be about 1,000 mg Na+/L or 2,500 mg NaCl/L. Serious effects, including death, become likely at 5,000 mg Na+/L. We recommend keeping drinking water Na concentrations at less than 1,000 mg/L, although short-term exposure to concentrations up to 4,000 mg/L should be well-tolerated.