Sulfur Summary

In ruminants, high dietary S may cause acute death, PEM, trace mineral (especially Cu) deficiencies, and/or chronic, as-yet-poorly-defined ailments that decrease production efficiency. All dietary sources of S (water, forage, concentrates, feed supplements) contribute to total S intake and thus to potential toxicity. The S contribution of water, usually as the SO₄^2- ion, varies dramatically with environmental conditions as water consumption goes up and down.

From a strictly theoretical standpoint, the NRC maximum tolerable dose of S for cattle is 0.5% of the total diet (0.3% for feedlot animals).⁵⁸⁹ Wyoming grasses are reported to contain between 0.13%-0.48% S.⁶³⁸ Assuming forage S concentrations of 0.2% and water consumption typical of young, rapidly growing cattle at summer temperatures (30 C), a water SO₄ concentration of 1,125 mg/L will meet or exceed the NRC’s maximum tolerance limit for S in cattle. Adult bulls, which consume half as much water, could theoretically be impacted by 2,250 mg/L, and lactating cows would fall somewhere in between. In practice, water SO₄ concentrations as low as 2,000 mg/L have caused PEM and/or sudden death in cattle. This observation is supported by many field cases investigated by the WSVL and other regional diagnostic labs since 1988. It seems to be contradicted by some of the early studies mentioned above, notably Digesti and Weeth585, but both probability and the morbidity of poisoning increase with progressively larger SO₄ concentrations; thus, studies with small numbers of animals easily overlook marginally toxic doses. Anecdotal data also indicate cattle are able to adapt to elevated S concentrations if introduced gradually to potentially toxic waters over a period of several days to weeks. The details (i.e. how rapidly dietary S can change) of this process and the effect(s) of other dietary factors such as energy and protein on the process are still a matter of conjecture.

Waterborne SO₄ is reported to decrease Cu uptake at concentrations as low as 500 mg S/L as SO₄^2-.^602,606 Whether overt Cu deficiency results depends upon the dietary concentration of Cu, and excess dietary Cu may compensate for some or all of the effect of SO₄^2-.³⁰⁸ Unfortunately, most Wyoming forages are marginally to drastically deficient in Cu for cattle. Elevated dietary S also interferes with the uptake of Zn and Se. Trace element deficiencies are multifactorial diseases that do not normally manifest themselves unless animals are exposed to other stressors such as bacterial pathogens, bad weather, shipping, etc. Therefore, it is difficult, if not impossible, to settle upon a single number that consistently results in deficiency or guarantees safety; however, the NRC recommends "the sulfur content of cattle diets be limited to the requirement of the animal, which is 0.2% dietary sulfur for dairy and 0.15% in beef cattle and other ruminants."⁵⁸⁹

Relatively low S concentrations (equivalent to 500-1,500 mg SO42-/L in water) have also impacted performance (e.g. ADG, feed efficiency) in feedlot and range cattle via a variety of mechanisms not completely understood.^{614,616,639,640} Loneragan et al.597 suggested that H2S produced from SO42-, eructated and then inhaled, resulted in pulmonary damage and increased susceptibility to respiratory infections. Elevated SO₄^2- also results in decreased water intake under experimental conditions. Finally, it is possible some, as yet unrecognized, interactions with other dietary components result in decreased utilization and feed efficiency. These effects have obvious implications for animal health, but they are difficult to quantify under field conditions.

Monogastrics, such as horses, are at less risk of S effects that involve ruminal generation of sulfide. In these species, the principle effect of elevated drinking water SO₄ seems to be diarrhea resulting from the osmotic attraction of water into the gut. The relative contributions of the SO₄^2- ion and its associated cation are unclear, but the literature indicates the effect 1) is transient and not life-threatening and 2) probably only occurs at concentrations considerably in excess of those toxic in ruminants. Therefore, concentrations that are safe in ruminants should provide adequate protection for horses.

Assuming normal feedstuff S concentrations, keeping water SO42- concentrations less than 1,800 mg/L should minimize the possibility of acute death in cattle. Concentrations less than 1,000 mg/L should not result in any easily measured loss in performance.

Back to to index