Question
Are high iron levels in drinking water a problem for horses? I wonder if my horse is getting too much iron from his water.
Answer
Without the test results from your horse’s water supply as well as a thorough accounting of your horse’s diet, it is difficult to give a conclusive answer to your question. However, I can supply some general commentary on the topic.
According to the Water Research Center, iron is a nonhazardous nuisance in water supplies. The Environmental Protection Agency classifies iron under its secondary standards (based on taste, odor, color, foaming, and staining properties), of which the secondary maximum contaminant level (SMCL) for iron in drinking water is 0.3 mg per liter (0.3 ppm). This indicates water with iron levels below that shouldn’t have an unpleasant taste, odor, or appearance.
According to Nutrient Requirements of Horses, last published by the National Research Council (NRC) in 2007, “from data in other species, the maximum tolerable concentration of iron has been set at 500 mg/kg ration.” Additionally, “dietary iron absorption in nonruminants fed adequate iron is likely to be 15% or less.” The only reported issues of iron toxicity cited in Nutrient Requirements of Horses involved a form of iron that has since been banned for use in neonates. Given to foals, which have greater absorption than mature horses, this supplemental iron product delivered 350 mg in each dose and was given at birth and again at three days of age.
With this information in mind, horses all over the world, drinking all sorts of water and eating fortified rations, don’t exhibit any signs of iron toxicity, so I think you can be confident that iron toxicity from a water source is not a looming threat. The danger would most likely only lie in oversupplementing with multiple products that feature highly digestible forms of iron on top of rations that already offer adequate intake.
Recommendations offered in Nutrient Requirements of Horses include 40 mg of iron per kg dry matter (DM) consumed. For a 500-kg horse eating 2% of its body weight or 10 kg DM, that would translate to 400 mg of iron per day.
The adjective “high” linked with iron may lead to the belief that there is a high level of a nutrient in the water versus a level of a mineral that is likely to result in perceived undesirable properties such as odor, taste, color, and staining. At 0.3 mg of iron per liter, the horse’s normal daily consumption of water about 5 liters per 100 kg body weight for horses fed a dry diet, which is 25 liters per day for a 500-kg horse would contribute (0.3 mg iron per liter x 25 liters) 7.5 mg per day toward the horse’s requirement of 400 mg daily.
