Iron for Animals

Iron has been recognized as an essential nutrient for over 100 years. Despite this fact, iron deficiency is still a major problem in several segments of the livestock industry.

Approximately two-thirds of body iron is present in hemoglobin in red blood cells and myoglobin in muscle, 20% is in labile forms in liver, spleen and other tissues with the remainder in unavailable forms in tissues such as myosin and actomysin and in metalloenzymes. In hemoglobin, which contains 0.34% iron, an atom of ferrous iron in the center of a porphyrin ring connects heme, the prosthetic group, with globin, the protein. The iron in hemoglobin is essential for the proper function of every organ and tissue of the body. Iron also plays a role in other enzymes involved in oxygen transport and the oxidative process, including catalase, peroxidases, flavoprotein enzymes and cytochromes.

Iron in blood plasma is bound in the ferric state (Fe⁺⁺⁺) to a specific protein called transferrin. Transferrin is the carrier of iron in the blood and is saturated normally only to 30-60% of it iron-binding capacity (215).

The duodenum is the main site of iron absorption. However, only 5% to 10% of what is consumed is absorbed and then only if it is in the ferrous state (Fe++). Once absorbed, the body tenaciously holds on to the iron for reuse. For example, most of the iron released from red blood cell breakdown is used to synthesize new hemoglobin. However, if the red blood cells are not replaced as rapidly as they are destroyed, anemia occurs. Anemia may occur at any stage of life, but it is especially likely to occur in certain species during the suckling period, since milk is very low in iron.

Iron is very low in the milk of cows, goats and sows. It varies from 0.5 to 1.0 ppm. Since pigs depend heavily on mother’s milk during the first two to three weeks of life, they need iron supplementation because their body stores are unusually low.

This magnitude of growth rate imposes a greater demand on iron needs than occurs with young ruminants. There is little evidence of an iron deficiency occurring with calves, lambs and kids raised under grazing conditions, except when blood loss or disturbance in iron metabolism occurs because of parasitic infection or disease. This is because they start early to eat food other than mother’s milk. Iron supplementation is needed, however, when young ruminants are fed an exclusive whole milk diet. Young nursing calves and lambs, receiving no supplemental source of iron, have responded to intramusculature injections of iron-dextran by improved hemoglobin levels and growth rate.

Table 23 gives information on the iron requirement of animals as well as the levels that may result in harmful effects.

Class of Animal	Iron Requirement in Total Diet (ppm)	Toxic Level in Total Diet (ppm)
Swine
Baby Pigs	140 –150	3,000
Growing-Finishing	80	3,000
Sows	80	3,000
Dairy Cattle
Calves	100	1,000
Other Cattle	25	1,000
Beef Cattle	50	1,000²
Sheep	30-50	500²
Goats	30-50³	⁴
Horses	40-50	⁴
Poultry	50-80	4,500

¹ These are the nutrients given by the latest NRC publication on nutrient requirements of animals, unless indicated otherwise.

The pig is born with about 47 mg of iron in body stores. The iron storage will last about a week, since it is estimated the pig will need 7 mg of iron absorbed daily for normal growth. In many cases, however, the iron stored is dangerously low by 3 or 4 days of age. Sow’s milk is very low in iron and contains about 1 ppm, whereas the baby pig’s iron requirement is 140 to 150 ppm in the total diet. Plasma volume increase markedly during the first 12 hours after the newborn begins nursing. However, because there is little change in red blood cell volume the piglet develops physiological anemia rapidly. Therefore, unless the pig is given extra iron, it will become anemic as early as 5-10 days of age. During the first two to three weeks of life, the baby pig depends mostly on milk for its food supply. Baby pigs usually double their weight by one week of age and redouble it again in another week or ten days. Therefore, the pig rapidly outgrows its iron supply. So, iron supplementation is very important for the baby pig (87, 92).

Some of the symptoms of an iron deficiency are poor growth, listlessness, rough hair coat, anoxia, wrinkled skin, paleness of mucous membranes, hypochromic microcytic anemia, enlarged heart and spleen, enlarged fatty liver and ascites. A characteristic sign is labored breathing after minimal activity from which the term "thumps" arose. Blood hemoglobin is a reliable indicator of the iron status of the pig. Usually, a level of less than 9 grams of hemoglobin per 100 milliliters of blood results in borderline anemia (87, 92).

Attempts to significantly increase the iron content of sow’s milk by feeding or injecting high levels of iron during late gestation and lactation have not been successful to date. Therefore, the practice of injecting the baby pig with 150 to 200 mg of iron during the first three days after birth is recommended. Moreover, iron supplementation of prestarter and starter diets is widely used by industry (104). Nursery pigs were fed diets supplemented with 0, 25, 50, 100, 150 ppm iron in the diet (as-fed basis) from ferrous sulfate. Whole body iron stores increased linearly due to increasing dietary iron concentrations (314). These results show that the basal diet was inadequate to maintain indices of iron status and that there was an increased risk for the pigs to develop anemia during the grower and finisher periods.

Proper iron nutrition is essential for optimum immune function. Nursing pigs made anemic by withholding supplemental iron were more susceptible to the lethal action of bacterial endotoxins than their littermates that had been given iron (158). Parsons et al. (159) reported that pigs receiving iron intramuscularly prior to four weeks of age were more likely to survive a transmissible gastroenteritis outbreak and recovered more rapidly than anemic pigs.

Iron reserves of the newborn calf are usually sufficient to prevent a severe iron-deficiency anemia if dry feeds are fed within a few weeks after birth. Iron supplementation is needed, however, when calves are fed exclusively a whole milk diet. With older cattle, an iron deficiency seldom occurs unless there is a loss of blood caused by a parasitic infection or disease.

Iron deficiency results in anemia, reduced gain, listlessness, inability to withstand circulatory strain, labored breathing after mild exercise, reduced appetite, decreased resistance to infection, blanching of visible mucous membranes and a pale color of the muscle meat (91, 156).

The iron requirement of calves is about 100 ppm and 50 ppm appears adequate for older cattle (157). Calves on an exclusive milk diet develop an iron deficiency. Anemia and decreased growth rate occur (85, 157).

Little is known about iron requirements for sheep. An iron deficiency anemia sometimes occurs with lambs raised on slotted, wooden floors. The anemia can be prevented by iron supplementation in the creep-feed or by intramuscular injections of iron-dextran. Two injections given three weeks apart, each of 150 mg iron, are effective (155).

Very little is known about iron requirements for goats. A deficiency may occur with young goat kids because of their low body stores at birth and the low iron content of milk. If iron deficiencies are observed and the kids are continued on a milk diet, injection of 150 milligrams iron-dextran at two-to-three-week intervals is recommended by the 1981 NRC publication, Nutrient Requirements of Goats (100).

Little is known about iron requirements for horses. An iron deficiency will result in anemia. Anemia may result with horses that are heavily parasitized. This is important, since horses are very susceptible to parasites and are treated for them more frequently than are other classes of livestock. Adequate iron is very important for racing and performance horses, which need a high hemoglobin level for proper oxygen transport and endurance (98, 154).

Iron deficiency produces a microcytic, hypochromic anemia in chickens. This type of anemia also occurs with all other animals. Iron deficiency also produces a complete depigmentation of the normally red and black feathers of the Hampshire chicken (141). The iron requirement of chicks fed a casein, dextrose, and isolated soybean protein concentrate-based diet was studied by Aoyagi and Baker (238). Hemoglobin data were used to estimate the chick’s requirement at 85 ppm in the diet. Heart hypertrophy was observed if less than 70 ppm iron were fed. Duck diets are usually formulated to provide 70-80 ppm of iron (300).

There is a lack of adequate information on iron needs and usage with most small animals. Iron requirement of the rabbit is not known. At birth, rabbits have a very large iron reserve and, therefore, are not as dependent on a supply of iron in the milk and are not as susceptible as some other animals to iron deficiency anemia during pre-weaning (89).

The dog needs 31.9 ppm iron in the diet. An iron deficient dog develops anemia and tissue anoxia (142). The cat requires 100 ppm iron in the diet. A deficiency of iron results in anemia (102).

Iron requirement of foxes is not known. In mink, a fur abnormality known as cotton fur, has been linked to an iron deficiency. A normal pelt was produced with 114 ppm iron in the diet, so this level might be used as a guide until a more adequate level is determined. Levels of 20 to 30 ppm iron are considered adequate by some, but others recommend levels of 60 to 88 ppm iron, and Scandinavian mink diets contain as high as 156 to 352 ppm iron (143). Iron deficiency symptoms in mink usually include growth retardation, anemia, severe emaciation, roughened fur and a lack of underfur pigmentation (achromotrichia) (143).

Iron supplementation increased trout growth and prevented anemia, but the iron requirement is not known. Trout may be able to absorb iron directly from water to meet requirements (144).

Iron requirement of non-human primates is 180 ppm in the diet. Iron deficiency results in anemia (101). Other laboratory animals need iron for anemia protection, but the requirement is not known in most species. The rat requires 35 ppm iron in the diet. The mouse requires 25 ppm for growth; a level of 120 ppm iron in the diet is the lowest level tested for reproduction and was satisfactory. The guinea pig needs 50 ppm in the diet for growth (103).