Microminerals or trace minerals need to be supplied in smaller amounts in swine diets, with the requirements for and dietary concentrations of trace minerals generally expressed as parts per million (ppm or mg/kg) or milligrams per pound (mg/lb) of diet. Because of the smaller amounts, trace minerals are often added in the diets in the form of a trace mineral premix. Zinc, copper, iron, manganese, iodine, and selenium are the typical trace minerals included in a trace mineral premix for swine. Iron is also provided via injectable iron in young piglets.
Zinc is an important component of many enzymes and participates in the metabolism of carbohydrates, proteins, and lipids. Zinc is found in grains and oilseeds at low concentration and mostly associated with phytate, which makes zinc unavailable to the pig. Zinc deficiency is characterized by a skin condition called parakeratosis, low growth rate, and impaired reproductive performance. Zinc toxicity depends upon source, dietary level, and duration of feeding, but generally the maximum tolerable dietary level for swine is set at 1,000 ppm with the exception of zinc oxide, which may be included at higher levels (NRC, 2012) for short periods of time immediately after weaning.
Pharmacological levels of dietary zinc between 2,000 and 3,000 ppm is a common recommendation to nursery diets to reduce post-weaning diarrhea and improve growth performance (Hill et al., 2000; Shelton et al., 2011). These effects have been consistently demonstrated with dietary zinc provided as zinc oxide (ZnO) (Hill et al., 2001; Hollis et al., 2005; Walk et al., 2015), while zinc sulfate (ZnSO4) has greater potential to induce toxicity (Hahn and Baker, 1993). Organic sources of zinc with greater bioavailability have not consistently demonstrated the same benefits as zinc oxide when organic zinc is added at lower levels (Hahn and Baker, 1993; Carlson et al., 2004; Hollis et al., 2005).
Copper is an important component of many enzymes and participates in iron absorption and synthesis of hemoglobin. Copper is found in grains and oilseeds probably at adequate quantities to meet the requirements, but determination of copper requirements of pigs is scarce. Copper deficiency signs include anemia and low growth rate. Copper toxicity occurs at levels above 250 ppm when fed for a long period of time (NRC, 2012).
Pharmacological levels of dietary copper between 125 and 250 ppm is commonly used in the diet to enhance fecal consistency in nursery pigs and improve growth performance in both nursery and grow-finish pigs (Bikker et al., 2016; Coble et al., 2017). The most commonly used source of dietary copper is copper sulfate (CuSO4) (Cromwell et al., 1998), but tribasic copper chloride (TBCC) is as effective as copper sulfate in promoting growth performance (Cromwell et al., 1998; Coble et al., 2017). Organic sources of copper with greater bioavailability, such as Cu-amino acid chelate, also seem to have the potential to influence growth performance (Pérez et al., 2011; Carpenter et al., 2018).
Iron is an important component of many enzymes and is essential for synthesis of hemoglobin. Iron is low in grains and thereby commonly supplemented from inorganic sources in a trace mineral premix. Newly born piglets develop iron deficiency during lactation and have to be provided with injectable iron.
Piglets develop iron deficiency in the first week of life due to limited iron storages at birth, low levels of iron in sow milk, and the rapid growth rate that occurs during this early stage of life. Iron deficiency is characterized by anemia, and anemic piglets evidence low growth rate, lethargy, pale skin, and rough hair coats. Iron in excess is also prejudicial, as iron affects gut health, stimulates proliferation of bacteria, and causes diarrhea (Li et al., 2016).
Hemoglobin and hematocrit are commonly used as reliable blood criteria to indicate iron status in pigs. Hemoglobin levels of 11 g/dL or above indicate adequate blood iron status, levels of 9 to 11 g/dL indicate borderline anemia, and levels of 9 g/dL or below indicate an anemic condition (Bhattarai and Nielsen, 2015). For hematocrit, values above 30% indicate adequate blood iron status (Perri et al., 2016).
Iron injection in piglets is a well-stablished practice to prevent iron deficiency and anemia. The injection is administered intramuscularly and preferentially in the neck area of piglets. The most commonly used sources of iron are iron dextran and gleptoferron, which have shown similar efficacy in preventing iron deficiency in piglets (Morales et al., 2018). However, absorption of iron seems to be greater with gleptoferron due to its potentially greater iron bioavailability (Morales et al., 2018).
A single dose of 200 mg of injectable iron around 4 or 6 days after birth maximizes growth performance and improves blood iron status at weaning and in the nursery (Williams et al., 2018a,b). On the other hand, providing an iron injection too soon (day 2) or too late (days 8 or 10) after birth seems to restrict pig performance (Williams et al., 2018b). The need for a second iron injection depends on the amount of iron given in the first injection. When using an injection of 200 mg of iron at 2 days of birth, an additional booster dose of 100 mg of iron midway through lactation can improve blood iron status, but it does not provide further benefits in growth performance (Williams et al., 2018a).
Manganese is a component of many enzymes and is involved in bone development. Manganese is found in grains and oilseeds at low concentration. Manganese deficiency signs include impaired skeleton development, lameness, and low growth rate.
Iodine is an important component of thyroid hormones and thereby is involved in regulation of metabolic rate. Feedstuffs grown in low-iodine soil, as in the case of sandy areas, are deficient in iodine. Also, canola or rapeseed may contain increased levels of goitrogenic compounds called glucosinolates that interfere with iodine metabolism. Iodine deficiency is characterized by goiter (thyroid enlargement), lethargy, and low growth rate.
Selenium is an important component of enzymes involved in antioxidant defense. Selenium and vitamin E have closely related functions, but requirements are independent of one another. Feedstuffs grown in low-selenium soil, as is the case of many areas in the United States, are deficient in selenium. Selenium deficiency signs are similar to signs of vitamin E deficiency, which includes white muscle disease, mulberry heart disease, sudden death, and impaired reproduction. Selenium toxicity at levels of 5 to 10 ppm selenium is characterized as chronic selenosis, with signs of low growth rate and separation of the hoof at the coronary band (NRC, 2012; Gomes et al., 2014). Selenium toxicity at levels above of 10 to 20 ppm selenium is characterized as acute selenosis, with signs of posterior paralysis and lesions in the central nervous system (NRC, 2012; Gomes et al., 2014). The amount of selenium inclusion is regulated in the United States and restricted to a maximum of 0.3 ppm added selenium in any swine diet.