Álvaro García
Micromineral nutrition is often discussed in the context of calcium, phosphorus, or zinc, yet two trace elements beginning with the same letter, iron (Fe) and iodine (I), remain critical and frequently underestimated in cattle diets. Despite their minute quantitative requirements, both minerals exert far-reaching physiological influence, shaping oxygen transport, endocrine regulation, metabolic rate, growth, and lactational performance. When either is deficient, the disruption extends well beyond individual pathways, impairing systemic homeostasis and overall productivity.
Iron forms the core of hemoglobin and myoglobin, ensuring effective oxygen transport and utilization. It also participates in numerous enzymatic reactions central to energy metabolism. Iodine, by contrast, is the indispensable component of thyroid hormones (thyroxine and triiodothyronine), which regulate metabolic rate, thermogenesis, and nutrient partitioning. Their interdependence is biologically striking iron-dependent enzymes such as thyroid peroxidase catalyze the iodination of tyrosine residues in thyroglobulin, linking both minerals in thyroid hormone synthesis. Consequently, iron deficiency can impair iodine utilization, and iodine deficiency can, in turn, limit iron metabolism—creating a cycle of metabolic inefficiency that undermines performance and health.
Deficiencies of these trace elements follow recognizable geographical patterns. Iron deficiency is most common in young or rapidly growing ruminants raised on sandy, acidic, or eroded soils, such as those found in sub-Saharan Africa, South Asia, and northeastern Brazil. Iodine deficiency predominates in mountainous regions (e.g., the Andes, Himalayas, and Alps) and rainfall-leached coastal zones where natural soil iodine reserves have long been depleted. In humid tropical environments, both deficiencies may coexist due to mineral leaching and dietary antagonists, such as goitrogenic plants, molybdenum, or excessive sulfate.
Iron and iodine: Interconnected micronutrients of dairy metabolism
Iron is essential for oxygen transport, cellular respiration, and immune function. About 60–70% of total body iron is contained in hemoglobin, 10% in myoglobin, and the remainder in enzymes and storage proteins such as ferritin. Despite its abundance in the earth’s crust, iron availability to ruminants can be low due to antagonistic minerals and feed composition. Absorption occurs mainly in the duodenum through the divalent metal transporter (DMT1) and ferroportin, with iron bound to transferrin for circulation. Ferroportin is a membrane protein that serves as the only known iron exporter in mammals. It plays a crucial role in iron homeostasis by controlling the release of iron from cells into the bloodstream. It transports iron (Fe²⁺) out of cells where it is stored or absorbed, primarily enterocytes (intestinal cells), macrophages (that recycle iron from red blood cells), and hepatocytes (liver cells). Once iron is exported by ferroportin, it binds to transferrin, the main plasma protein that carries iron to tissues.
Calves reared on whole milk or milk replacers are particularly vulnerable to deficiency because these diets are inherently poor in bioavailable iron.
In lactating cows, iron supports mammary oxygenation and mitochondrial energy production—key processes sustaining milk yield and quality. Deficiency leads to anemia, poor thermoregulation, reduced feed intake, and impaired immune defense. Conversely, excessive iron intake, especially from water sources, can generate oxidative stress and interfere with copper and zinc metabolism.
Iodine’s principal role lies in the synthesis of thyroid hormones, which regulate basal metabolic rate, growth, reproduction, and thermoregulation. Its metabolism, however, depends on iron status because thyroid peroxidase, the enzyme responsible for hormone synthesis, requires iron in its heme structure. When iron is deficient, this enzyme becomes less active, reducing iodine incorporation and the formation of thyroxine (T₄) and triiodothyronine (T₃). Similarly, inadequate iodine can indirectly suppress iron metabolism by lowering thyroid-driven erythropoiesis and intestinal absorption.
This interaction creates a metabolic bridge between oxygen delivery (iron) and energy use (iodine). In high-producing dairy cows, particularly during the transition and early lactation phases, marginal deficiencies in either mineral may reduce milk synthesis, delay estrus, and increase health risks, often without obvious clinical signs. These subtle but chronic impacts make the two “I’s” among the most strategically important micronutrients in dairy nutrition.
Practical insights: Supplementation and regional deficiencies
A recent study by Salles et al. (2025) in Frontiers in Animal Science examined the effects of supplementing milk replacer with iron, selenium, and vitamin E in Holstein calves during their first 60 days of life. Calves receiving the full supplement (200 mg Fe/kg DM) displayed improved metabolic and immune resilience compared to controls. Glutathione peroxidase activity, a key antioxidant marker, increased significantly (p < 0.01), while plasma lactate and urea concentrations decreased, suggesting more efficient energy and protein metabolism.
Health outcomes also improved markedly: the incidence of diarrhea fell from 67% in control calves to 51% in those supplemented with iron (p = 0.03), and pathogen loads of Anaplasma marginale were lower. While growth differences were modest, the reduction in health-related losses translated into tangible economic savings. A 15% reduction in neonatal diarrhea could save approximately $300–400 per 100 calves through reduced veterinary costs and improved weight gain, demonstrating the financial value of strategic micronutrient supplementation.
Such findings are particularly relevant in regions prone to soil mineral deficiencies. Iron deficiency tends to occur in areas with acidic or highly leached soils, common across sub-Saharan Africa, Southeast Asia, and parts of Latin America, where forages and grains are naturally low in trace minerals. Iodine deficiency remains prevalent in mountainous and inland zones, such as the Himalayas, Andes, and Northern Europe, where long-term glaciation and rainfall have stripped soils of iodine. In many tropical areas, overlapping deficiencies of both minerals lead to poor growth, rough hair coats, weak thermoregulation, and delayed reproduction in cattle.
Managing the two “I’s” in dairy herds
Identifying and correcting trace mineral deficiencies requires both observation and diagnostic support. Cows’ marginal in iron often displays pale mucous membranes, dull coats, fatigue, and reduced feed intake. Calves may suffer from poor growth, diarrhea, or respiratory disease. Iodine deficiency manifests as enlarged thyroid glands (goiter) in newborns, prolonged gestation, weak calves, or low milk yield despite adequate feeding. Milk iodine content below 150 µg/L typically signals herd-level deficiency.
Blood testing offers confirmation. Low serum ferritin (<15 µg/L) or transferrin saturation (<20%) indicates iron deficiency, while low thyroxine (T₄) with low or normal triiodothyronine (T₃) points to iodine deficiency or impaired conversion due to iron shortage.
Effective supplementation should address both minerals simultaneously when risk factors are present. Iron can be supplied through ferrous sulfate, chelated iron, or injectable forms such as iron dextran for calves. Mineral mixes containing 300–500 mg Fe/kg typically ensure adequacy without promoting oxidative stress. Iodine is best provided via potassium iodide, calcium iodate, or iodized salt, targeting 0.5–1 mg/kg of dry matter in lactating cows. Organic iodine forms like EDDI (ethylenediamine dihydroiodide) improve bioavailability, particularly under environmental or heat stress.
Because high iron levels can antagonize copper, manganese, and zinc absorption, mineral premixes should remain balanced rather than overly fortified. Preventing subclinical deficiencies yields measurable economic returns: improving milk yield by 2–3% and reducing health-related culling by 5–10% easily offsets the cost of targeted mineral programs. Beyond productivity, balanced trace mineral nutrition enhances fertility, udder health, and calf vitality, keys for sustainable herd management.
The double “I” advantage: Iron + iodine for dairy cows
Iron and iodine are small nutrients with outsized influence. Together, they synchronize oxygen delivery and energy utilization, the twin engines of dairy productivity. Deficiency in either seldom causes dramatic illness but gradually erodes herd efficiency through subtle declines in milk yield, reproductive success, and calf vigor. Recognizing their interdependence allows producers and nutritionists to shift from merely preventing deficiency to actively optimizing metabolic performance.
In modern dairy systems, where sustainability and precision feeding define success, ensuring adequate iron and iodine represents a modest nutritional investment with a disproportionately large biological return. By mastering these two essential elements, farmers can strengthen both the productivity and resilience of their herds, proving that sometimes, progress begins with getting the basics right.
The full list of references used in this article is available upon request.
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