Álvaro García
Selenium (Se) is a micro nutrient with a significant impact on dairy cow performance, health, and milk quality. Deficiency is common in many regions of the world, including the Middle East, North Africa, China, New Zealand, northern Europe (especially Scandinavia and the Baltic states), sub-Saharan Africa, and parts of South America such as Brazil and Chile, because soils and forages in these areas naturally contain low levels of this essential trace mineral.
Under heat stress, cows’ bodies work harder to maintain normal temperature and metabolism, which leads to the buildup of harmful molecules called free radicals. This condition, known as oxidative pressure or oxidative stress, can damage cells, weaken the immune system, and make the udder more vulnerable to infection. Selenium helps prevent that damage by forming part of key antioxidant enzymes such as glutathione peroxidase, which neutralize free radicals and protect tissues. However, not all selenium sources are absorbed or used by the cow with the same efficiency.
For decades, most dairy diets relied on inorganic sodium selenite. This form is highly reactive in the rumen, where part of it is converted to elemental selenium, a biologically unavailable form. As a result, only a fraction is absorbed and stored. Organic selenium, such as selenium yeast or the purer hydroxy-selenomethionine (OH-SeMet), behaves differently. Because it is bound to an amino acid like methionine, it can be absorbed and incorporated into body proteins, forming a natural selenium “reservoir” that supports antioxidant activity and immunity when cows face stress, whether from calving, disease, or high environmental temperatures.
What research shows about selenium sources?
A recent trial by Hachemi and colleagues (2023) at the University of New Hampshire helps clarify the difference. In that study, mid-lactation Holstein cows were fed one of four diets:
- Control: with background selenium from feedstuffs (about 0.2 mg/kg),
- Treatment 1. With 0.3 mg/kg selenium from selenium yeast.
- Treatment 2. With 0.1 mg/kg selenium from OH-SeMet.
- Treatment 3. With or 0.3 mg/kg selenium from OH-SeMet.
The cows were healthy and not selenium deficient at the start, which makes the results even more relevant to well-managed commercial herds.
The analysis showed that cows fed OH-SeMet had higher selenium in both plasma and milk, and the transfer efficiency from blood to milk was greatest at the 0.3 mg/kg level. Milk selenium concentration averaged 104 µg/kg for cows receiving OH-SeMet 0.3 mg/kg, compared with 85 µg/kg for those on selenium yeast and only 50 µg/kg in the control. Statistical modeling showed that about 0.2 mg/kg of selenium from OH-SeMet achieved the same milk selenium as 0.3 mg/kg from selenium yeast, confirming that the hydroxy-selenomethionine form was more bioavailable.
Beyond mineral status, the benefits extended to udder health. Somatic cell counts (SCC) dropped dramatically in cows fed the higher OH-SeMet dose. Average SCC in the control and selenium yeast groups ranged between 59,000 and 81,000 cells/mL, while cows receiving 0.3 mg/kg of OH-SeMet averaged just 29,000 cells/mL. Milk and energy-corrected milk yields also increased slightly for all selenium-supplemented cows compared with the control, but only the OH-SeMet group showed a simultaneous rise in milk protein content and a decline in SCC, two characteristics of better udder health and tissue integrity.
The authors proposed that this effect was due to the unique structure of OH-SeMet. Because nearly all its selenium (more than 98%) is present as selenomethionine, it is readily incorporated into proteins in mammary and immune cells. During infection or stress, these reserves can be mobilized to strengthen antioxidant defenses, stabilize cell membranes, and moderate inflammation. The result is less epithelial cell damage in the udder, lower SCC, and better milk quality. This finding agrees with research in other livestock species showing that OH-SeMet helps protect mammary cells from heat- or stress-related damage and supports stronger immune defenses when cows are under oxidative stress.
The economics of selenium efficiency
For producers, these biological differences have real financial implications. Lower SCC means more saleable milk, higher premiums, and fewer health costs. The economic value of low SCC extends beyond premiums alone. Each rise in bulk tank SCC reduces milk yield and feed efficiency even before any penalty is applied. High-SCC cows divert nutrients toward immune responses and lose milk solids, effectively producing less from the same feed (García, A. 2020).
Economic analyses and field data summarized by García show that reducing SCC from around 300,000 to 200,000 cells/mL typically adds about two pounds of milk per cow per day while improving component percentages (García, A. 2020). At the same time, many processors offer quality bonuses of $0.10 to $0.20 per hundredweight for milk under 200,000 SCC. Combined, these improvements could generate about $190,000 in additional income per 1,000 milking cows each year. The premium portion accounts for a third of that value, while the hidden production gain represents the rest.
In that same herd, switching from selenium yeast to OH-SeMet might increase feed costs by three cents per cow per day, or about $5,500 annually, a modest expense compared with the potential return if lower SCC and better milk persist through the summer months. Even small improvements in milk yield, feed efficiency, and herd health would more than offset the supplement cost.
The broader benefits under heat stress
While the New Hampshire study was conducted under temperate conditions, the implications for hot climates are particularly relevant. In regions where the temperature–humidity index (THI) often exceeds 80, heat stress triggers oxidative stress, immune suppression, and a higher incidence of mastitis. Under these conditions, selenium requirements rise because antioxidant enzymes must work harder to neutralize reactive oxygen species.
Organic selenium sources like OH-SeMet provide an advantage because they build internal reserves that can be drawn upon when cows eat less or face heat stress. Studies in tropical and subtropical herds have reported better heat tolerance, higher conception rates, and lower inflammatory markers when cows receive organic selenium compared with inorganic forms. Thus, the combination of improved selenium status, stronger immune response, and reduced SCC not only improves milk quality but also enhances resilience and longevity in challenging environments.
From biology to the bottom line
For producers, the message is clear: selenium choice affects more than a mineral number on a lab report. It influences udder health, milk yield, cow longevity, and the farm’s economic resilience. While sodium selenite remains the cheapest option per ton of feed, it offers the least return in terms of animal performance. Selenium yeast is a step up, improving bioavailability and transfer to milk, yet its variable composition, only about 60–65% of its selenium is in the active selenomethionine form, limits its efficiency. Organic selenomethionine, though slightly more expensive, is a purer and more predictable source that yields higher tissue and milk selenium levels, lowers SCC, and provides better protection against oxidative and infectious stress.
Maintaining low SCC is not only about avoiding penalties. It is about protecting daily milk output, optimizing feed efficiency, and reducing drug and labor costs associated with mastitis. The cumulative effect of these small daily advantages can determine the difference between a herd that breaks-even and one that remains sustainably profitable, especially in regions where heat stress and disease pressure erode margins.
Last thoughts
The research by Hachemi et al. demonstrates that feeding 0.3 mg/kg selenium from organic selenomethionine can enhance milk selenium, improve udder health, and reduce somatic cell counts more effectively than selenium yeast or lower selenium inclusion levels. These physiological improvements translate into measurable economic benefits when coupled with meticulous herd management.
When selecting a selenium source, producers should think beyond the mineral price per kilogram and focus instead on the returns per cow. A slight increase in feed cost can pay back severalfold through higher milk yield, better component premiums, and lower culling and treatment costs. Eventually, improving antioxidant and immune capacity through an efficient selenium source like organic selenomethionine is not just a nutritional decision, it is a strategic investment in herd health, milk quality, and profitability.
The full list of references used in this article is available upon request.
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