Alfalfa fertilized with selenium can increase milk’s organic selenium levels by fourfold
A new study in Applied Animal Science traces the essential mineral selenium from forage to the final milk product, including what form it takes and the overall efficiency of the transfer
Champaign, IL, September 28, 2026---The trace mineral selenium plays a critical role in the diets of animals and humans alike, and there is increasing interest not just in animal supplementation, but also in producing selenium-enriched foods--meat, milk, and eggs--for human consumption. In a new Applied Animal Science study, a team of researchers demonstrates that dairy cows grazing alfalfa fertilized with selenium produced milk with roughly four times the selenium of cows grazing untreated pasture, and almost all of that selenium arrived in its most bioavailable organic form, adding critical data to the body of selenium research.
“We know selenium is essential,” explained corresponding author Alejandro R. Castillo, PhD, PAS, of the Cooperative Extension, University of California (Merced, CA, USA), “and its deficiency is an important widespread practical problem.” Although you can find the mineral naturally occurring in the soil and in plants, much of the world’s cropland falls below what grazing animals need. Producers have several ways to close the gap: free-choice supplements, injections, drenches, ruminal boluses, or fertilization of the pasture directly. “We knew from a previous study that applying selenium directly to alfalfa plants can increase overall forage levels and especially the more bioavailable organic forms of selenium, so we wanted to explore whether that increase also translates into the resulting milk from grazing dairy cows.”
Castillo and an international group of researchers set out to get answers by studying 20 late-lactation Holstein cows under differing grazing conditions: One set of alfalfa plots was fortified with a direct application of sodium selenite, and the control group received none.
Caption: A recent Applied Animal Science study examines direct pasture fertilization of selenium--a critical trace mineral for animals and humans--as a route to increase selenium amounts in dairy cows’ milk (Credit: Alejandro R. Castillo).
For the pasture itself, this fertilization led to a 21-times increase in selenium concentration compared with the control group and a 19-fold increase in overall selenium intake for the animals in the study. Forage quality measures, including crude protein and neutral detergent fiber, indicated that the application of selenium did not compromise the feed value. The milk selenium concentration from the selenium-supplemented group increased by a factor of four, and the team found organic selenium made up roughly 80% of the selenium in milk, a higher proportion than in the alfalfa itself. No inorganic selenium species were detected in either the forage or the milk.
However, the team observed that far more selenium in the cows’ diet did not produce a proportional increase in the milk. The mineral’s transfer efficiency dropped from 8.0% in the control cows to 2.3% in the fertilized group.
“Our results indicate that this kind of fertilization application can increase selenium concentration in pasture and in the resulting milk, without affecting pasture nutrient composition,” explained Castillo. However, he and the team cautioned that longer-term studies are needed to evaluate effects on milk yield, animal health, and production efficiency. “Anyone considering pasture fertilization in future research needs to work from their own unique figures,” Castillo said. “Selenium already in the feed and water, interactions with other minerals, the animals’ requirements, and the regulations where they operate all have to come first.”
Bill Weiss, PhD, PAS, Dipl. ACAS, editor in chief of Applied Animal Science, said: “Adequate intake of selenium contributes to the health of animals and humans. This article discusses how fertilizing pasture with selenium influences concentration and, especially important, the chemical form of selenium in milk when cows graze the pasture. This information could lead to improved methods of increasing selenium in human diets.”
The article appears in the October 2026 issue of Applied Animal Science.
Notes for editors
“Evaluation of alfalfa pasture biofortified via foliar selenium applications and lactating dairy cows under grazing conditions,” by Mónica del Carmen Gaggiotti, Fernando F. Muñoz, María Jesús Sánchez-Martín, Roberto Boada, Laura Simonelli, Manuel Valiente, María G. Guevara, Mercè Llugany, Carlo Marini, Alejandro Smulovitz, Camila Miotti, Pablo Roskopf, Julio A. Di Rienzo, Federico Marini, Pramod K. Pandey, and Alejandro R. Castillo (https://doi.org/10.15232/aas.2025-02787), Applied Animal Science, volume 42, issue 5 (October 2026), published by FASS and Elsevier.
This article is openly available at https://doi.org/10.15232/aas.2025-02787.
To schedule an interview with the author(s), please contact Alejandro Castillo at arcastillo@ucanr.edu.
About Applied Animal Science
Applied Animal Science (AAS) is a gold open access, peer-reviewed scientific journal and the official publication of the American Registry of Professional Animal Scientists (ARPAS). In continuous publication since 1985, AAS is a leading outlet for animal science research. It has a Journal Impact Factor and is indexed by Scopus, Clarivate, CABI, and AGRICOLA. AAS caters to a wide and comprehensive audience, and its readers are typically university, extension, animal-industry, and government employees; members of professional societies and related organizations; producers, commodity organizations, and related feed and foods industries; researchers and students; and consultants and companies providing products and services in animal agriculture. The journal welcomes novel manuscripts on applied technology, reviews on the use or application of research-based information on animal agriculture, commentaries on contemporary issues, short communications, and technical notes. Topics that will be considered for publication include (but are not limited to) feed science, farm animal management and production, dairy science, meat science, animal nutrition, reproduction, animal physiology and behavior, disease control and prevention, microbiology, agricultural economics, and environmental issues related to agriculture. Themed special issues also will be considered for publication. www.appliedanimalscience.org
About the American Registry of Professional Animal Scientists (ARPAS)
The American Registry of Professional Animal Scientists (ARPAS) is the organization that provides certification of animal scientists through examination, continuing education, and commitment to a code of ethics. Continual improvement of individual members is catalyzed through publications (including the AAS journal) and by providing information on educational opportunities. ARPAS is affiliated with five professional societies: American Dairy Science Association, American Meat Science Association, American Society of Animal Science, Equine Science Society, and Poultry Science Association. www.arpas.org
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