Lactating Steers and Hormones in Beef: What a 20,000-Head Study Found

Key Takeaways

• A 2024–2025 research project from West Texas A&M University evaluated 20,481 finished cattle from 33 feedlots and found 6,078 of them lactating at slaughter, including 14.55% of native beef steers and 19.66% of dairy-cross steers.
• Steers are castrated males. Milk collected from these animals contained measurable fat, protein, and lactose.
• Most conventional feedlot cattle receive slow-release hormone implants in the ear containing compounds such as estradiol, trenbolone acetate, and zeranol.
• The study did not prove implants caused the lactation, but earlier controlled research shows these same compounds can trigger mammary development in beef cattle.
• Commercial milk carries its own hormone load, and human research shows those hormones are absorbed when milk is consumed.
• A generic "grass fed" label does not prohibit hormone implants. Certified organic and certified American Grassfed beef do.
• The most reliable way to avoid implanted beef is buying directly from a farm whose practices you can verify.

This article is for educational purposes only and is not medical advice. It does not diagnose, treat, or prevent any condition. Consult a qualified practitioner about your individual health needs.

The Study Why Steers Lactate What Implants Are Human Health Hormones in Milk Label Guide Find a Farm FAQ

A castrated male animal producing milk falls outside normal biology. Farmers know this, veterinarians know this, and until recently nobody had counted how often it happens in commercial beef production. Now someone has, and the numbers deserve your attention before your next trip to the meat counter.

The Study: 6,078 Lactating Cattle

Researchers from West Texas A&M University, working with a co-author from Cargill Meat Solutions, evaluated 20,481 finished cattle across 33 feedlots. The findings were first presented as an abstract at the 2024 American Meat Science Association conference, then expanded in a 2025 Plains Nutrition Council poster titled "Outcomes of Unintended Consequences: Frequency of Lactation and Intact Males and Their Impact in Finished Cattle."

Of the cattle evaluated, 6,078 were lactating. The breakdown matters:

• Native beef heifers: 28.77% lactating
• Dairy-cross heifers: 55.81% lactating
• Native beef steers: 14.55% lactating
• Dairy-cross steers: 19.66% lactating

Heifers are young females that have never calved. Lactation without a pregnancy is already abnormal for them. The steer numbers are the ones that should stop you, because steers are castrated males. Roughly one in seven native beef steers, and nearly one in five dairy-cross steers, was producing milk at slaughter. The researchers did not simply note swollen tissue. They collected milk samples from a subset of animals and confirmed measurable fat, protein, and lactose.

Small independent processors tell a different story. Farmers and slaughterhouse operators working with cattle from family ranches and regenerative operations report never having seen a lactating steer, an observation documented in an August 2026 investigation by rancher and journalist Mollie Engelhart. That contrast is anecdotal rather than controlled, but it points at a difference between production systems that nobody has formally studied.

Why Would a Castrated Male Produce Milk?

The study documented the lactation without identifying its cause, and the researchers proposed one possibility themselves: extremely fat cattle produce estrogen in their fat tissue, which could contribute to milk production. That explanation is worth taking seriously, and it is not reassuring, since it would mean modern finishing programs push cattle physiology far enough that castrated males lactate from body fat alone.

The other suspect is the hormone implant program used across conventional feedlots, and the surrounding science makes that suspicion hard to dismiss. Inducing lactation in cattle that have never been pregnant is not a theoretical idea. Researchers have done it on purpose since at least 1975, using protocols built around estradiol and progesterone. A 1991 controlled experiment published in the Journal of Animal Science went further and tested the exact compounds used in growth implants. Beef heifers treated with estradiol, trenbolone acetate, zeranol, and combinations of the three showed measurable mammary changes, including increased teat development, compared with untreated controls.

So the established facts stack up this way. Hormones can switch on the mammary gland without a pregnancy. Growth implants deliberately alter endocrine signaling, and controlled research shows implanted steers carry higher circulating estradiol and trenbolone metabolites than unimplanted ones. Compounds used in implants have produced mammary development in beef cattle under experimental conditions. A large commercial survey now finds lactation in a meaningful share of finished steers. What no one has published is the direct comparison of lactation rates in implanted versus never-implanted cattle. Until that study exists, the accurate position is that implants are the leading hypothesis, not a proven cause, and that either explanation reflects a production system operating outside normal biology.

What Hormone Implants Actually Are

A growth implant is a slow-release pellet placed under the skin of the ear. The ear is chosen because it does not enter the food supply, but the hormones do not stay there. The compounds release into circulation over weeks to months, shifting the animal's endocrine system toward faster growth and leaner tissue on less feed. That is the entire point of the product, and it works, which is why the practice is standard economics in conventional feeding.

The compounds include estradiol (an estrogen), trenbolone acetate (a synthetic androgen), zeranol (an estrogenic compound derived from a mold toxin), progesterone, and testosterone. Doses are not trivial. The manufacturer's label for one widely used product, Revalor-XS, lists 200 mg of trenbolone acetate and 40 mg of estradiol per implant. Cattle in aggressive programs may be implanted more than once between placement and slaughter.

How common is the practice? A USDA feedlot survey reported that roughly 90% of feedlot steers and heifers received at least one growth implant. Treat that figure with the caution owed to any government-published statistic, since independent verification is limited, but no independent source disputes that implanting is near-universal in conventional feedlots. The lactation study did not report implant histories for its 20,481 animals, though cattle drawn from 33 commercial feedlots would be expected to broadly resemble the industry norm.

What This Means for the People Eating the Beef

Long-term human studies on developmental outcomes from eating implanted beef do not exist. Regulatory approval rests on residue thresholds set decades ago, and the European Union found that evidence unconvincing enough to ban implanted beef in 1989, a ban it has defended against US trade pressure ever since.

The human data we do have is limited but pointed. An NIH-funded study published in Human Reproduction in 2007 examined 387 men in relation to their mothers' diets during pregnancy. Sons of women who ate beef more than seven times per week had sperm concentrations 24.3% lower than sons of mothers who ate less, and were three times as likely to fall below the subfertility threshold. The authors identified anabolic steroids in beef as a suspected explanation. This is observational evidence, one tier below controlled trials, and it cannot prove causation. It is also the only serious attempt anyone has made to ask the question in humans, which says as much about research priorities as it does about the answer.

Whether or not implants explain the lactating steers, the study reveals something consumers were never told: a meaningful share of conventional beef comes from animals in a visibly abnormal endocrine state at slaughter. You are allowed to want better information than that before feeding beef to your family.

The Parallel Problem: Hormones in Milk

Beef implants are one hormone pathway into the food supply. Milk is another, and it works differently. Dairy cows do not receive ear implants. Instead, modern dairy management keeps cows in milk through nearly their entire pregnancy, and a pregnant cow's milk carries her pregnancy hormones with it. Estrogen and progesterone concentrations climb steeply through gestation, which means commercial milk pooled from herds milked deep into pregnancy contains hormone levels that pre-industrial dairying never produced.

A 2010 study in Pediatrics International measured what happens when people drink that milk. Japanese researchers gave commercial cow's milk to men, women, and prepubertal children, then tracked their hormones. Serum estrone and progesterone rose significantly after intake. In the men, luteinizing hormone, follicle stimulating hormone, and testosterone all dropped. Urine estrogens increased in every subject, children included. The authors concluded that milk estrogens are absorbed, that they suppress the body's own gonadotropin signaling, and that "sexual maturation of prepubertal children could be affected by the ordinary intake of cow's milk." They pointed to pregnant-cow milk as a plausible contributor to earlier puberty onset.

For evidence grading: this was a small acute study of 18 subjects drinking substantial volumes, one tier below a controlled trial, and it has not been replicated at scale. It remains the most direct human measurement anyone has published, and the mechanism it demonstrates, absorption of milk hormones intact into circulation, is the part that matters most.

The takeaway mirrors the beef story. The hormone load in milk is a product of management decisions, not of milk itself. A small dairy running seasonal calving and a traditional dry-off period produces milk with a different hormone profile than a confinement operation milking cows until weeks before calving. Knowing your dairy farmer matters for exactly the same reasons knowing your rancher does, and the questions worth asking are the same: how the animals are managed, how long cows are milked into pregnancy, and whether you can see it for yourself.

What the Labels Do and Do Not Tell You

Most shoppers assume "grass fed" rules out synthetic hormones. It does not. The claims operate independently, and knowing the difference protects your grocery budget from marketing.

• Grass fed (generic): Describes diet only. A grass fed animal can legally receive hormone implants unless another certification prohibits them.
• No added hormones / raised without hormones: A meaningful claim for beef when verified, though it relies on producer affidavits rather than testing in most programs.
• Certified American Grassfed (AGA): Prohibits hormones and antibiotics, requires pasture for life, and audits farms. One of the stronger third-party standards.
• Certified organic: Prohibits hormone implants outright.
• Natural: Refers to processing after slaughter. It tells you nothing about hormones, feed, or how the animal lived.

Labels are a floor, not a ceiling. The certification systems above are worth knowing, but every one of them is a substitute for the thing that actually answers your questions, which is a farmer you can talk to.

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How to Find Beef Raised Without Implants

The consolidated commodity system erases the story of how an animal was raised. Direct relationships preserve it. When you buy from a small farm, you can ask five questions that no supermarket label answers:

• Do you ever use hormone implants or growth promotants of any kind?
• What do your cattle eat from birth to finish?
• Where are your animals processed, and do you attend?
• How long do your animals live before slaughter?
• Can I visit the farm?

A farmer with nothing to hide enjoys these questions. Hesitation is itself an answer.

Where to start looking

• Montana: Oxbow Cattle Company in Missoula raises cattle on regenerative pasture without implants, and their tallow is what we use in our own products.
• Montana: Mannix Family Beef in Helmville, a fifth-generation ranch in the Blackfoot Valley, raises 100% grass fed and finished cattle without hormone implants or antibiotics, processed at 24 to 36 months and sold at the Missoula, Lincoln, and Seeley farmers markets and the Good Food Store.
• Nationwide shipping: White Oak Pastures in Bluffton, Georgia abandoned hormone implants in 1995, slaughters on the farm in its own USDA-inspected abattoir, and holds American Grassfed, Animal Welfare Approved, and Certified Humane certifications. They ship anywhere in the US.
• American Grassfed Association producer directory: Searchable by state, every listed farm certified hormone-free and pasture-raised for life. americangrassfed.org
• EatWild state-by-state directory: The longest-running independent listing of pasture-based farms in North America. eatwild.com
• Local farmers markets: Vendors selling their own beef will nearly always tell you their implant policy on the spot.

Expect to pay more per pound and less per animal. Buying a quarter or half beef directly from a verified farm routinely beats supermarket prices on premium cuts while funding the kind of agriculture that does not produce lactating steers.

Frequently Asked Questions

Does the US use hormones in beef?

Yes. Hormone implants are legal and standard in US beef production. A USDA feedlot survey reported that roughly 90% of conventional feedlot cattle receive at least one implant before slaughter. The European Union banned implanted beef in 1989 and does not accept US hormone-treated beef imports.

What hormones are used in beef cattle?

Approved implant compounds include estradiol, trenbolone acetate, zeranol, progesterone, and testosterone. They are combined in slow-release pellets placed under the ear skin. One common product, Revalor-XS, delivers 200 mg of trenbolone acetate and 40 mg of estradiol per implant.

Why are steers lactating?

No published study has proven the cause. Hormone implants are the leading hypothesis because estradiol, trenbolone acetate, and zeranol have produced mammary development in beef cattle in controlled research. The study authors also suggested estrogen produced by excess body fat as a possible contributor.

Do hormones in beef affect humans?

Long-term human studies do not exist. One observational study found that sons of mothers who ate beef more than seven times weekly during pregnancy had 24% lower sperm concentrations, with anabolic steroids in beef named as a suspected cause. The finding needs confirmation, and independent research remains scarce.

Is grass fed beef hormone free?

Not necessarily. A generic grass fed claim describes diet, not hormone use, and implants remain legal under it. Certified American Grassfed and certified organic beef both prohibit hormone implants. The surest option is asking the farmer directly.

Does milk contain hormones?

Yes. Commercial milk contains natural estrogens and progesterone because modern dairy cows are milked through most of their pregnancy. A 2010 human study found these hormones are absorbed when milk is consumed, raising estrogen levels and suppressing testosterone in men. Milk from small dairies with traditional management carries a lower hormone load.

Sources

1. Petit TC, Eckhardt ME, Lucherk LW, Schwartz TE, Brown TR, Lawrence TE. Frequency of Lactation and Intact Males and Their Association with Carcass Outcomes in Finished Cattle. AMSA 2024 Abstract 115; expanded poster, 2025 Plains Nutrition Council Spring Conference. Primary poster proceedings (PDF)
2. Moran C, et al. The effect of estradiol, trenbolone acetate, or zeranol on growth rate, mammary development, carcass traits, and plasma estradiol concentrations of beef heifers. J Anim Sci. 1991;69(11):4249-4258. PubMed
3. Fulkerson WJ, McDowell GH. Artificial induction of lactation in cattle. J Endocrinol. 1975. PubMed
4. Swan SH, Liu F, Overstreet JW, Brazil C, Skakkebaek NE. Semen quality of fertile US males in relation to their mothers' beef consumption during pregnancy. Hum Reprod. 2007;22(6):1497-1502. DOI
5. vom Saal FS. Could hormone residues be involved? Hum Reprod. 2007;22(6):1503-1505. DOI
6. Maruyama K, Oshima T, Ohyama K. Exposure to exogenous estrogen through intake of commercial milk produced from pregnant cows. Pediatr Int. 2010;52(1):33-38. DOI
7. Engelhart M. When a Steer Produces Milk, Farmers Should Pay Attention. Acres U.S.A., August 2026. Independent firsthand reporting by a working rancher. Article
8. Revalor-XS official product label, DailyMed. Manufacturer-published dosage documentation. Label
9. USDA APHIS Feedlot 2011 survey, implant practices. Institutional source, lower confidence; included as the only available prevalence figure. Report (PDF)