Growth Promoters and the Environment Revisited 🎙️
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This article written by Dr. Reynold Bergen, BCRC Science Director, originally appeared in the November 2025 issue of Canadian Cattlemen magazine and is reprinted on BeefResearch.ca with permission of the publisher.
In October 2021, this column described a research project that examined how long residues from growth promoters persist in the feedlot environment. They learned that residues from trenbolone acetate (TBA; used in some growth implants to mimic testosterone) and melengestrol acetate (MGA; sometimes fed to heifers to suppress estrus) dissipate very quickly after they’re excreted. However, residues from ractopamine (a feed additive that improves feed efficiency, weight gain and leanness late in the feeding period) could be found on the pen floor for up to five months after it was last fed.
Jon Challis and collaborators at Agriculture and Agri-Food Canada and the University of Saskatchewan recently published a follow-up study to learn whether manure composting, stockpiling or soil incorporation help break down ractopamine residues and whether ractopamine residues can affect hormone levels in other organisms that may come in contact with them in the environment (“Chemical and bioassay-based characterization of the growth promoter ractopamine in beef cattle manure;” doi.org/10.1093/etojnl/vgaf211/8252989).
What They Did
Four pens of feedlot steers were raised using TBA implants and fed ractopamine for the last 42 days of the feeding period. Four pens of steers were raised without either growth promoter. Pens were cleaned as soon as the 273-day feeding period ended, and manure was windrowed on 12 separate pads (6 windrows per treatment). Half of the windrows (three per treatment) were stockpiled (left undisturbed) for 28 days. The other windrows were composted for 28 days and turned on day seven and 14. Each windrow was sampled at seven timepoints for residue analysis in the lab. Raw manure was also applied to soil plots that hadn’t received manure before. Manure was applied at a rate equivalent to 24 long tons per acre, incorporated, and sampled at nine different timepoints.
Ractopamine and TBA residues were extracted from the samples, and analyzed to see whether stockpiling, composting or soil application help break ractopamine down. Samples from days zero, four and 28 were also used in bioassays to see whether ractopamine occurred at high enough levels to impact hormone levels in even the most sensitive organisms. One bioassay used a breast cancer cell line that was extremely sensitive to factors that could drastically increase or inhibit androgen (e.g., testosterone) production. A second bioassay used a breast cancer cell line that was extremely sensitive to factors that could drastically increase or inhibit estrogen production. These cell lines are even more sensitive to hormonal disruption than the aquatic organisms that are sometimes seen to have skewed sex ratios or other abnormalities when exposed to environmental contaminants.
What They Learned
Levels of TBA (and its metabolites) were too low to detect, even on day 0. That’s not surprising – their previous study found the same thing.
Stockpiling vs. composting: Both manure handling approaches had similar effects. Levels of ractopamine dropped by at least 95% by the end of the 28-day trial. Most of the reductions occurred within the first 10 days.
Soil application: Ractopamine levels were much lower after raw manure was incorporated into soil, partly because the soil diluted the manure. But ractopamine levels also dropped by 80% over the next four weeks. Again, most of the decline happened within the first 10 days after manure was applied to the soil.
Androgen bioassay: Exposing manure extracts to the highly sensitive cancer cell line did not affect androgen production.
Estrogen bioassay: Manure extracts affected estrogen, but this was not because of growth promoters. Manure from steers that received TBA and ractopamine had the same effect on estrogen as manure from steers that didn’t get any growth promoters. So, what happened? The estrogen effect from the manure was probably stimulated by naturally occurring estrogen produced by the animals, phytoestrogens that came from the diet, or some combination of both. All animals produce estrogen, and many plants contain phytoestrogens, so manure from other livestock, pets or humans would very likely have the same estrogenic effect as cattle manure.
Bottom Line
A month of stockpiling or composting effectively reduced ractopamine residues to harmless levels in feedlot manure. Stockpiling or composting manure longer would virtually eliminate the remaining residues. These are both common manure management practices. In 2021, a Canadian Roundtable for Sustainable Beef survey of 333 cattle producers across the country found that 17% of producers compost manure and 66% stockpile manure before spreading it.
What Does This Mean to You?
It’s not always possible, but it’s always best to have answers before tough questions arise. Growth promoters are a tremendously valuable tool for the feedlot sector. We’ve known for decades that beef from cattle raised using growth promoters is just as safe and nutritious as beef from cattle raised without them. This research looked for – and failed to find – potential effects on environmental health. Common manure management practices like stockpiling and composting make manure lighter and easier to haul and spread, while also helping to protect the environment.
The Beef Cattle Research Council is a not-for-profit industry organization funded by the Canadian Beef Cattle Check-Off. The BCRC partners with Agriculture and Agri-Food Canada, provincial beef industry groups and governments to advance research and technology transfer supporting the Canadian beef industry’s vision to be recognized as a preferred supplier of healthy, high-quality beef, cattle, and genetics. Learn more about the BCRC at www.beefresearch.ca.
- References
- Challis, J.K.; Sura, S.; Cantin, J.; Curtis, A. W.; Shade, K. M.; McAllister, T. A.; Jones, J. P.; Giesy, J. P.; Larney, F. J. (2021). Ractopamine and Other Growth-Promoting Compounds in Beef Cattle Operations: Fate and Transport in Feedlot Pens and Adjacent Environments. Journal of Environmental Science and Technology 55 (3). Available here.
- Challis, J. K.; Cantin, J.; Thresher, J.; Curtis, A. W.; Jones, P. D.; Brinkmann, M.; Hogan, N.; Giesy, J. P.; McAllister, T. A.; Larney, F. J. (2025). Chemical and bioassay-based characterization of the growth promoter ractopamine in beef cattle manure. Environ Toxicol Chem. 44(11):3149-3158. Available here.
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