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.
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