>Actually, it appears that the beneficial effects of antibiotics as growth
promoters do not diminish over time which suggests at least two things: the
effect is the consistent on naive animals, which haven't lived long enough
to become inured to it, and/or the effect is not related to suppression of
evolvable microbes; possibly it's a hormetic response to the toxins in
antibiotics.

Thanks Pete (Charlie, you apparently missed the point of my question).  If
the mode of action was to suppress certain microbes, then one would expect
that after enough generations of use, that the antibiotic would start to
lose its benefit at promoting increase in muscle mass.

So I did a quick search to see confirm whether there was data to support
the widespread use of antibiotics as growth promoters for livestock.  It
appears that their economic benefit may indeed be diminishing. From a
review:

"However, there is growing evidence to suggest that antibiotics used as
growth promoters do not have as much economic benefit as previously
thought40, particularly in countries with advanced farming techniques. This
undermines the economic arguments in favour of using antibiotics for growth
promotion: that the farmer will suffer productivity losses if he or she
does not use growth promoters. Recently published papers suggest that the
benefit from a growth perspective of using antibiotics sub-therapeutically
in animals has declined over time, due to the changing microbial
composition of animals fed antibiotics41. They also tend to be most
effective when the conditions the animals are kept in are poor42, with low
standards for infection control and cramped conditions. General
improvements in these standards seem to have reduced the effectiveness of
antibiotics as growth promoters. Studies in the US, Denmark and Sweden,
after the 2000s, showed that growth promoters had less effect than they had
done in earlier decades on the growth rate, and feed efficiency of animals.
The impact after the 2000s was typically less than five percent43. "

https://amr-review.org/sites/default/files/Antimicrobials%20in%20agriculture%20and%20the%20environment%20-%20Reducing%20unnecessary%20use%20and%20waste.pdf

If you wish to read the citations (41-43), they are open access:
40. Graham J, Boland J, Silbergeld E, Growth Promoting Antibiotics in Food
Animal Production:An Economic Analysis. Public Health Reports, 2007 122
(1), 79-87.
41. Laxminarayan, R., Van Boeckel T, Teillant A, The Economic Costs of
Withdrawing Antimicrobial Growth Promoters from the Livestock Sector. OECD
Food, Agriculture and Fisheries Papers, 2015, No. 78, OECD Publishing.
http://dx.doi. org/10.1787/5js64kst5wvl-en.
42. Ibid.
43. Laxminarayan R, Van Boeckel, Teillant A, Global antimicrobial use in
the livestock sector. OECD, 2015, Working Party on Agricultural Policies
and Markets, Trade and Agriculture Directorate, Committee for Agriculture,
TAD/CA/APM/WP(2014)34/FINAL.

Lots more at https://amr-review.org/Publications.html


-- 
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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