The main reason that they give is the higher risk of dark cutters I think.Did you see the Country Calendar calve rearing farm I posted @gone up the hill ?
Why is it supermarkets and abattoirs in the UK don't want anything over 16 months?
More likely is it gives them more control over farmers and they don't want bulls in slowing down kill lines, not that I have great deal to do with that end of the industry.
Most places grazing large numbers of bulls isn't particularly practical here anyway