Hormones
Conventionally grown meat is cheaper for several reasons. First of all, hormones make calves, piglets and other animals grow faster and get to market more quickly. Hormones are a major no-no for organic food fans.
Growth hormones are banned in the EU, but there is very little research on the health effects on humans. Anecdotal reports of girls developing earlier than forty years ago and an increase in obesity when hormones became common in the meat industry.
Coincidence or causation?