Demand pushes to have more meat, but animals need time to grow, sometimes years. To earn more money, companies need to grow animals faster, which allows them to accelerate sales and minimize feeding time (feed is a huge part of the cost). To make animals grow faster, they are injected with hormones, the residue of which stays in the meat as well as pollutes the environment.
Environmental impact of hormonal treatment for animals is seen the most in fish, when contaminants (known as endocrine disruptors) leak into waters (among others with inadequate manure treatment). Aquatic life reduction is attributed to the injection of slow release hormone capsules injected under the skin of the cows.