By P.J. Huffstutter and Tom Polansek CHICAGO (Reuters) – Merck & Co Inc wants to feed its controversial feed additive Zilmax to 240,000 U.S. cattle to prove it is safe. Merck plans to conduct the biggest ever test of its kind in an effort to reintroduce the weight-adding drug into the United States and Canada after suspending sales last August. I can’t afford to give away a steer, let alone hundreds.” Cargill Inc and Tyson Foods Inc, two of the world’s largest beef processors, told Reuters their stance on Zilmax had not changed since last autumn, when they stopped accepting cattle fed the drug following reports it may cause lameness.