Flaxseed likely won't become a staple in the diets of beef cattle. It's too fat, and too high in unsaturated fat, which is toxic to the bacteria in the cow's rumen (first stomach and huge fermentation vat). Plus, fat tends to coat the fibre of the hay and stop it's digestion by bacteria, which is a really bad idea for a ruminant. It's a nice thought, but in practice, it's not realistic. If you really want the benefits of flax, it's best to just eat it yourself :)