The Pollan book looks interesting, however, I question the statement about cattle not having evolved to eat corn.

Back in the 1930s-1940s, we preferred corn-finished beef (which we called Kansas City beef because at the time KC was a center for feedlots and slaughtering), although it was expensive, to grass-finished, which comprised most beef. Statements that all beef before WW2 was grass-finished are not true, although it certainly was by far the more abundant. Especially in warmer areas (Georgia to Texas) grass-fed beef was strong in taste and the fat was yellow. Warm-climate grasses are lower in protein content as well.

We never heard that the cattle had trouble digesting the corn; although this was before the surge in antibiotics. Corn-finished beef is fattier, therefore more tasty, but research shows that grass-finished beef is healthier- some of this research about lower cancer risk, etc., is incomplete and questionable.

See this article, which pushes grass-fed, antibiotic-free cattle: Know your meat