Back before people had nutritional science figured out, the key to success was to let livestock (and people) pick and choose from a wide variety of foodstuffs. Confined animals (and people) fared poorly. Sailors suffered from scurvy at sea, and people in institutions suffered from pellagra, but the same people never had these problems when given a little freedom, even though they knew nothing about nutrition. They just listened to their cravings.
Nutritional science means that you can get away with giving livestock (and people) a balanced diet without any food choices, but that doesn’t mean it’s always the right thing to do.