If you're American or Canadian or Australian (maybe others?) the secret is to make sure you visit "Accredited" zoos and aquariums.
They have regular inspections and assessments to ensure they meet Requirments for enclosure space sizes and maintenance and cleanliness quality, behaviour and mental enrichment for the animals (of which this would be something, providing natural prey instead of ground meat), health checks and veterinary care, documentation of all of the above, etc.
If somewhere isn't accredited is risky as to whether or not the animals are properly cared for.
It is AZA in America, CAZA in Canada, and AAZA in Australia. There may be others for other countries but those are the ones I'm familiar with.
(Source: former zookeeper, AZA)
Corn is a grass, but you are typically feeding them the grain of the grass rather than the stalks and leaves which they should be eating.
Normally grass grains aren't the majority diet of cows, it's the leaves.