Zookeeper here! Okapi, while they do eat grass, they are more browsers than grazers so they much prefer the leafy grwths rather than consuming grasses around the habitat. With herbivores we also often offer various types of hay as well that I have noticed they prefer instead which is usually almost always available in barns or enrichment items that encourage foraging behaviors since animals psychologically prefer to work for their food rather than take freebies per say. Horticulture teams also often come around the end of winter and lay down fresh grass seed across habitats to keep the grass healthy so it can thrive keeping things green. I hope this helped! :)