Sounds like Robert Silverberg's "Sundance" (1969).
One member of the group notices herds of small, yellow creatures that feed on the grass of the planet.
They are spherical, bulky, slow-moving creatures, covered by masses of coarse orange fur.
The man tries the grass and finds it is hallucinogenic.
The Eaters are nibbling the lemon-colored perforated globes, nibbling the stalks as well. They thrust their plants at me. It is a religious ceremony, I see. Take from us, eat with us, join with us, this is the body, this is the blood, take, eat, join. I bend forward and put a lemon-colored globe to my lips. I do not bite; I nibble, as they do, my teeth slicing away the skin of the globe. Juice spurts into my mouth while oxygen drenches my nostrils.
He eats it several times, only to find he no longer knows what is real and what isn’t. The story ends with the phrase “then the floor drops out.”
It is like falling through many trapdoors, looking for the one room whose floor is not hinged. Everything that has happened on this planet is your therapy, you tell yourself, designed to reconcile an embittered aborigine to the white man’s conquest; nothing is really being exterminated here. You reject that and fall through and realize that this must be the therapy of your friends; they carry the weight of accumulated centuries of guilts and have come here to shed that load, and you are here to ease them of their burden, to draw their sins into yourself and give them forgiveness. Again you fall through, and see that the Eaters are mere animals who threaten the ecology and must be removed; the culture you imagined for them is your hallucination, kindled out of old churnings. You try to withdraw your objections to this necessary extermination, but you fall through again and discover that there is no extermination except in your mind, which is troubled and disordered by your obsession with the crime against your ancestors, and you sit up, for you wish to apologize to these friends of yours, these innocent scientists whom you have called murderers. And you fall through.