Settlers land on a supposedly unoccupied planet, and raise livestock on an early life stage of a sentient species. The early stage looks like grass. After the sentient species begins its next stage of life, they realize that they have to give back the cells they took from the species.
Sounds like Velvet Fields, by Anne McCaffrey, first published in the speculative fiction magazine "Worlds of If" in its November-December 1973 issue and then in "The Girl Who Heard Dragons", a collection of McCaffrey's short stories.
The planet is apparently deserted:
Although Survey had kept a watch on the planet for more than thirty years standard and the cities were obviously on a standby directive, the owners remained conspicuous by their absence.
The settlers raise livestock on the grass:
We pastured the cattle in neatly separated velvet fields. Martin Chavez worried when close inspection disclosed that each velvet field was underpinned by its own ten-meter-thick foundation of ancient, rock-hard clay. Those same foundations housed what seemed to be a deep irrigation system.
But the grass turns out to be the first stage of the native sentient species:
“You mean, the plants are the people?”
“What else have I been saying? They are born from the Trees.”
And in the end, the settlers who took nutrition from the planet give back parts of their bodies:
We had to give back to the soil what we had taken from it. The handless Zobranoirundisi, recognizing his missing member from the cells now incorporated into the fingers of a young colony child nurtured on milk from cattle fed in the velvet fields, had every right to reclaim what was undeniably his own flesh. The legless Zobranoirundisi could not be condemned to a crippled existence when the Terran child had used the same cells to run freely for seven years on land where previously only Zobranoirundisi had trod.