Prairie Oyster Harvest is a side mission in Far Cry 5. It takes place in John Seed's region of Holland Valley. This quest is unlocked by completing the side mission Testy Festy for Casey Fixman at the Spread Eagle Bar and then talking to him again.
Talk to Casey
Casey is glad you got the trailer back, but he's still missing the most important ingredient to a food-based festival: the food.
Fortunately, Casey has a plan on how to make this year's festival memorable: three new varieties of cooked testicles.
Reach Davenport Farm
Once you've reached the farm, you can obtain the testicles in any order.
Kill a Bull with a Tractor Mulcher
The easiest to complete, simply get in the nearby Tractor Mulcher, and shred a bull by running it over.
Kill a Bull with Fire
The bull must die to a fire damage over time for this to count.
Kill a Bull that is Mating
And now for the Blue balls: kill a bull while it is mating.
There is a gate nearby that contains several cows. Shooting the lock off the gate will allow the cows to run free, and the bulls will begin their mating ritual.
Once you hear the music (Sexual Healing by Marvin Gaye) kick in, it is safe to kill the bull for his testicles.
Talk to Casey
Now that you have his ingredients, return to Casey at the Hope County Cougars baseball field, and deliver the goods. It's time to party.
Have a Shot
Have a shot of whiskey, and enjoy the festival.
Shoot the targets (Optional)
While in a drunken, testicle-fueled stupor, take shots at the balloon targets with a Slingshot. This is an optional objective, and completing it does not affect the outcome.
After waking up from a booze-and-balls induced blackout, the mission will complete, and you will be given your rewards.