I agree that 86% of those Calories would otherwise go to waste. But considering how bad animal agriculture is for the environment just in terms of direct impact

I agree that 86% of those Calories would otherwise go to waste. But considering how bad animal agriculture is for the environment just in terms of direct impacts, I doubt that feeding them to animals is an improvement over the alternative you mentioned, and I highly doubt that it's the optimal solution across all the potential alternative disposal methods we could come up with. The plants that cattle and many other livestock eat - including grasses, corn, and other feed crops - contains protein. The exact amount depends on the plant, its age, and growing conditions, but it's often 10 to 15% by mass. Cows apparently consume about eight grams of protein for every gram of protein that makes it to humans' plates, with the rest of the amino acids presumably making up inedible organs and/or being used for bodily functions while they're alive.