The real problem is that you need hemispherical hairless cows. ;)

I suspect a photosynthetic cow would still need to eat a lot of grass

If you eat fructose, you end up turning some of that into uric acid and excreting it, wastefully. It does other varied harm on its way out.

That's exactly my point. Some percentage of that ADP and ATP will be lost and the adenine/ribose broken down or reused for something else - cells are messy environments. Some of that will be lost permanently to waste. The more turnover in this process, the more opportunity for that loss.

That's a good opportunity to join forces with Sabine Hossenfelder and together move some of excess funding from, say, particle physics, and allocate it to fundamental research on which all physics depends: creation of frictionless, vacuum-rated spherical cows.