What would the best course of action be in your opinion? Our hunting property is a working cattle farm, its closed off to cattle from Oct through Feb. In late Sept. or early Oct we amend the soil based on soil test results. Plant a couple green fields in it. We use white clover and WMS Deer Magnet. Roughly 80-100 cows have been grazing it since March.
Is it worth our efforts to fertilize these green field areas now - hoping for healthier regrowth when it cools off? My fear is that cattle traffic will destroy these planted areas and hay will choke it out and we’ll be back to square one come October.

There's no way I'd plant a warm-season plot and spend time and money on something cows can and will eat. Save your money for fall and plant a good plot then.
Any fertilizer you use now will basically be gone in 6 months (probably sooner).

0-20-20 now would be great for your clover in that field

I'm not talking about planting anything new. I'm just talking about fertilizing what's already planted.

I gotcha. I still wouldn't do it, but that's just me, I'm cheap

I wouldn't do it now, maybe when i planted.