Having assisted in cornering and capturing a few barn cats, I can't imagine spaying one.
It would be like trying to sedate a running buzz saw.
I guess a vet could get a needle in it while it tried to tear his assistant's arm off.
Last time I cornered and captured a barn cat, I had the good sense to be wearing a thick pair of heat resistant welding glove that came up to my elbows.
Damned bastard still managed to bite a finger thru the gloves!
I tossed him in the manure holding pit. All 325,000 gallons of it. Problem solved.