Okay I'm finally back to check in. I had to leave her to pick my daughter up from work, and stopped to buy a flashlight (since all mine have mysteriously disappeared) on the way home.
Went out and found her in the field with very little progress over a few hours time. But earlier she wouldn't let me get close. This time I walked right up, and with a little help from the cow managed to finally, with much effort, get the head and half the body out.
I thought when she stood again the calf would drop, but he didn't. She walked clumsily several more paces and practically fell on him (well, just one leg). When she began to push again, I began to pull again, and between her and me we managed to deliver a big bull calf.
Cow and calf are both alive but laying exhausted in the grass. I'll go back out and check on them again before I hit the bed.
Man, it sure is harder to pull a calf than to pull a kid goat.
And, yes, I know it would have been better if I had been able to help earlier. I kept telling the cow I could help her if she'd let me. But that's the thing about pasture cows. She wouldn't let me until it was dark and she was exhausted.