I wouldn't think it is safe, every bull is probably unique.
Don't wear red or bright colors. I think that excites them.
Ask the owners/land owners of the bull and cow and get their permission to collect milkweed in their pasture. I think it is entirely likely a bull could charge if he thinks you are invading his territory.
You might have to sign a liability waiver and assume your own risk.
You might also get a cowboy chasing you with a gun if you don't have permission to enter pasture.
Hopefully you can find some milkweed on the road side of the fence. I'd put a fence between me and the bull, but if the bull wants you I'm not sure a fence will stop him.
Saving one egg, cat and butterfly at a time