After my first cutting I’ve gotten a lot of Johnson grass in my hay fields. There’s a lot of conflicting info online about whether or not to feed it due to cyanide toxicity. Is it safe to mow and bale or should I just mow and not bother with a second cutting? 

Im not really sure, but I remember for a few years our hay guy, who happens to be my hubby's cousin, had a ton of Johnson grass in his fields. He baled it up. The only thing was my horses hated it! They wouldn't eat it, theyd eat around it, but my neighbors horses ate it up just fine, we ended up giving them quite a few bales that had the JG in it. He finally got it under control last year. 

How did he get rid of the JG?... it’s taking over my pasture and my horses won’t eat it.

You might check with your county extension agent.  There is a specific time when the Johnson Grass turns from just a nuisance to noxious- but I don’t trust my memory, so check with someone who deals with it yearly-

I'll have to ask, I cant remember off the top of my head, I know my husband went over there and helped him. I'll ask him and get back to you. 

Article about Johnsongrass toxin and horses:

https://thehorse.com/150281/toxin-topic-johnsongrass-poisoning-in-horses/

It's toxic after the grass has been under stress, such as after a frost or drought.

When it's first coming up it's fine for them to eat. Most horses don't like it when it gets really tall because it becomes tough. Once the grass starts to dry out it will get a white film on the blades and that is what is toxic to horses. Most of the time this occurs when it's really dry and hot. The more you cut it down the less it will spread and it will eventually die off. You have ot let it get about 12-18in tall each time and then mow it. We also would spot spray with round up to kill it off.