Welcome to GoY.

I've found this on the internet re cattle.

'It is believed to be so, yes, according to a commonly-held belief. However, more studies need to be done to confirm if pyracantha or Firethorn is indeed poisonous to livestock or if it's merely a myth that has been passed down through the ages.'

Community.horse.com
says it is toxic to horses. Berries causing collic.

MAFF's 'Poisonous Plants in Britain and their effects on Animals and Man' just mentions it as an 'also thought to be poisonous' at the end of the listing for cherry laurel. The Horticultural Trades Association doesn't list it as potentially harmful.

I haven't tasted the berries so can't say if they would be good to eat though I doubt it but, I think, it is the spines that would, probably, make it unattractive.

The International Poisonous Plants Checklist doesn't have any references to it specifically though it seems to be mentioned in a couple of general papers from the 1970s.

I would check with your vet to see what would be safe as a hedge bordering a pasture. The local cooperative extension would probably also have some good ideas.