Cow and sheep cannot trip balls because they are "Ruminating animals". Meaning they have 4 seperate parts to their stomachs and in each stomach they have a different set of aggressive enzymes that specially evolved for breaking down all sorts of alkaloids/poisons that would either make us sick, kill us or make us trip balls.

The reports of people getting head-butted/attacked are probably due to the fact that about this time of year the cows are known as "yearlings" as in a year old. Much the same as feisty teenagers, these guys are very curious and boisterous and full of hormones and I am not surprised they don't take kindly to random walkers in their fields. Especially considering at that age farmers don't separate the males from the females so the males are going to be starting to be very protective of their girls.
Its only at about 15-18 months where the cows are able to reproduce and the farmers will remove the bulls because they start getting very dangerous if you let your guard down, or stand in between the bull and the other females. (I'm a farmers son)

Sheep are the same, they have a 4 part stomach that is excellent at breaking down poisons and other chemicals.

On the subject of reindeer's they do eat amanita's but again their ruminant digestive tract simply disposes of the muscimol through the urine. that is why the natives will feed them amanita's and drink the urine. This has the added beneficial effect of the reindeer's having already broken down the other toxins contained in the amanitas.


That is to say, there are certain plants that pose a threat to sheep and cow in the UK anyway, interestingly ergot can be fatal.

read here for a comprehensive list: http://lavenderfleece.com/poison.html And http://www.sheepandgoat.com/poison.html