As mentioned, they're more of a problem when the rotors are not turning, but based on my own experience with power lines, if it's any help:
With horses, go up, without banking or causing blade slap. Once you are at 200 ft, they shouldn’t move. Cows generally don’t bother, but when they do, prefer you to be as low as possible, because they can’t get their head high enough to see what’s going on. If you have to go around animals, make sure they run towards open ground.
The University of Bristol carried out trials in 1963 and 1964 on their own farm, using a Bell 47 against horses, cows-in-calf, heifers, in-lamb ewes, and chickens and cows inside buildings. Experienced stockmen were used, and observations were also made on the production of milk or eggs. Passes were made by the helicopter at 60 and 35 feet, at speeds up to 25 mph. It was concluded that completely housed farm stock is not affected at all (although you would still be advised to avoid such buildings wherever possible). When out of doors, reactions are very temporary, after a fleeting period of bewilderment when the animals could injure themselves by hitting fences or falling into ditches. Poultry (and ostriches!) out of doors, however, present the most problems, and will react even to the helicopter’s shadow.
But, risk management wise it's best to avoid them anyway.