Strange - I never found horses to be much of a problem - if we saw them, we just used go slightly higher (to about 200 ft) and they would soon settle down, especially if you were out of their way fast and didn't produce any blade slap. Cows, on the other hand, seem to prefer you to go lower, because they can't raise their necks very high.
For information, Bristol University 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.
While on the subject - how's this for overgrowth?
Phil