LONDON (Reuters) – The deaths of no fewer than four people after being trampled by cows in the past two months has prompted Britain's main farming union to issue a warning about the dangers of provoking the normally docile animals. Cows can become aggressive and charge, especially when calves are present and walkers are accompanied by dogs, said the National Farmers Union (NFU). The union and the Ramblers' Association both advise that walkers release dogs from their leads when passing through a field of cows. "The cattle are interested in the dog, not the walker," said Robert Sheasby, Rural Surveyor...