I remember a thread a while back about dogs unrestrained in the back of utes. Not sure about other breeds but working dogs are extremely agile and I never heard of any "falling off" any item of farm machinery, ever. We lived for 15years in beef/dairy country in South Gippsland and the dogs only ever wanted to be where the action was.
p.s. These guys do not wear doggy jackets, have personal trainers or eat pro-biotic yogurt.