There are several ways to encourage your dog to leave your chickens alone. Some dogs will be discouraged by your displeasure, just tell them "NO" and swat them when they are looking interested in your chickens (note this only works for a certain type of dog).
The other ways are to fence your dog (not really a great method on a farm) or tie a chicken carcass on your dog and let it stay there until it rots. The theory behind this is your dog will remember the end result of its kill and decide not to play with chickens anymore.
Another option is a training collar, working on the same method as the first solution you basically wait for your dog to show interest or chase a chicken, then zap them. Some people are uncomfortable using electronic shocks for training, but it is very effective.