Final answer:
The farmer should use stratified sampling to calculate average milk yields for different types of cows, providing a proportional and representative sample from each group in the herd.
Step-by-step explanation:
The best sampling technique for a farmer to determine the average milk yield of each cow type in their herd, which consists of Ayrshire, Friesian, Galloway, and Jersey cows, would be stratified sampling. This approach involves dividing the herd into strata, or groups, based on the type of cow, and then taking a proportional sample from each group. This method ensures that every cow type is represented in the sample and that the average milk yield is accurately calculated for each type.