Jersey Cows will only become available once you ship your first piece of S-rank wool. Once a cow wins the Cow Contest, it will start to produce S-rank milk.
Mirabelle sells Jersey Cow for 15,000G each. Although expensive, they will produce better quality milk. The milk from a Jersey Cow is worth more than double what regular milk is. Brushing cows with the Brush will increase their friendship and their product quality.
If you use a Miracle Potion on a cow, you will need a Birthing Pen and the cow must be an adult. After 21 days the baby cow will be born and it will take another 18 days to be a full grown. Cows have a life span about 6 years.