However, the important thing you must remember here is that the cows you will get as cattle. Therefore, you must take care of them for a few days until they turn into adults. It is just like a real-life, but a little bit faster, that is why its called a simulation.
Raising Heart Ratings and Caring Cows

You must interact with all of the cows, every day in-game, this will help you to gain extra heart points if you make them happy. Take your cows outside to feed them grass, however, on rainy days, keep them locked in the barn. However, in-game, you will have to grow grass by yourself, therefore buy and spread a few grass starter kits from Pierre at 100G.
Make sure that there is enough grass for all of your cows; each day, a cow can eat a tile of grass. Also, keep sure that the grass is not too far from the barn, otherwise while hungry cows can get hungry, and the heart rate may drop.
Take your cows outside daily if it’s not raining, they prefer to feed outside. The cows will return to the barn in the evening. Make sure that you close the barn door after the cows are in. After that, you can go and sleep, and otherwise, if you forgot to lock the door before you sleep, there will be a penalty. The game has seasons, and winter is one of them Grass began to die in winter. Therefore, before winter, collect grass by cutting with a scythe to feed the cows in winter.
However, the cows will be adults before winter if you have taken care of them properly. On rainy days, don’t open the barn doors, make sure that use a heater in the barn. Use the hooper to feed them.
Once all your cows are old enough, you must use the milk pail to gain more heart points. If you are milking them, you must know that it may sometimes fail, probably because they are outside of your range. Lastly, after they are adults, you will have to give affections and control their reproduction.
Getting Milk and Large Milk From Cows in Stardew Valley

To get milk from a cow, firstly, you must buy a Milk Pail from Marnie. After that, you can milk your cows every day if you have keep their heart level at check by taking proper care.
This is how you get milk in Stardew Valley its really simple, now to get large milk, all you have to do is to milk a cow which is very happy. The happier the cow, the more chances you have getting a Large Milk.
If you are hoping to earn more money by the normal milk, then turn the milk into cheese with multiple presses. Moreover, you can provide the milk to the village for cooking or just proving as a gift.
If you turn large milk into cheese, it is even more profitable. The cost of the normal milk is 100G, and when it is turned into cheese, it is 160G. On the other hand, Large milk costs 150G and when turned into cheese costs 240G.