Dairy Farm

The Dairy Farms are buildings available on FarmVille. This building can be purchased at the market for either coins or FarmVille Cash. However, you need to have at least 8 neighbors to be able to buy one with coins. A standard Dairy Farm will hold up to 20 cows (or 19 cows and 1 bull) and lets a farmer collect from them all at once. A fully upgraded Dairy Farm (a "Huge Dairy Farm") will hold up to 40 cows (or 39 cows and 1 bull). A farmer can own up to five Dairy Farms paid by coins, but more can be purchased for 20 farmville cash each. To place a cow in a Dairy Farm, a farmer must select the move option on the cow and then click on the Dairy Farm.

Any single Dairy Farm building can contain a maximum of one bull, regardless of the size. If a MDairy Farm contains a bull, then every time milk is collected there is a chance that there will also be a Baby Calf produced, up to a maximum of one per day per farm. There is also a chance of receiving one bag of Fertilizer per day to post to your feed that doubles the amount of fertilizer available to a neighbour visiting your farm. There is also a very small chance of receiving a Cow Bell which can be shared with your neighbours. The odds of receiving any of these items is low from each cow so having a lot of cows increases the odds of getting something. When the building is harvested, the program iterates through the list of cows giving each one a chance (probably somewhere in the order of 1 in 100), until one produces the particular item. There is no advantage to clustering all the cows of a particular type into any one dairy though you may want to avoid having them in the last building as it will have the lowest odds if you have 5 fully expanded dairies and they are all full. The odds of a particular animal producing a calf seems to be reduced if the cow has been in the Dairy for less than 30 minutes.

Although originally unable to do so, the Dairy Farm can now hold Referee Cows.

Multiple Harvests
When a cow is put inside a dairy, the ready status of the cow will be applied to it until you leave the farm. For example if a Dairy Farm is 37% ready and a cow that is 50% ready is put inside this dairy, the dairy will also be 50% ready, and once you reload the game or visit a neighbor, the dairy will be 37% ready again (and contain the cow put inside)

This can be made use of to harvest the entire dairy farm with all the cows inside multiple times by putting a 100% ready cow inside a dairy.

While this would entail a considerable amount of work to make a usable amount of coin from most types of cows, with the Belted Cow this can yield millions a day with only a small number of Belted Cows.

If you have a dairy with 10 Belted Cows and 29 regular Cows for example, and various harvest-ready cows outside the dairy, each harvest-ready cow you put inside to harvest the dairy and remove again will yield over 30.000 coins (10*3000 coins + 29*8 coins + once the harvest value of the cow put inside)

Trivia
Before an update on Decenber 10, 2010, this trick only used to work if the dairy farm was entirely empty, which means a harvest-ready cow would have to be put inside an empty dairy, then each other cow that you would want to harvest as well (their harvest status wouln't matter at this point since the dairy status would be unaffected by further cows). The way it used to be was far harder to make profit since a dairy needed to get entirely emptied again for each further harvest.

With this update, Zynga made it easier for farmers to use this method for getting money, which also proves that this is officially cosidered a legit way to get big amounts of coins.

Expanding
Dairy Farms can now be extended after the June 21st update. Dairy Farms are expandable, first to 30, then to 40 cows. The procedure is similar to a barn raising. Extension of dairy farms causes 1 additional milk jug to appear on the side of the building per extension. If you have 5 Dairy Farms, the total cows that you can put in the Dairy Farm is 200 cows.