The Cows Collection is a collection. You can acquire the items when a neighbor posts a Cow Collection item to their News feed. You can also acquire the items when harvesting from Cows, either your own farm and Dairy Farm, or that of a neighbor.

Cow Bell Milking Bucket Milking Stool Milk Bottle More Cowbell
Cow Bell-icon Milking Bucket-icon Milking Stool-icon Milk Bottle-icon More Cowbell-icon
Common Common Common Uncommon Rare

Collection Level Coins XP Rewards
Level 1 Coin-icon 500 coins XP-icon 50 experience Fuel-icon
Level 2 Coin-icon 1,000 coins XP-icon 100 experience Cow Topiary in Bloom-icon
Cow Topiary in Bloom
Level 3 Coin-icon 1,500 coins XP-icon 150 experience Rainbow Cow-icon
Rainbow Cow
Level 4 Coin-icon 2,000 coins XP-icon 200 experience Fuel-icon
Level 5 Coin-icon 2,500 coins XP-icon 250 experience Cow Topiary in Bloom-icon