Bull with balls
They might not be the strongest or the fastest, but there's just something about these bulls that makes them the most bull of them all! Get one in your farm, and you will have a steady production of DNA (according to their skin rarity). Use it to breed new bulls, or sell it to less fortunate brothers looking for a mate!
Balls are the cornerstone upon which the breeding system is built. They provide DNA according to skin rarity, meaning they will transmit their color to 45% of their children. The standard production time for 1 unit of DNA is 10 days, but it can be reduced quite a bit!
In order to increase your yield, look for bulls with miner synergies. There are also some testosterone items which serve the same purpose, but those are very rare and only found in chests during special events. Be sure to have a couple high level miners for the best chance to find one!