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The bigger the cow is, the more you have to feed her. Although, a bigger cow may produce more milk, you don’t want a monthly feed bill that exceeds what you’re able to spend.  Holsteins are the biggest eaters and will cost the most. Brown Swiss cows are somewhere in the middle. Jerseys are the smallest and need the least amount to eat. What kind of cow do you think you can handle? Can you deal with nervous cows or would you need to have very docile ones? Take this into consideration when selecting a breed.  Brown Swiss cows are the most docile dairy cows. Holsteins tend to be a bit nervous. Jerseys can be notorious kickers. If you are raising a cow to be part of a dairy farm, obviously milk production will be a crucial factor. On the other hand, if you only need enough milk for your family, you may not need such a high-production breed. You may wish to mix your herd if it's for commercial reasons, such as adding a few Jerseys to a predominantly Holstein herd to increase the Holstein's butterfat content.   If you're looking to maximize milk production, Holsteins are the way to go. If you're only wanting milk for you and your family, you may want to consider a cow that doesn't give as much milk (such as a Jersey). Breeds that are dual-purpose (produce both milk and meat) may be ideal, but this will produce less milk overall. You may also want a dairy cow for butter and cheese making. Jerseys and Brown Swiss produce milk higher in butterfat and protein content, making them ideal for cheese and butter making. Dairy cows that have been culled from conventional dairy operations are cheaper. However, such cows may have injuries, illnesses, or inability to breed back.  If you are a dairy farmer just starting out, avoid these cull animals. As a hobby farmer you may not mind the time, money and feed invested in rescuing and caring for one or two cull dairy cows.
Consider the cost of feed. Gauge the cow’s temperament. Decide what you’ll use this cow for. Look at cull cows.