The dairy BCS is a five-point scale, with cows at 5 being fat and cows at 1 being skinny. You want your cows to calve when their body condition is about a 3.5 to 4. After she calves, she will slowly lose condition because of the intense demands that lactation places on a her body. It is really hard to put weight on a cow when she is milking. Farmers often find that the more they feed a cow, the more milk she gives, rather than improve her body condition. By making sure that she is in good condition before she calves, you will be able to have her at 2.75 or 3 when it is time to breed her back around three months after calving. This is the optimal body condition for her reproductive system to be reliably cycling, producing viable eggs and sustaining a pregnancy.
The Dry Period
If you don't provide enough high-quality food for your dairy breed of cow, she will probably end up too skinny. If your cow is nearing the end of her lactation and she's looking like an old bag of bones, it might be a good idea to dry her up a few weeks early. The dry period is prime time to improve her body condition by giving her some high quality groceries. However, if your cow looks fairly good at the end of her lactation, you probably don't need to offer her any supplemental grain or alfalfa until a couple weeks before her calving date.
Fat Dairy Cows
Most dairy breeds of cow will not be fat during the early part of lactation. So much of their energy has to go to milk production that they will be slimming down when they are producing the most milk. However, as they reduce the volume of milk they are making toward the end of lactation, a wise farmer will also begin to gradually scale back the grain that he's feeding the cow if she has good body condition. If you continue feeding her at the same rate, she will end up too fat.
Fat is just as hard on cows as it is on humans. Fat cows may have trouble with calving, experiencing a higher rate of malpresentations. They also are at a higher risk of certain metabolic issues post-calving. Additionally, it just doesn't make financial sense to overfeed your cow. Feeding her an excessive amount of grain just wastes money.
When to Judge
You should make a note of your cows' body conditions several times throughout the year. By checking at particular times, you can adjust the cow's diet to reflect her needs at the time. Additionally, keeping an eye on your cow can help you spot health problems. A cow can have an illness that melts her fat away, but she may not experience a drastic difference in her milk supply.
Your cow should be checked a few weeks before calving. This allows you some time to put some weight on her if she's too thin or cut back on rich feed if she's too fat. She should be at a 3.5 or 4 at calving.
Next, you should check your cow a few weeks after calving. This is when the demands of lactation are going to be the hardest on your cow. If she's looking pretty thin, you should feed her more grain or other high energy feed. Don't forget to keep plenty of good-quality hay in front of her too. Feeding her properly during lactation means that she will hit her peak milk production and have enough nourishment to breed back soon after calving.
Last, check out your cow's body condition around the time that you are ready to dry her off. Even if she's giving a decent amount of milk, you should give her a rest between calves. You will be able to make some final adjustments to her diet at this point to help her have a good condition for calving.
For more on the specifics of judging your dairy cow's body condition, you can check out the University of Nebraska's guide for judging dairy cows' body condition.