Milk is the most important product that dairy cows have to offer, and it has become sort of a necessity for dairy farmers to being breeding heifers as soon as they are able to. A cow cannot produce milk until after calving or parturition (or giving birth). So at around two years of age, the heifers are bred to give birth and officially become dairy cows that can produce milk.

Dairy cows will reach its milk production peak at around 40 to 60 days after giving birth. After that, production will steadily decrease. Eventually the milking will cease when the cow is dried off after about 305 days. The dairy cow can calve again a year after her first birth. For high production cows, though, it is viewed that 13 or 14 month cycles is appropriate because high production cows are more difficult to breed.

While dairy cows will continue to be productive and have many more lactations, there is still a great probability that problems will arise and the cow will be culled. More than half of all cows are actually being culled, mainly because of the following reasons:

Infertility: Dairy cows are at the peak of fertility between 60-80 days after giving birth. After this period, it will increasingly become more difficult to breed if it is still not with calf. Infertility can happen as a result. If the previous pregnance was difficult, or if the cow came down with milk fever, or failed to cleanse, infertility is a big possibility. Other occurrences can be metritis, or infection of the uterus, both of which can be cured.

Mastitis: Mastitis is a mammary gland infection that can result in high somatic cell counts and loss of production. This is characterized by a reddening and swelling of the infected part of the udder. There is also the presence of whitish clots or pus in the milk. The cow can have a fever and may lead to its death. There are treatments available to stop the spread of the infection but it all depends on the cow's immune system.

Lameness: The cows may suffer from persistent foot infection or leg problems which could cause infertility and loss of production. Lameness and foot infections can be caused by the acidic conditions in the cow's rumen (caused by highly digestible carbohydrate). This can actually be avoided with proper feeding techniques. Laminitis and subsequent lameness can increase horn production and lead to abnormal foot growth. If this is not cut back regularly the cow may become open to foot infections and ulcers. Cows suffering from lameness are in pain and will not stand or walk, which will decrease food intake.

Production: There are just some cows that can't seem to produce the average levels of milk to justify their feed costs. If the cow only produces around 12 to 15 liters of milk a day, then that's bad news. Usually, the cow begins lactating at 20 liters, peak at more than 40 liters, and be dried off before she can be calved.

Because of the big number of cows being culled due to the above reasons, dairy farmers are looking into embryo transfer. Embryo transfer can help increase the number of calves instead of the normal single.