Heifers don't officially become cows until they give birth to a calf. For dairy heifers, that's about the age of 2 years. These young mothers aren't fully grown themselves at the time of their first calving. Generally, dairy heifer pregnancies last around nine months, but various breeds have different specific pregnancy lengths.
Heifers generally experience their first estrus between the age of 11 and 13 months. Puberty hits when heifers reaches a certain weight. When they reach about 60 percent of their mature weight, usually between the ages of 14 and 16 months, they are bred. The majority of dairy operations use artificial insemination for breeding cows; approximately 30 percent of heifers are bred naturally -- turned out with a bull -- according to a University of Wisconsin website. Since they have not yet given birth, heifers aren't in a dairy herd.
Breed and Pregnancy
The black-and-white Holstein is by far the most common dairy breed in the United States. According to a West Virginia University website, the average Holstein pregnancy lasts 279 days, and that of the Jersey is one day less. The Ayrshire's average pregnancy also lasts 278 days; the Guernsey's gestation averages 284 days; the Brown Swiss', 290 days. Much depends on the individual cow. A dairy operation keeping good records knows that certain cows give birth a certain number of days after breeding, every time. Since this is the heifer's first pregnancy, no pattern has been established.
Approximately 3 percent of pregnant heifers will abort that first calf. The primary causes of abortion in dairy heifers are bovine viral diarrhea, infectious bovine rhinotracheitis and leptospirosis. These diseases cause abortion at about the halfway point of the pregnancy. Vaccinating her against these diseases as a calf and before breeding are effective means of eliminating their risk. Dairy heifers bred naturally, rather than through artificial insemination, might contract the venereal disease known as vibrosis. Vaccinating the heifer for vibrosis before breeding cuts down considerably on the possibility of abortion, which occurs in the first trimester.
A heifer's udder starts enlarging a couple of months before her delivery, whereas a cow's established udder may become noticeably fuller only shortly before delivery. When the heifer's first labor starts, she separates herself from the herd, becomes restless and lacks appetite. After her water breaks and hard labor begins, the calf arrives. The first calf is probably the most difficult one she'll ever deliver. Because of inexperience and relative pelvic size, the mortality rate for heifers and their calves is higher than that of mature cows. At the time of the dairy heifer's first delivery -- the moment she becomes a cow -- she has reached approximately 82 percent of her mature size.