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حمیده شیخ زین الدین کارشناس ارشد رشته ژنتیک و اصلاح دام از دانشگاه تهران هستم.
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مدیر وبلاگ : حمیده شیخ زین الدین
A young dairy animal is known as a calf. A female calf which has not given birth to a calf and is less than thirty months old is called a heifer. When the heifer is seven months pregnant or has reached the stage in pregnancy where the udder starts to swell, it is known as a springer. After more than thirty months old, a female dairy animal is known as a cow. The process of birthing a calf is known as calving or parturition. A male dairy animal is called a bull at any stage of life, unless castrated, in which case it is known as a steer until it is four years old, then it is called an ox. "Ox" is also the term for any bovine trained for draft work; this is usually a steer. A dairy animal's mother is known as its dam. Similarly, a dairy animal's father is known as its sire.
 Modern times
Historically, there was less distinction between dairy cattle and beef cattle than is the case now, with animals of the same species often being used for both meat and milk production. Dairy cattle are now specialized animals, and most of them belong to breeds which have been bred specifically to give large volumes of milk. This milk is made into various products, including cheese, yogurt, butter, ghee, cottage cheese, whey, and ice cream, and is consumed around the world.
 Dairy farms
Dairy cattle may be found in herds on farms where dairy farmers own, manage, care for, and collect milk from them. These herds range in size from small farms of fewer than five cows to large conglomerates of 25,000 cows or more. The average dairy farmer in the United States manages about one hundred cows but this varies from an average of 800 cows in California to under 80 in the North East states. Herd sizes vary around the world depending on landholding culture and social structure. In many European countries the average herd size is well below 50. In the UK it is over 100 in New Zealand 185 and Australia 280. Dairy farming is closely linked to areas settled by North Europeans and these areas (Europe, N. America, Australasia) dominate commercial dairy production and trade in dairy products. However, the demand for dairy produce and benefits of dairying are increasingly being realised by developing economies.
 Life of dairy cattle
Pure bred heifer calves are usually reared as herd replacements, and as such are of great value to the dairy farmer. Sales of crossbred heifers and bull calves are subject to the demand for beef animals within transport range of the farm. Surplus calves are generally sold at two weeks of age and bulls may fetch a premium over heifers due to their size and potential. Calves may be sold for veal, or for one of several types of beef production, depending on available local crops and markets. Such bull calves may be castrated if turnout onto pastures is envisaged, in order to render the animals less aggressive. Purebred bulls from elite cows may be put into progeny testing schemes to find out whether they might become superior sires for breeding. Such animals may become extremely valuable (20,000 euros or more for Holsteins).
The aim of most farms is to separate the calf from its dam within 24 hours of birth. Contrary to popular perception, this early separation eases the stress on cow and calf as "bonding" is prevented. A restless cow bellowing for her lost calf can be avoided by earlier separation; the calf, in turn, may be more easily taught to drink milk from a bucket, as its "sucking reflex" strengthens with time spent suckling the cow. The dam's first milk, called colostrum, is rich in antibodies and is required for newborn calves to protect them from infection. A calf must drink two quarts (2 L) of colostrum within twelve hours of birth, as it has no immune system of its own for the first two weeks of life. The antibodies are directly assimilated into the bloodstream by the calf's digestive system, a phenomenon which shuts down permanently from 12 to 24 hours after birth. The colostrum changes into milk suitable for commercial use within three or four days after calving. Most young stock then subsist on commercial milk replacer, a feed based on dried milk powder and reconstituted using hot water, until old enough to start consuming solid foods at 3 to 4 weeks old. Milk replacer is cheaper than milk from the cow. A day old calf can only drink around 2 litres of milk per day, whereas the average Holstein cow will produce 30 litres or more. Even at weaning, at 8 weeks old, a calf will only be consuming around 6 litres per day.
 The bull
A select few high genetic potential pedigree bulls sired by elite bulls out of elite cows, will be reared for breeding purposes. These bulls will generally have excellent production indices, based on the results of their progeny testing. They will also demonstrate superior type conformation (for the breed), as measured by their daughters udder quality, feet and leg quality etc., compared with the general cow population.
Herd bulls, are bulls kept on the farm to provide natural breeding for the herd. A bull may service up to 50 or 60 cows during a continuous breeding period. Any more and the semen will become too diluted, leading to cows "returning to service". A herd bull may only stay for one season since over two years old their temperament becomes too unpredictable.
More recently, since the 1950s, artificial insemination,or AI, has become practically ubiquitous. Through AI, fewer than a thousand elite bulls can serve as sires for an entire world generation of cows. Although conception is dependent upon effective herd management and heat detection which increases the time the dairy farmer must spend with the cows, a few factors have prompted farmers to use AI nearly exclusively. The foremost is the high quality of cows produced through AI. AI also limits the need for farmers to maintain their own bulls, which contributes to safety, as bulls can be dangerous animals to keep on the farm. Some dairy farms however, still use herd bulls, as it is difficult to cover the entire year's breeding program using AI, and there is rarely a need to breed the entire herd with quality purebred bulls.
 The cow
Dairy heifers are of great value to their breeders, as they will become the next generation of dairy cows. As a cow cannot produce milk until after calving (giving birth), most farmers will begin breeding heifers as soon as they are fit, at about fourteen months of age for Holsteins. A cow's gestation period is about nine months (279 days long), so most heifers give birth and become cows at about two years of age.
A cow will produce large amounts of milk over its lifetime. Certain breeds produce more milk than others; however, different breeds produce within a range of around 4,000 to over 10,000 kg of milk per annum. The average for dairy cows in the US in 2005 was 8,800 kg (19,576 pounds).
Production levels peak at around 40 to 60 days after calving. The cow is then bred. Production declines steadily afterwards, until, at about 305 days after calving, the cow is 'dried off', and milking ceases. About sixty days later, one year after the birth of her previous calf, a cow will calve again. High production cows are more difficult to breed at a one year interval. Many farms take the view that 13 or even 14 month cycles are more appropriate for this type of cow.
Dairy cows will continue to be productive members of the herd for many lactations. 10 or more lactations are not uncommon. The chances of problems arising which may lead to a cow being culled are however, high; the average herd life of US Holsteins is today fewer than 3 lactations. This is unfortunate as it requires more expensive herd replacements to be reared or purchased. Over 90% of all cows are culled for 4 main reasons:
Cow Cost - Dairy cows cost anywhere from $10.00 USD to $5,000 USD.
Herd life is strongly correlated with production levels. Lower production cows live longer than high production cows, but are arguably less profitable all the same. Cull cows are sent for slaughter. Their meat is of relatively low value and is generally used for processed meat such as hamburger beef.
More recently, certain practices have been developed to enable the multiplication of progeny from elite cows. Such cows are given hormone treatments to produce multiple embryos. These are then 'flushed'. 7-12 embryos are consequently removed from these donor cows and transferred into other cows who serve as surrogate mothers. The result will be between 3 and 6 calves instead of the normal single, or rarely, twins. This process is called embryo transfer.
Many other breeds are used nearly exclusively for dairy, or for both dairy and beef purposes.
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