No to the first and somewhat yes to the second. Cows will begin producing milk a few weeks weeks to several days prior to giving birth to a calf, or immediately after the calf is born. The first milk being produced is called colostrum, and is a crucial substance for the health of a calf in aiding its passive immunity and giving the calf the much-needed antibodies and immunoglobins for the first few weeks to months of its life. Colostrum is produced for only 24 to 36 hours, then gradually replaced by "normal" milk, which is when the dairy cow must be separated from her calf, unfortunately, and be put on the milk production line with the rest of the herd.
The cow is bred around 90 days later, which is the time given to her to come back into normal heat (estrus) cycling, and for her stretched uterus to involute or go back to its normal, pre-pregnancy size. She is still lactating at this point. Gestation lasts for 285 days (or 9.5 months), and she is still lactating until she is dried up 60 days prior to her set calving date to allow her to rest and recharge, and focus her energy on growing a calf inside her during her last trimester. Lactation peaks at the time she is ready to breed, and declines gradually afterwards. Her lactation period lasts for 10 months.
Thus, a cow is lactating during the first five months of her pregnancy. For beef cows, this would be shorter, because they have their calves weaned at around 6 months old, which means only 3 months are spent being both pregnant and lactating.
But the clincher is that cows won't keep lactating if they are not milked regularly, nor have a calf suckling from them frequently. Thus a cow can't be producing milk all the time if nothing or no one is there to make sure they are milking "all the time." Lactation has evolved so that milk is continually replenished if there's a demand for it. Just like with economics, if there's a demand for something, then a supply must be had in order to satisfy this demand. Otherwise, if there is no demand, then supply will dry up and not be available. Milk will not be available if there is no demand for it (no calf to suckle, no one to milk the cow). That is, until a new demand comes about, like if the cow births a new generation. However, it takes two to three weeks for a cow to completely dry up or stop producing milk altogether. She cannot simply nor suddenly stop producing milk if there is suddenly no demand for her milk.
Thus, cows will continue to produce milk if they are regularly stimulated to do so. A cow can be regularly milked for 2 or more years without having to produce a calf, so long as she is milked regularly and not "forced" to dry up. The reason that most dairy operations can only milk their cows for 10 months--and not all at once, only a small group of cows are dried up and set to calve while the rest are still in production, or another group is just going into production--is because they have been bred a few months after they have calved, and need that two-month window of rest to focus on the fetus inside them and not lactation.
Cows neither produce milk all the time, nor do they only produce milk when they are pregnant. In general, cows begin to produce milk when they give birth and will continue to do so until they are "dried-off", which happens when their calf is weaned (beef cows) or when the farmer stops milking them (dairy cows). For most cows, milk production increases until about 90 days after they give birth, and then slowly declines during the rest of the lactation. If a dairy cow is re-bred successfully at 60-90 days following calving, she will usually be "dried-off" after a lactation of about 305 days so that her mammary gland has a chance to re-charge for a couple months before she gives birth to another calf. (A cow's gestation period is about 280 days.) Some cows that do not re-breed can be milked continuously for 2-3 years, but production declines sufficiently over time that this is not usually economically feasible. Consequently, cows with fertility problems get culled from the herd. More input from WikiAnswers Contributors: * Only when pregnant like other mammels....except they keep getting milked in the farms which means that their bodies will continue to produce it as long as it is being used. Milk doesn't know that it is going into a jug and into a store or the difference between that and if a calf is drinking it. Viola. * Some cows from strong milk lines (most commonly in JERSEY and HOLSTEIN breeds) young cows can and do produce milk without being pregnant. This is called Spontaneous Lactation, Virgin Lactation or Maiden Milk. This also happens with Dairy Goats such as Nubian and LaMancha breeds.
Oddly enough, cows. I think what you mean is "how do cows produce milk", which is another question entirely. Cows, and other female mammals, produce milk as a natural product of their physiology when they reach the age at which they can bear young. All female mammals have the capacity to produce milk.
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