Do cows produce more milk than it is required for their calves? It seems like cows are able to provide milk all the time (all year around). Is it so? Or do they, like other mammals, produce milk only in ammounts requeired by their offspring?
Dairy cows are bred, or selected to give milk. So they do produce excessively. The normal bovine wild type, like other mammals, not only produce less milk, but also will tend to stop lactating when the calf is not nursing.
I would be interested to know if dairy cows stop giving milk if they are not milked every day. I understand they can get very uncomfortable if they are not milked, but i don't know if they stop giving milk or if they are injured in some way.
The average domesticated dairy cow produces far more milk than would be required to feed their calf. All cows, wild and domesticated, will only lactate in the period between their calf's birth and weaning. Milk is calf-food, and when there's no calf, there's no evolutionary advantage in producing milk.
On dairy farms, cows are milked twice daily, from spring (when they give birth) until late autumn. This mechanical milking 'fools' the cow into continuing to lactate. When a cow has stopped lactating, they will only start again after giving birth. This means that you can't just start milking a cow and expect to get milk.
Generally farmers milk their cows from spring (birth) until late autumn. The reason for stopping in autumn is simply because the grass grows much slower in winter, so there isn't enough food to support lactating cows. However, it is entirely possible to milk longer than a year; I know of farmers who milk their cows continuously for two years. These farmers will have to purchase a lot of supplimental food (like hay or silage) during winter. The advantage of milking for longer than one season is that the cows do not have to give birth every spring, but instead only every second spring.
I believe (but can't guarantee) that in winter, most milk purchased in a shop comes from the opposite hemisphere. I do know that here in New Zealand, we export a lot of milk to northern-hemisphere countries.
If you were to suddenly stop milking a cow, they might get sick but generally they will survive. It's still something to avoid! Although if the calf is left with their mother then the cow would be relieved.
I do not have an 'official' source for these facts. However, I grew up on a dairy farm, so this was my life.
Cows only produce milk after a calf is born and their lactation period lasts approximately 10 months. In many instances, farmers have given growth hormone (GH)¹ to cows in order for them to produce more milk. It must be noted that there are many side effects associated with this, not to mention the residual effects it may have on us humans as consumers. Primarily though, the cows would have decreased immune efficiency and this would leave them vulnerable to all types of parasitic infections or disease.