Black Angus Beef - Circle K Ranch

When reading this, think of a Beef Cow's life cycle as being similar to a human's in many regards.  
Once born, the cow is actually called a calf.  The calf will instinctively start feeding on its mothers milk nearly immediately after being born.  They will be very wobbly of course, and are fun to watch sort of like someone who has way too much to drink.  The mother's initial milk is extremely important during the calves first 3 days of  life as it consumes colostrum.  If a calf won't get milk on it's own, you need to find a way to get it to them to ensure their survival.  This first milk contains antibodies, which boost the calf's immune system as well as vitamins, minerals, protein and energy. The antibodies are absorbed by the calf's intestinal tract. After that three day time period, some production farms will start feeding powdered milk along with grain, hay, and water until weaning.  At the Circle K Ranch, and many other Natural Ranches, we let nature take its course, and let the calf feed with its mother until the weaning period at around 6-10 months old. 
At the 6 to 10 month period, the calf magically turns into a Heifer (female cow that has not calved).  By now, the heifer should be adjusted to grazing on green fresh grass and is in the grass filled pasture, or in production facilities eating corn, grain, and hay in its pen.
The puberty stage takes place at the 11-16th month periods and normally at 15 months they are at a size at which they are ready to be mated.  Some heifers might have been born later in the year, or might have smaller size so they will not be able to be mated during their first go round.  It is important that they get a good, healthy start in life, to allow for a maximum number of calves in their life for the benefit of the rancher's production. For more technical information, there are some good estimates as to the age and weight thresholds for Angus Beef Cattle in particular.  The age is around 396 days, and weight should be 60% of her mature weight for Angus Cattle for instance.
When mated, the heifer changes her name to Cow, once again magically.  Now for the breeding information. Cows have the same gestation period (pregnancy time) as humans which is of course, 9 months.  With this in mind, if you follow the 15 month allowable mating time line, the cow will give birth at around 24-27 months (depending on how busy they get) and have a weight of 500 Kg.
To prepare for our Cow to have a calf, she will start producing milk, or lactating.  Once the calf is born, the cow will feed her baby for about 3 months, get a couple of months of much deserved rest, and then continue the cycle.   The average cow will stay productive in a breeding herd for 7 to 9 years if no disease or physical problems develop.  A typical Black Angus cow will reach between 1200-1600lbs in her life and live around 8-12 years depending on health and stress.
There are more details I could give you, but I'm trying to keep it interesting.  For some very technical information, here is an excellent site that is especially good for the ranchers out there.  It is applicable to calving and nutrition during the life cycle.  If you're not interested, it could also provide a good nap.