Like humans cows have a gestation period of around 9 months. This means if you want your cows to calve around September/October time you should be getting them in calf at around November/December. 23 November mating works out to be late August/early September.
RUNNING WITH A BULL
You can get your cows in calf naturally by running them with a bull, this is the lowest input option but relies on you having access to a bull. Remember that bulls can pass disease on to your own cows if you are borrowing one from elsewhere. You should research the health status of where he is coming from as well as his vaccination history. Be aware, bulls can also cause havoc on farm fences etc.
The other option is performing artificial insemination. This is a process where semen is taken from a bull and stored within a specialised straw. When the cow is in season the straw is inserted into her vagina and through her cervix where the sperm is then deposited. The benefits of artificial breeding are that there is less of a disease transmission risk (all sperm is screened for disease) and it also means you can chose your bull depending on what characteristics you would like your calves to have (semen can be used from bulls all over the world). The fall backs of artificial breeding are that you need a trained technician to perform the insemination and you need to watch your cow closely for signs of heat.
HOW TO KNOW WHEN YOUR COW IS IN HEAT
Signs of heat are shown by a cow before she ovulates, they are important to watch our for so that the artificial breeding timing is optimal, this will increase the chance of pregnancy. You will need to watch your cows closely for signs of heat. Signs of heat include mounting other cows and allowing herself to be mounted, bellowing, restlessness, standing away from the crowd and showing a 'string' of clear mucus from her vulva. Cows need to be watched throughout the day to detect this. Alternatively you can try using tail paint, apply liberally to her tail head and this will become smudged when she is mounted by others- indicating that she is in heat. If your cow is in heat in the morning, the technician should artificially breed (serve) her that afternoon. If she is in heat during an afternoon or evening the technician should serve her immediately the following morning.
If you have several cows that you want to calve in close succession it is a good idea to synchronise their ovulation times. This procedure involves administering a schedule of hormones and can only be carried out by your veterinary surgeon. Synchronising cows is also a good option if you do not have the time to watch your cows for signs of heat, with some hormone protocols you can artificially breed them without needing to watch for these signs. This also works in small numbers and gives a fixed time for AI. Please talk to a vet about this first.
HOW DO I KNOW MY COW IS PREGNANT
Firstly you should look out for signs of heat after your cows have been served. A cow which comes back into heat after an artificial breeding is likely not pregnant. Your veterinarian can use an ultrasound scanner to diagnose a pregnancy and confirm an estimated due date.