Cattle need teeth. But don't ever look in the mouth of a ruminant and worry that it has lost all its top incisors: it never had them. They have lower incisors, which close against the hard top dental pad, to tear the grass they then chew with top and bottom molars and pre-molars, often referred to as the cheek teeth.
Like humans, young cattle have deciduous teeth (sometimes called milk teeth), which fall out at varying stages of their development, to make way for the permanent teeth which will (hopefully) last them a lifetime. The true molars, which are the back three teeth top and bottom, on each side in mature animals, are not present at birth but grow as the animal ages, the first two within the first year or so of life and the third a year or two later. Mature cattle have 32 teeth.
A calf's teeth at birth or soon after comprise eight incisors along its bottom gum line and three pre-molar teeth, top and bottom, on each side, twenty teeth in all. All of those are deciduous and when they're brand new, are very sharp. It is extremely unwise to allow your finger to get in the way of the premolars as the animal closes its mouth and be aware that the incisors can easily cut your hands if you have to help a newborn calf feed.
A calf's incisors look just as large in its mouth as an adult's permanent incisors do but by the time the calf is a yearling, those milk teeth seem much smaller and when the permanent incisors begin to come through, those are obviously much larger teeth.
It is possible to estimate the age of an animal by observing which of the deciduous teeth have been replaced by the permanent teeth. Once an animal has a full quota of permanent teeth, it is difficult to use its dentition to as accurately estimate age.
The easiest teeth to see are the incisors, the eight teeth along the bottom gum line and those are the most useful for indicating an animal's age. Sometimes you can easily see these teeth as an animal quietly masticates but often that movement is too fast to really get a good look at what's going on in. A fast multi-shot option on a digital camera can allow you to get good pictures of the teeth at various stages of mouth movement, so you can see how many incisors they are, whether they're deciduous or permanent and what state they are in.
By the time an animal reaches 18 months or so, its first permanent incisors will be erupting, in the centre of its bottom jaw. This probably causes some change to the efficiency of grazing by that animal and if she's a pregnant R2 heifer, she needs particular care in feed provision, to ensure she's not under extra stress because of the uneven line of her teeth.
Often the new incisors come up crooked, sometimes while the milk teeth are still in place and it can take a while for them to straighten out to provide a solid line to enable efficient grazing.
The next pair out from the centre emerge at around two to two-and-a-half years; the third pair out at around three years and the last pair at three-and-a-half to four years of age and should be nicely evened out by five years. A mature cow's full set of teeth will often appear to be a straight line of enamel along her lower jaw. Obvious wear on the incisors will be seen over time, from about seven years and on.
It's generally a good idea from then on to have a look at your animals' teeth at least once a year, to check that they're all still present. You don't want to leave checking until you see an animal losing condition, to find she's lost some or all of her teeth. Adult cattle, particularly if pregnant, need to be able to eat efficiently, especially during the winter, when things are a bit hard. A cow who has lost some teeth and is on marginally sufficient feed through the winter, can lose a lot of condition quite quickly and then hit real problems come calving.
There's really only one serious conformation fault an animal can have in regard to its dentition, where the lower jaw does not line up with the upper jaw, so that the teeth don't properly meet the upper dental pad and the animal cannot harvest feed efficiently. Usually if you can easily see the "chin" of an animal from the front, it has an effective lower jaw. If you can't see the lower lip when the cow is chewing, have a careful look from the side. If you still have doubts, it is wise to check the animal when restrained in a head bail Make sure the teeth meet the dental pad right at the front. Any animal whose teeth and upper jaw don't meet properly should not be bred and may need to be looked after carefully when grass is short.