The number, condition, and order of eruption of the permanent incisors of sheep are the main indicators of their age. There is, however, a wide variation in the time of eruption of the permanent incisors caused by variations in breed, strain and environment, and particularly by nutrition.
The teeth of a sheep are divided into two distinct sections, namely, eight permanent incisors in the lower front jaw and twenty-four molars, the latter being divided into six on each side of the upper and lower jaw. Sheep have no teeth in the front part of the upper jaw which consists of a dense, hard, fibrous pad.
When born, the lamb usually has no teeth. Within a week after birth, the milk teeth or temporary teeth appear in the front lower jaw and by the time the lamb is two months old these, eight in all, have erupted.
These temporary teeth are replaced by permanent incisors, which appear in pairs, commencing with the two central teeth, followed by one on either side at intervals, until the eight temporary teeth have been replaced. During the period the teeth are growing, sheep are referred to by the number of permanent incisors present, such as two-tooth, four-tooth, six tooth, eight-tooth or full mouth.
Only a rough estimate of a sheep’s age can be made by looking at its teeth. When estimating the age, it is important to bear in mind whether the breed is early or late maturing.
British breeds, for example, mature at a faster rate, and their teeth erupt at an earlier age.
The condition of the teeth will vary according to the type of feed and country grazed on. On long, soft feed the teeth will grow long from lack of wear, but remain in good condition. On short feed, where close grazing is necessary, particularly if the soil is sandy or gravelly, the teeth will wear down.
After the eight permanent incisors have appeared, the next stage is known as ‘broken mouth’. This is a progressive deterioration, the rate depending on the conditions under which the sheep was grown.
Estimation of age at this stage is very difficult. The teeth gradually become longer with wide spaces eventually falling out, or they may wear down, become loose and fall out. After the teeth have fallen out the sheep is known as a ‘gummy’. (Source: NSW Agriculture)