Tooth deterioration in sheep : a study of farms in southern North Island of New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Veterinary Science at Massey University
Progressive deterioration of sheep's teeth on New Zealand farms has become more apparent as farming has undergone intensification. In particular it would appear that on farms where stocking rates are highest the sheep have been most seriously affected. Historically the amount of wear of the incisor teeth has been used as a method of culling in most sheep flocks. In many affected flocks it has been common practice to cull ewes after three lambing seasons. In other situations sheep farmers have been encouraged to retain older ewes with severely deteriorated mouths but to give them preferential grazing treatment. This has amounted to culling by age and production rather than condition of teeth. Nevertheless, many producers still continue to cull by the former method believing that ewes with badly deteriorated incisors will fail to maintain condition and production during competitive management periods. Whether impairment of production is real or perceived, traditional culling by the condition of the incisors continues to be a common practice. As a result tooth deterioration has become a very severe waste to the sheep industry. Tooth deterioration is a problem that was first described by Barnieoat (1947). Since its recognition there has been a multiplicity of aetiological factors proposed for this condition. Barnieoat stated, "Abrasion due to mechanical causes is well known both here and abroad. This problem is quite distinct from that of progressive deterioration of mouths on improved country, which is the central problem of this investigation" (Barnicoat, 1957). Other researchers disregarded this supposition after linking the problem to soil ingested and hypothesised that tooth wear resulted from abrasion by ingested soil (Healy & Ludwig, 1965). Since its presentation this theory has been generally accepted (O'Hara, 1983). It seemed to be a logical explanation of the cause of the problem but tended to disregard some of the clinical manifestations.