Incisor development, wear and loss in sheep and their impact on ewe production, longevity and economics: A review

The review investigates the use of permanent incisor wear and loss by farmers culling sheep from flocks, summarises investigations into incisor development, factors affecting incisor wear and loss, impacts on production and economics and practices designed to prolong incisor life and makes suggestions for future research. Periodontal disease is outside the scope of the review. Sheep farmers place considerable emphasis on the soundness of their ewes’ mouths and cull sheep for a wide range of faults in their incisors. The length of the productive life in sheep is essentially determined by the state of their permanent incisors. It is normal for there to be a range in the number of incisors present at different ages probably related to maturity and live weight, but most reports fail to quantify these factors. Incisor wear and loss (“broken mouth”) are affected by stocking rate, soil ingestion, farm of origin, breed and gender of sheep, pasture type, internal parasites, mineral nutrition, supplementary feeding and age at first mating of ewes. Incisor wear and loss affects feed intake of sheep, and in most studies reduces live weight gain, milk and wool production. Under certain conditions sheep are not affected by worn or lost incisors. Premature culling of sheep, as a result of incisor wear and loss, increases overhead and replacement costs and reduces lifetime productivity, genetic gain and income from sheep sales thus reducing profitability of sheep farming. A number of practices have the potential to increase longevity of ewes in flocks by reducing incisor wear and loss including: genetic selection; increasing dietary calcium intake during growth and lactation; and supplementary feeding during periods of short pasture. The evidence suggests the mechanical shortening of incisors provides no benefits. Incisor wear and loss in sheep has been accepted as an inevitable outcome of advancing age. There has been little investigation of these issues during the past 20 years. Suggestions are made for future research including: quantification of economic benefit/cost ratio of different farm practices including mineral nutrition, on incisor condition and animal production; genetic selection to improved incisor retention and wear; and development of interactive economic models related to different culling strategies based on incisor condition. Suggestions are made on ways to improve the design and conduct of research in this field including adequate controls, size, duration and statistical power.