Sarda ewes, ∼4·5 million animals producing 500 000 tonnes milk annually, are one of the most important breeds of dairy sheep in the Mediterranean area. Several studies (Casu & Labussière, 1972; Labussière et al. 1981; Gallego et al. 1983; Rebello de Andrade et al. 1989; Bencini, 1993) have shown that milk production is influenced by mammary gland size and cistern dimension. The size of the mammary cistern affects both milk secretion rate and milk emission kinetics during milking.

Milk secretion rate is controlled at the mammary gland level mainly by a protein feedback inhibitor of lactation (FIL), which is produced by mammary epithelial cells and secreted together with milk into the alveoli (Wilde & Peaker, 1990). As the alveoli are the site of action of the FIL (Henderson & Peaker, 1984), the FIL affects the rate of secretion when the milk is stored in the secretory tissue, whereas it is inactive in the milk stored in the cistern. As a consequence, the action of the FIL should be less in animals with a greater cistern volume, because a large proportion of milk is stored in the mammary cistern and the time during which the milk is in contact with the alveoli is reduced. This hypothesis is supported by the finding that the milk production of cows (Dewhurst & Knight, 1992; Knight & Dewhurst, 1992, 1994) and sheep (Karam et al. 1971; Enne et al. 1972) with large cistern storage capacities was almost unaffected by changes in the frequency of milking.

Cistern volume also affects milk emission kinetics and the proportion of stripped milk obtained at milking (Labussière, 1988). Cisternal milk is immediately available for removal, whereas alveolar milk is available only after operation of the ejection reflex, necessary in dairy ewes for complete udder emptying (Bruckmaier et al. 1997). Therefore, in animals readily able to expel alveolar milk into the cistern before the whole cisternal milk fraction is removed (Pazzona et al. 1978; Bruckmaier et al. 1997), a larger cistern volume enables milking time to be reduced by eliminating or restricting the need for stripping.

On the basis of the above considerations, the volume of the mammary gland cistern could be proposed as a selection objective to improve milk production and milking ability of dairy ewes. For this purpose, a quick, accurate and economic method for measuring it is needed. The ultrasound technique allows the internal structure of the mammary gland cistern to be observed clearly in sheep (Ruberte et al. 1994; Pulina et al. 1996; Bruckmaier et al. 1997), cows (Bruckmaier et al. 1994b) and goats (Bruckmaier et al. 1994a). Cistern size has been measured by ultrasound in dairy cows (Bruckmaier et al. 1994b) and sheep (Pulina & Nudda, 1996), where a positive correlation between milk yield and cistern area calculated from the ultrasound images of mammary glands was found. However, area estimation requires the use of expensive ultrasound equipment or of a digitizing tablet. In both cases, area measurement is difficult owing to the irregular shape of the cistern.

The aim of this study was to test the use of linear measurements taken directly from ultrasound images to estimate cistern size in dairy ewes.