Observations on the length and angle of declination of the vulva and its relation to fertility in the mare.

An instrument has been designed to measure the effective length (l) and angle of declination (a) of the vulva in the mare. The product, la, provides an index (Caslick Index) suitable for determining the necessity for Caslick's operation in mares not exhibiting the classical symptoms associated with pneumovagina. The value l showed a significant increase (P less than 0.05) with increased age in breeding mares. Studies on 9020 mares revealed that all caslicked mares, and mares with a Caslick Index of less than 150, had a significantly higher pregnancy rate than non-caslicked mares of similar age and a Caslick Index greater than 150. Values of 100 and over should be an indication for more careful clinical appraisal of the genital tract in those mares not as yet clinically affected. Nulliparous mares caslicked for racing had a significantly higher pregnancy rate than non-casliked maiden mares in their first year at stud.