Extension factors for incomplete lactations of Serra-da-Estrela sheep

Data for 6537 lactations of 2335 Serra-da-Estrela ewes representing 41 flocks (1966 to 1985) were obtained from the Milk Recording Scheme conducted by the Portuguese Ministry of Agriculture. The suckling period averaged 32 d, and ewes were milked for approximately 200 d after the suckling period. Individual test day yields for 1548 lactations of 600 ewes from 32 flocks (1975 to 1985) were used to define the shape of the lactation curve. A third-order polynomial and an exponential function were equally accurate in describing the shape of the lactation curve in the postsuckling period. A considerable number of incompletely recorded lactations occur, and, with extension factors, these incomplete lactations could be used in genetic evaluation. Factors based on both partial lactation yield and last test day yield were developed. Use of these factors should be restricted to partial lactations of at least 75 d with 2 test days. An alternative of recording only postweaning yield was proposed.