Application of two models to the lactation curve of Massese ewes.

The experiment was carried out on 52 lactations of 37 multiparous Massese ewes. Milk yield was recorded weekly, starting before the 15th day from lambing. Due to the mating practice of the breed, three types of lactation were detected: Type 1, a short lactation initiated in autumn; Type 2, a short lactation initiated in spring, following the Type 1 lactation; Type 3, a long lactation initiated in autumn. The Wood model (W) and a bi-exponential function (CB) were applied to fit milk yield data of individual lactations. Milk yield of the whole lactation was estimated both by integrating the W and CB functions, and by using the Fleischmann official method. The W model, after the restriction of b>0, fitted the original data better than the CB model, particularly in late lactation. However, the CB model was more adequate for describing the initial sudden rapid increase of milk yield. Differences among the three methods for the estimate of total milk yield were very small, even though the CB model slightly overestimated late lactation. Among the three types of lactations, Type 2 had a much earlier peak, a higher peak yield and a lower persistency than the other two types. The ewe's age at lambing influenced only the total milk yield, which reached the maximum at about 4 years of age.