Comparison of Different Lactation Curve Models to Describe Lactation Curve in Moroccan Holstein-Friesian Dairy Cows

In this study, the incomplete gamma function, an exponential function, a mixed-log function and a polyno-mial function were evaluated to describe the lactation curve in Moroccan Holstein-Friesian dairy cows. Data from 1990 to 1999, comprising 77130 monthly milk yields of 6029 dairy cows in 280 dairy herds, were used. Edits were carried out by considering the lactation length ( < 305 d), days in milk (DIM) at which the first test-day was recorded ( > 5 d and < 50 d) and the interval between two consecutive recordings ( < 60 d). The models were compared based on the mean square error, the adjusted coefficient of determination and the estimated difference between actual and predicted milk yields. The estimated mean square errors of the models were slightly equal differing only in the first decimal for the incomplete gamma function (33.8) and for the other three models (33.7). The adjusted coefficient of determination was estimated to be 0.18 in the exponential function, mixed-log function and polynomial function but 0.91 for the incomplete gamma function. The estimated difference between actual and predicted milk yields was zero for all the models. It was concluded that all models provided an acceptable level of accuracy in describing the shape of the lacta-tion curve for Moroccan Holstein-Friesian dairy cows, but Wood model is observed to be the most suitable.