Evaluation of models for somatic cell score lactation patterns in Holsteins

Milk somatic cell score (SCS) typically reaches a minimum early in lactation and then rises. Nonlinear mixed effects models were used to describe this trajectory while accounting for between and within-cow variation. A total of 2387 SCS records from 217 Holsteins were analyzed. Four nonlinear and two linear functions were studied. Approximate maximum likelihood estimates indicated that cows free of intramammary infection had sharper SCS decreases after calving, and lower overall levels. Lactations starting between October and December had the highest fall of SCS levels at the beginning of lactation, and the smallest increases thereafter. In general, there was significant variation between cows' individual trajectories. For some parameters and models, however, this variation was small. A four-parameter model suggested by Morant and Gnanasakthy was supported better by the data than the other five functional forms. (C) 2000 Elsevier Science B.V. All rights reserved.