Short communication: Estimating lactation curves for highly inhomogeneous milk yield data of an F 2 population (Charolais × German Holstein).

Fitting of lactation curves is a common tool to obtain the entire milk yield as well as to estimate the main curve characteristic (such as day of peak milk yield) for a lactation. These models are primarily designed for dairy cattle, but have been applied to nondairy cattle breeds and also for other species. In this study we considered milk yield data of 197 F2 crossbred cows of Charolais and German Holstein (founder breeds) for the first and the beginning of the second lactation. The F2 cows showed a high variability regarding the length of lactation, which varied between 7 and 406 d in milk for the first lactation. Thus, the data also show high variation regarding the daily and overall milk yield. To obtain complete lactation curves, we evaluated the lactation models of Ali-Schaeffer and Wilmink. To compare the 2 lactation models, we evaluated the goodness of fit using 6 evaluation criteria. The results show that the model of Ali-Schaeffer performs better on these highly inhomogeneous data, in contrast to the model of Wilmink. We discuss our findings from a statistical point of view and present possible biological reasons for the high variability regarding milk yield within the F2 population. Hence our findings may be helpful when milk yield data of crosses between dairy and beef cows (dual purpose) are investigated, whose lactation curves may not show the typical characteristics of dairy cattle.