A data set containing 2-year test day milk yield records was used. Random regression model solutions established with a covariance function (CF) approach was used. Persistence and lactation curve explanation performances of polynomial, Wilmink and Ali-Schaeffer curve definitions were compared for CF. When comparisons were made for R-2 values, polynomial (0.9835) definition headed the list, followed by the Ali-Schaeffer curve (0.9534), and Wilmink curve (0.6023) definitions. Among the persistence predictions in respect of lactation orders, the Ali-Schaeffer curve provided predictions with the minimum variance, followed by the Wilmink and polynomial curve approaches. All of the curve definitions were in accord with the literature.