Lactation curves of autumn- and spring-calved cows in pasture-based dairy systems

A linear-based and a non-linear-based method for the analysis of lactation curves were evaluated in this study to investigate the effects of calving s eason on the shape and length of the lactation curve, total milk yield, mil k composition and somatic cell counts of autumn- and spring-calved cows. La ctation records from a S-year systems study in which cows calved either in the autumn or in the spring, were analysed by either split-plot analysis of test-day data, or by fitting the diphasic equation of Grossman and Koops [ J. Dairy Sci., 71 (1988) 1598-1608] prior to analysis by linear models. Ave rage lactation curves produced by both methods were similar. Lactation curv es of spring-calved cows were 'normal', with a peak followed by a steady de cline. However, lactation curves of autumn-calved cows were different in sh ape, with lower yields at peak of lactation but higher yields in mid and la te lactation, which resulted in a significant interaction (P < 0.01) betwee n calving season and stage of lactation. The greater yields in mid and late lactation by the autumn-calved cows, together with their longer lactations (despite the fact that the same guidelines were applied for deciding the d rying-off of all cows), resulted in greater total milk yields for these cow s. Somatic cell counts were greater (P < 0.05) in mid and late lactation fo r the spring-calved cows than for the autumn-calved cows, due probably to a dilution effect. A conceptual model is presented, in which the observed di fferences in the shapes of the lactation curves of autumn- and spring-calve d cows are referred to the cows' potential yield. (C) 2001 Elsevier Science B.V. All rights reserved.