The effect of age and liveweight at first calving on the shape of the lactation curve of dairy heifers.

Abstract

2A The effect of age and liveweight at first calving on the shape of the lactation curve of dairy heifers R. Dobos1, K. Nandra2, K. Riley2, S. Morris3 and W. Fulkerson4 1 2 3 4 NSW Agriculture Beef Industry Centre, Armidale NSW 2350 NSW Agriculture, Elizabeth Macar thur Agricultural Institute, Camden NSW 2570 Wollongbar Agricultural Institute, Wollongbar NSW 2477 MC Franklin Laborator y, University of Sydney, Camden NSW 2570 robin.dobos@agric.nsw.gov.au Variation in milk production during lactation is commonly described as a lactation curve. The production by dairy cows typically increases with time until a peak is reached at 4 to 8 weeks after calving and then steadily declines until lactation ceases. The combined effects of age (AFC) and liveweight at first calving (LWFC) on the yield and composition of milk in the subsequent first lactation have been reported by Dobos et al. (2001). However, it is not known how these two factors affect the shape of the lactation curve. Weekly milk yields during the first 12 weeks of first lactation from dairy heifers grown to achieve different AFC and LWFC were used to determine if AFC and LWFC affected the shape of the lactation curve. Data from 111 heifers were used. Empirical (linear or non_linear), mechanistic or non_parametric models fitted to the data have been used to study the curve of dairy cows. We chose to use the method of Verbyla et al. (1999). Briefly, a mixed model for the response of milk yield to time and treatment group was fitted. Fixed terms in the model were main effects of AFC, LWFC and time (weeks) as well as all their interactions. That is, a linear model relating milk yield to time with intercepts and slopes was allowed to vary according to treatment group. Curvature in the response was incorporated by inclusion of a cubic spline term for time as a random effect. Lack of fit was also included in the model as random deviations due to time increments. Model fitting was accomplished using ASREML (Gilmour et al. 1999). The analysis indicated significant (P<0.05) effects of AFC and LWFC on milk yield but there was no interaction. A linear response over time was found to depend on AFC, while curvature about that response was dependent on LWFC. Neither AFC nor LWFC effected the overall shape of the lactation curve. However, the position of the curve differed depending on treatment (see Figure 1). Within each treatment the gain in production from 550 to 620 kg was small. This suggests an optimum LWFC that is between 550 and 620 kg. The optimum LWFC for milk yield was reported by Dobos et al. (2001) to be 559 kg. Dairy managers will now be able to implement feeding strategies during early lactation to account for AFC and LWFC of dairy heifers. Dobos, R.C., Nandra, K.S., Riley, K., Fulkerson, W.J., Lean, I.J. and Kellaway, R.C. (2001).The effects of age and liveweight at first calving on first lactation milk, protein and fat yield of Friesian heifers. Australian Journal of Experimental Agriculture 41, 13_19. Gilmour, A.R., Cullis, B.R., Welham, S.J. and Thompson, R. (1999). ASREML Reference Manual. NSW Agriculture Biometric Bulletin no. 3. NSW Agriculture, Orange NSW, Australia. Verbyla, A.P., Cullis, B.R., Kenward, M.G. and Welham, S.J. (1999) The analysis of designed experiments and longitudinal data using smoothing splines (with discussion). Applied Statistics 48, 269_311. Predicted Milk Yield (L) 16 18 20 22 24 14 AFC 24 months 29 months 34 months 2 4 6 LWFC 480 kg 550 kg 620 kg 8 10 12 Week of Lactation Figure 1 Lactation cur ves for first 12 weeks of first lactation. Recent Advances in Animal Nutrition in Australia, Volume 13 (2001)