Effects of genetic improvements on efficiency of energy utilization in dairy cows

In the last 3 decades, much progress in genetic improvements in milk production has taken place and numerous studies have been conducted on energy metabolism in dairy cows. This study investigated the effects of these improvements on key parameters of ME systems. These are net energy for maintenance (NEM), efficiency of utilization of ME for milk production (kl), growth (kg) and efficiency of utilization of body stores for milk production (kt). A large data set was collated by Kebreab et al. (2003) [J. Dairy Sci. 86:2904–2913] and further updated with data from the Netherlands. The data set contained a total of 701 individual cow observations from 38 calorimetry studies on Holstein-Friesian dairy cows. All energy related variables were selected for the study. Kebreab et al. (2003) estimated the 4 key parameters by deriving a function based on a linear relationship between milk energy and ME intake and correcting for tissue energy loss or gain. The function served as the basis of a full Bayesian hierarchical model where the between study variability in the 4 parameters was modeled by a multivariate normal distribution and the within study variability by a student t-distribution. The time trend was included as categorical variable with 3 levels differentiating the cows in experiments conducted before 1990, 1990–1995 and after 1995. In the analysis, an informative prior was introduced for the population parameter (NEM ~N(0.45, 0.04)). The deviance information criterion (DIC) was used to compare models with varying complexity. There was a difference of 5 DIC units between the 2 models, favoring the simple model. Based on the data and an informative prior, the posterior distribution of NEM, MEM, kl, kg and kt were estimated to be 0.34 (0.028) MJ/(kg0.75 BW d), 0.58 (0.034) MJ/(kg0.75 BW d), 0.58 (0.021), 0.89 (0.056), and 0.69 (0.047), respectively. The analysis does not support the hypothesis that genetic improvements in milk production from 1986 to 2007 have significantly altered key parameters. However, the MEMvalue was higher than NRC recommendations (based on older data) and may reflect higher requirements in modern cattle.