A theoretical framework for deriving direct economic values for body tissue mobilization traits in dairy cattle

The aim of this study was to simplify the description of body tissue mobilization during first lactation into 3 parameters for which breeding values can be estimated. The traits defined were 1) the rate at which a bull's daughters lost body energy early in lactation, 2) the maximum drop in body energy content, and 3) the rate at which a bull's daughters regained body energy after nadir. A theoretical framework was developed to derive economic values for the 3 traits of body tissue mobilization. The UK population parameters were used in the economic framework to derive economic weights for body tissue mobilization during lactation. Assuming the average calving pattern in the UK, the economic value for the maximum drop in body energy content is small and negative at -0.14 pence (p/MJ). The economic value for the rate of loss of body energy in early lactation is also negative (-3.1 p/MJ) and for the rate of body energy gain in later lactation is positive (19.7 p/MJ). Results showed that the economic values for the 3 traits of body tissue mobilization change dramatically depending on the date of calving due to changes in feed costs in different seasons. The framework was developed to accommodate various system parameters (e.g., the costs of differing energy sources throughout the year and calving pattern), thus allowing for farm-system-specific economic values for body tissue mobilization. The framework provides a method to select sires based on an index of body energy mobilization of their daughters.