Modeling net energy efficiencies as quantitative characteristics in lactating cows.

To date, researchers have measured net efficiencies of energy conversion using data from animals in energy chambers. The expense of this approach prevents the establishment of a large data base for quantitative studies. Our purpose was to investigate models that would enable us to use data collectable in normal field conditions to compare dairy cattle for their net energetic efficiency. Data from 357 Holstein cows in seven herds and in various parities consisted of daily measures of DM intake, net energy intake, milk production, biweekly measures of milk components, and bimonthly BW. Eighteen alternative multiple regression models were fitted to each of the cows to estimate simultaneously net efficiency of energy conversion for maintenance, lactation, pregnancy, and BW change during positive energy balance period, negative energy balance period, and whole lactation. Results from several fitted models approximated closely literature results based on data from cows in energy chambers. These comparative results suggest that it is possible to estimate efficiency of energy conversion on individual cows using data obtained from normal animal management situations.