Fiftieth Anniversary of the California Net Energy System Symposium: What are the energy coefficients for cows?1

The same model structure used to describe energy metabolism in the growing animal is often used to model energy metabolism in the cow. Energy requirements of the cow are modeled as the summation of energy required for maintenance and recovered energy, where recovered energy is the summation of energy for the conceptus, milk, and tissue energy. Energetic requirements of the cow fluctuate throughout the production cycle depending on whether they are pregnant, lactating, or both. The current model requires energy cost to be associated with either net energy of maintenance or the partial efficiencies of conceptus growth, milk production, and tissue energy change. Mathematically, they are not independent. Incorrectly estimating one will result in an erroneous estimate in the other. Most of the current models in production agriculture allocate energy use into maintenance, and synthesis of tissues making it difficult to assign energy utilization by tissues that provide support functions to pregnancy, lactation, and weight fluctuation. The consequence is the assignment of partial efficiencies that reflect whole animal efficiencies rather than tissue efficiencies. Historically, these models have been predictive of energy metabolism, but caution should be used when inferring the energetic efficiency at the tissue level. Alternative modeling approaches more thoroughly describe tissue energy metabolism and have been used to estimate whole animal metabolism. These models resolve the problems associated with developing coefficients that lack biological meaning but are more complex. There is a critical need for independent data sets to test new components of the model for cows.