An interactive, mechanistic nutrition model to determine energy efficiency of lactating dairy cows

Selection for milk production alone may result in some cows that have lower energy efficiency (increased energy efficiency index - EEI - Mcal of metabolisable energy (ME) required per kg of milk) on the diets available due to limitations in dry matter intake (DMI), increased body size relative to the amount of milk produced, or inadequate ability to mobilize and replete energy reserves. Ideally, an energy efficient dairy cow would produce the same amount of milk with less DMI by mobilizing reserve tissues during negative energy balance and replenishing reserve tissues during positive energy balance in a more efficient and faster manner. The objective of this paper is to describe a nutrition model that identifies differences in ME required (MER) for the observed milk production under farm conditions where individual DMI is not known. This model is comprised of four submodels that account for energy requirements for maintenance, growth, pregnancy, and lactation. A fifth submodel accounts for the dynamic mobilization and replenishment of body reserves throughout the reproductive cycle. The model was evaluated with a database containing records of 241 Holstein cows individually fed for a complete lactation at four locations in the US (New York, Utah, Florida, and Arizona). A Monte Carlo simulation was used to assess the distribution and variation. Influential variables on the simulation output were accomplished by using standardized regression coefficients. Model predicted MER accounted for up to 83% of the variation in actual DMI consumed. We conclude this model can be used to identify differences in feed requirements for the observed milk production when individual cow DMI is not known.