Analytical procedures for estimating milk intake and yield in steady-state and nonsteady-state systems

A tracer method for determining milk intake, introduced earlier, was based on the transfer of tritiated water from a lactating female to a nuring offspring via milk. The analysis of the tracer data assumed a steady state system, i.e., the total body water of the offspring was assumed to be constant over the measurement period. This paper discusses the potential errors in the milk intake estimates in applications where steady-state does not exist. Three analytical procedures are considered and include the application of 1) steady state equations for a nonsteady-state system, 2) analytical solutions to nonsteady-state equations, and 3) a computer modeling program, SAAM-27. The application of steady-state equations is the simplest procedure and may yield acceptable estimates if the growth rate of the nuring offspring is low. The analytical solution procedure yields acceptable estimates at high growth rates of the nursing offspring but becomes unacceptable at low growth rates. The SAAM program requires sophisticated computer hardware and programming; however, the procedure yields the best estimates of milk intake in applications ranging from steady-state to high growth rates.