When calculating feed efficiency, milk yield needs to be converted to 3.5% fat-corrected milk (FCM) yield.
The formula for correcting for milk fat is listed below for 3.5% fat corrected milk (which is the number I use).
Lb 3.5% FCM = (0.4324 x lb milk) + (16.216 x lb of milk fat)
Be sure to use pounds of milk fat (lb milk x % fat test = lb of milk fat in your herd).
For more information on calculating and evaluating feed efficiency, please see Feed Efficiency and Its Impact on Feed Intake found here in the DAIReXNET Web site under nutrition or nutrient management.
Mike Hutjens, Extension Dairy Specialist, University of Illinois – Illini DairyNET