Efficiency of Selection on Milk Yield To a Fixed Age

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Abstract
Selection among cows on an early record of total milk to a fixed age may be expected to be an effective method to achieve genetic gain in total milk produced up to specific ages later in life. Ranking cows on total milk produced early in life, regardless of age at 1st calving, will lead to efforts to maximize reproductive efficiency and daily milk production for the lifetime of the cow. A moderate negative genetic correlation between age at 1st calving and total milk produced to later ages is compatible with a selection criterion which favors cows with an early age of 1st calving. Selection on total milk to a fixed age would emphasize the positioning of the lactation periods along the age scale of a dairy cow and account for production rate during the lactation periods.