Changes in Milk Production with Age and Milking Frequency

Abstract

Age-correction factors were derived from 43,573 Holstein-Friesian Herd Improvement Registry lactations of 5374 cows milked twice daily and 5627 cows milked thrice daily. Each cow had at least 3 lactations. Successive lactations by the same cows were compared. The biases which culling of cows may cause in age-correction factors are examined. The results agree moderately well with the widely used Bureau of Dairy Industry factors. The-latter seem a tiny bit too large at ages under 35 mos., and somewhat too small at ages around 45 to 66 mos. They appear much too large at ages past 9 yrs., but this is unimportant because few records are made by cows at those ages. Different correction factors are needed for late 1st calvers and for early 2d calvers at the ages (around 33 to 42 mos.) when these both occur. 3X records starting at 36 months or older can be multiplied by 5/6 to make them fairly comparable with 2X records. At younger ages a smaller fraction, such as 4/5 is more nearly correct.