465. Studies on variations in the solids-not-fat content of milk: I. Variations in lactation yield, milk-fat percentage and solids-not-fat percentage, that are due to the age of the cow

Abstract

1. The coefficient of variation of lactation milk yield was approximately 25% for all age groups. That for lactation milk-fat percentage approximately 10% and that for lactation solids-not-fat percentage only 3%. The last two coefficients tended to increase with the age of the cow.2. Lactation milk yield was least with first calvers and rose to a maximum for sixth calvers. Milk quality was highest with first calvers.3. No tendency to vary with the season of calving could be found for any lactation record of any age group.4. During the first years of the herd's existence there was a small negative correlation between lactation milk yield and lactation milk-fat percentage. This has disappeared in more recent years.5. There was no correlation between lactation milk yield and lactation solids-not-fat percentage.6. There was a small (r = +0·40 approximately) positive correlation between lactation milk-fat percentage and lactation solids-not-fat percentage.