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Sources of Variation in Ratios of Total to Part Yield

Dairy Cattle Research Branch, USDA, Beltsville, Maryland

ABSTRACT

The influences of breed, age, and season of calving on cumulative and noncumulative ratios of total to part milk and milk fat yields were studied in 49,034 lactations representing five dairy breeds.

Average ratios were formed for each of the ten test days, both individually and cumulatively, for all combinations of breed, month of calving, age class, and yield trait (milk and milk fat), but herd and year of of calving were ignored. Brown Swiss cows had the highest ratios throughout lactation, while Ayrshires had the least. Differences among the other breeds were not very pronounced. Ratios were uniformly highest for two-year-old cows among all breeds, but variation among cows four years or more of age was small and inconsequential. Three-year-olds were intermediate, but resembled older cows more than two-year-olds. Ratios for cows calving in summer (July–September) and fall (October–December) changed the least over the lactation, while those for animals calving in spring varied the most. The pattern of response was similar among breeds, although it differed in magnitude. Ratios for milk fat yield were higher than those for milk production, indicating a greater persistency of milk fat yield.

Analyses of variance of Holstein data indicated that the influences of age, season, and trait on ratios were significant (P<.001) for both cumulative and noncumulative ratios. Age was the most important influence. All two-factor interactions were statistically significant (P<.001), but the age by test day, trait by test day, and trait by season interactions were much more important than the others. The age by season by test-day interaction was the only notable higher order interaction.

It was concluded that optimum sets of ratios for extending breed, age, season, and milk and milk fat. Where operational considerations limit the number of sets of factors that can be used, the most important stratifications are by breed, by age (two years vs. all older), and by trait.