Genetic variance components for alternative definitions of fatty acids in dairy cow milk expressed either as a concentration or yield

The extent of genetic variability in fatty acids in bovine milk has, to date, generally focused on its concentration in either milk or milk fat. Selection for ratio traits, such as fatty acid concentration, is statistically and biologically problematic because it can distort relationships between component traits and lead to unintended genetic responses. The objective of this study was to explore the degree of genetic variability in the total yield of individual fatty acids, including when adjusted to a common fat yield. Animal linear mixed models were used to estimate variance and covariance components for a series of fatty acid phenotypes in part-day milk samples predicted using milk infrared spectroscopy; predictions for a total of 16 individual fatty acids and 13 groups of fatty acids were available for 68,353 test-day samples across 47,904 lactations from 27,023 cows. When expressed as a concentration in fat, the coefficient of genetic variation for the individual fatty acids varied from 2% to 11% with a mean of 5%; when expressed as a concentration in milk, the coefficient of genetic variation for the individual fatty acids varied from 7% to 13% with a mean of 10%. The yield of individual fatty acids per milking had a coefficient of genetic variation varying from 7% to 14% with a mean of 9%; the respective values when phenotypically adjusted to a common fat yield were 2% to 11% with a mean of 5%. The genetic correlation between a given fatty acid expressed as a concentration in fat versus as a concentration in milk varied from -0.03 to 0.82. The genetic correlation between the total yield of a given fatty acid for a milking and that expressed as a concentration in fat varied from 0.19 to 0.74, whereas the genetic correlation varied from 0.35 to 0.75 when expressed as a concentration in milk. The genetic correlations between the total yield of individual fatty acids in a milking and the associated fat yield in that milking varied from 0.48 to 0.96, with the genetic correlations varying from 0.18 to 0.65 between the total yield of individual fatty acids in a milking and the associated milk yield in that milking. Irrespective of definition, exploitable genetic variability was detected for most of the (groups of) fatty acids; the choice of which definition of fatty acid to use is a function of how it will be deployed in a breeding scheme but also its end use.