Short communication: Heritability of milk fatty acid composition and stearoyl-CoA desaturase indices in dairy cows

Activity of stearoyl-CoA desaturase (SCD) in the mammary gland is important for determining the relative proportions of saturated and monounsaturated fatty acids in milk and the concentration of the conjugated linoleic acid isomer rumenic acid (RA; cis-9,trans-11 18:2). Previous studies identified a large degree of between-cow variation in SCD activity, which was consistent across diets and suggests a genetic influence. The objectives of this study were to quantify genetic and phenotypic variations in fatty acid concentrations and SCD indices in milk fat and to estimate their heritabilities in a population of United Kingdom dairy cows. Milk samples were collected from 2,408 daughters of 597 Holstein-Friesian sires on 325 commercial farms for determination of fatty acid profiles. Indices of SCD activity were calculated by expressing each SCD product (cis-9 14:1, cis-9 16:1, cis-9 18:1, and RA) as a proportion of the precursor plus product [e.g., SCDI14 = cis-9 14:1/(14:0 + cis-9 14:1)]. For individual fatty acids, phenotypic variance was considerably greater than additive genetic variance, resulting in small and nonsignificant heritability estimates (+/- standard error) for all except 6:0 (h(2) = 0.27 +/- 0.10), 8:0 (h(2) = 0.27 +/- 0.09), 12:0 (h(2) = 0.13 +/- 0.07), cis-9 14:1 (h(2) = 0.28 +/- 0.10), and cis-9 18:1. (h(2) = 0.12 +/- 0.07). Heritability estimates of desaturase indices were significant for SCDI14 (h(2) = 0.38 +/- 0.11), SCDI18 (h(2) = 0.19 +/- 0.09), and SCDIRA (h(2) = 0.21 +/- 0.09), but not for SCDI16 (h(2) = 0.05 +/- 0.06). This study provides evidence that additive genetic effects are responsible for a significant proportion of the phenotypic variation in SCD activity in dairy cows. It is concluded that because heritability of desaturase indices is moderate and significant in many cases, these indices could be investigated further for use in future breeding programs to increase concentrations of monounsaturated fatty acids and RA while decreasing concentrations of saturated fatty acids in milk fat.