Breed differences in milk fatty acid composition across lactation in grazing Holstein-Friesian, Jersey, and Holstein-Friesian × Jersey crossbred cows.

This study evaluated breed differences in milk fatty acid (FA) composition and assessed whether breed differences in FA composition change across stages of lactation in Holstein-Friesian (F), Friesian × Jersey (F × J), and Jersey (J) cows managed under a pasture-based system. Ninety spring-calving cows (33 F, 31 F × J, and 26 J) were sampled during the 2023-2024 lactation season. Monthly herd test records (n = 790) were analyzed to characterize milk yield and composition. Composite milk samples were collected for determination of fat, protein, and mid-infrared spectra. Fatty acid concentrations (g FA/100 g of milk) were predicted using partial least squares models calibrated against 120 samples with FA measured by gas chromatography. Lactation patterns were described using random regression with orthogonal polynomial functions. Stage of lactation was the dominant source of variation, with short-chain, medium-chain, and saturated FA increasing from early to late lactation, while preformed FA declined. Jersey cows had the highest total fat concentration (6.29%), followed by F × J (5.50%) and F (5.32%), and produced approximately 23% more saturated FA than F cows (+0.80 g FA/100 g of milk). When averaged across lactation, F × J generally resembled F for most FA groups, although the crossbred response was FA-specific: F × J was intermediate for short-chain FA but resembled F for medium-chain FA. Breed differences were more pronounced within lactation stages than in pooled analyses. Breed × stage interactions reflected changes in the magnitude of breed differences rather than changes in ranking, with the saturated FA difference between J and F widening by about +0.11 g FA/100 g of milk from early to late lactation. These results demonstrate that crossbred FA profiles are FA-specific and stage-dependent and cannot be predicted as simple parental averages.