Factors associated with the concentration of fatty acids in the milk from grazing dairy cows.

Bovine milk fat contains ∼400 different fatty acids, with their relative concentration being highly variable and influenced by (or associated with) intrinsic and extrinsic factors such as diet, breed, and lactation stage. Using predicted fatty acids from milk mid-infrared spectra, the objectives of the present study were to (1) quantify the association between cow parity, lactation stage, breed, heterosis, and recombination loss with milk fatty acid composition, and (2) quantify the association between parental average genetic merit for both milk fat concentration and yield with a series of different fatty acids. The dataset used included 644,752 milk test-day records from 303,089 cows across 2,406 commercial dairy farms. The concentration of 16 individual fatty acids as well as 16 fatty acid indices were predicted from spectral analyses of milk samples. Factors associated with all the investigated traits were individually explored using linear mixed models. The fixed effects considered in all models were the interaction between parity and stage of lactation, the calendar month of test, breed composition, heterosis, and recombination; the random effects were contemporary group and a within-lactation repeated effect. In a separate series of analyses, cow genetic merit for fat yield and fat percentage were separately included in the model as linear covariates. The concentration of the different fatty acids changed throughout lactation and across calendar months, coinciding with the seasonal profile in pasture quality, especially during the summer months. Multiparous cows were characterized by milk with a higher concentration of saturated and short-chain fatty acids, along with a lower concentration of unsaturated, medium-chain, and long-chain fatty acids. Jersey bloodline was associated with a higher concentration of milk saturated and short-chain fatty acids, along with a lower concentration of unsaturated, medium-chain, and long-chain fatty acids compared with Holstein-Friesians. Genetic merit for fat yield or fat percentage was associated with a higher concentration of saturated fatty acids and a lower concentration of unsaturated fatty acids.