Genetics of protein fractions and free amino acids predicted by mid-infrared spectroscopy

Milk protein fractions and free amino acids (FAA) are important traits for the dairy industry specialized in cheese and milk powder production. Despite this, they are not included in any breeding objective due to the lack of quick and cost-effective routine determination of these compounds. In the present study, mid-infrared spectroscopy prediction models previously developed on bovine milk samples in Ireland were applied to stored spectral data to predict both protein fractions and FAA concentrations. After edits, 134,546 test-day records from 16,166 lactations and 9,572 cows were available. Variance components were estimated using repeatability animal models including the fixed effects of contemporary group, milking time, cow breed proportion, general heterosis, recombination loss, and parity-by-stage of lactation. Random terms were additive genetic effect of the animal, and within- and across lactation permanent environmental effects. Means of αs1-casein, αs2-casein, β-casein, and κ-casein were 13.70 g/l, 3.66 g/l, 12.96 g/l, and 6.03 g/l, respectively. About 83% of total whey proteins (6.13 g/l) were represented by β-lactoglobulin. Concentrations of FAA exhibited average values that ranged from 8.09 μg/ml (glycine) to 1.52 μg/ ml (valine), except for glutamic acid (30.93 μg/ml). Heritability estimates for protein fractions ranged from 0.36 (αs2-casein and κ-casein) to 0.46 (β-lactoglobulin A), except for α-lactalbumin (0.19). The range of heritability estimates for FAA was smaller (0.15 for glycine to 0.36 for aspartic acid). Genetic correlations between casein fractions and whey proteins were all positive. Genetic correlations between FAA ranged from -0.44 (aspartic acid and lysine) to 0.97 (glutamic acid and total free amino acids). Breeding strategies for genetically improving protein fractions and FAA may represent a viable solution to meet the requirements of specialized dairy industry.