We determined milk fatty acid (FA) concentration and yield responses in relation to diet nutrient composition in dairy cows. Our analysis used individual observations (n = 1202) from 212 lactating Holsteins cows from 14 studies. Milk FA were classified according to their source: < 16-carbon FA from de novo synthesis in the mammary gland and > 16-carbon preformed FA originating from extraction from plasma. Mixed source FA (16-carbon FA) originate from de novo synthesis in the mammary gland and extraction from plasma. Data were analyzed using a mixed model multiple regression, including diet nutrient composition (% of DM of starch, NDF, forage NDF, 16:0, 18:0, 18:1, 18:2n6, 18:3n3 and total FA) and DMI and DMI² (kg/d), and quadratic terms as fixed effects. Cow, block, period, and study were included as random effects. Variables were removed from the model using backward elimination with significance criteria of P < 0.05. The best fitting prediction equations for the concentration and yield of milk FA are presented in Table 1. Our results demonstrate that DMI is important for predicting the concentration and yield of de novo and preformed milk FA. The dietary concentration of 18:2n6 is negatively associated with the concentration and yield of de novo and mixed FA, while 16:0 is positively associated with the concentration and yield of mixed FA. Table 1. Best fit equations for the concentration and yield of milk FA in relation to diet chemical composition

Variable	Concentration, g/100g FA				Yield, g/d
De novo	Mixed	Preformed	De novo	Mixed	Preformed
Intercept	39.8 ± 4.265	36.2 ± 11.900	33.8 ± 2.674		144 ± 57.645	−66.7 ± 13.111	361 ± 60.703
DMI (kg/d)	0.55 ± 0.111	—	−0.66 ± 0.153		19.2 ± 3.455	8.02 ± 0.924	4.99 ± 1.208
DMI × DMI	−0.01 ± 0.00197	—	0.008 ± 0.0027		−0.21 ± 0.0611	—	—
Starch (%)	0.36 ± 0.0481	−0.74 ± 0.194	—		—	−3.73 ± 1.618	—
NDF (%)	—	−0.60 ± 0.231	0.25 ± 0.0329		−1.51 ± 0.717	—	3.39 ± 1.266
Forage NDF (%)	−1.43 ± 0.240	2.05 ± 0.308	—		—	27.4 ± 7.858	—
Total FA (%)	−1.48 ± 0.108	—	−1.41 ± 0.139		—	—	—
16:0 (%)	—	2.93 ± 0.165	—		—	64.1 ± 4.018	—
18:0 (%)	1.43 ± 0.200	—	1.74 ± 0.273		—	—	—
18:1 (%)	2.02 ± 0.687	−2.93 ± 0.735	3.10 ± 0.938		—	−79.9 ± 24.495	—
18:2n6 (%)	−1.84 ± 0.403	−3.08 ± 0.507	8.65 ± 0.548		−56.1 ± 11.351	−54.4 ± 17.556	—
18:3n3 (%)	—	—	—		−25.5 ± 12.795	—	—
RMSE	1.31	1.52	1.79		22.5	50.1	31.5
R2	0.91	0.86	0.91		0.96	0.88	0.91