We determined the effect of dietary fatty acid (FA) composition on digestibility of dry matter (DM), neutral detergent fiber (NDF), and FA (DMD, NDFD, FAD, respectively) and digestible energy intake (DEI). Our database comprised 423 individual observations from 124 lactating Holstein cows receiving diets that varied in FA composition from 5 studies. Dietary lower and upper quartiles of C16:0, C18:0, C18:1, C18:2, and C18:3 were 0.61 to 1.68, 0.07 to 0.23, 0.63 to 0.79, 1.29 to 1.81, and 0.18 to 0.21% DM, respectively. Starch and forage NDF (fNDF) varied from 26 to 29 and 19 to 21% DM, respectively, and DMI varied from 14 to 40 kg/d. Energy values were determined in feed ingredients, orts, and feces by bomb calorimetry using iNDF as a marker. Meta-regression used the following fixed effects: all the FA variables described above, respective quadratic terms, 2-way interactions between each FA category and starch, fNDF, and DMI. Random effects were study-specific intercepts and slopes on DMI, cow (study), period (study), and block (study). Best fitting prediction equations are presented in Table 1. C16:0 and C18:0 were the dietary FA with the most significant impact on digestibility and DEI. C16:0 increased NDFD and DEI when increased in the diet from 0.33 to 1.27% of DM, while C18:0 decreased DEI when it was increased from 0.35 to 0.88% of DM. Results could aid the development of models to predict DEI from dietary ingredients including dietary FA. Table 1 (Abstr. 424). Best-fit equations for digestibilities and DEI. Coefficients model the effect of deviations from the mean for each variable

Effect	DMD	NDFD	FAD	DEI
Intercept	66.4 ± 0.98	42.5 ± 0.86	76.5 ± 0.60	96.0 ± 1.47
DMI (linear, L)	−0.01 ± 0.06	0.06 ± 0.10	−0.37 ± 0.13	2.83 ± 0.07
DMI (quadratic, Q)	−0.02 ± 0.01	—	−0.03 ± 0.01	—
Starch	1.58 ± 1.03	—	—	3.86 ± 0.94
fNDF	−0.76 ± 1.16	0.96 ± 1.00	−1.49 ± 0.69	0.36 ± 1.02
Palmitic (L)	3.02 ± 0.30	1.71 ± 0.66	−1.71 ± 0.50	0.39 ± 0.85
Palmitic (Q)	−5.66 ± 1.08	—	—	−31.6 ± 3.72
Stearic (L)	−6.48 ± 1.19	16.4 ± 5.40	−11.0 ± 1.10	−16.7 ± 2.01
Stearic (Q)	—	−34.3 ± 10.6	—	—
Oleic (L)	—	5.12 ± 2.11	—	15.2 ± 5.45
Oleic (Q)	—	—	—	−33.9 ± 14.1
Linoleic (L)	—	−4.87 ± 1.24	—	21.4 ± 2.86
Linolenic (L)	—	—	—	325 ± 45.6
Linolenic (Q)	—	—	—	−4527 ± 608
DMI × fNDF	0.20 ± 0.08	0.27 ± 0.13	—	0.25 ± 0.08
Starch × Palmitic	−1.26 ± 0.46	—	—	−3.60 ± 0.80
Starch × Stearic	9.37 ± 2.67	—	—	−18.1 ± 5.95
fNDF × Palmitic	—	1.38 ± 0.65	—	—