Supplementing fat to lactating cows may reduce total-tract NDF digestibility (ttNDFd). The objective was to analyze the effects of different types of fat across studies. The numerical difference between ttNDFd and dry matter intake (DMI) for the supplemented minus control diet were calculated within study and these differences were analyzed in SAS 9.4 using Proc Mixed (Table 1). Studies were weighted based on the inverse of the standard error of ttNDFd squared. The model contained the fixed effect of fat type and study as a random effect. Fatty acid content of treatment diets never exceeded 10% of DM (mean = 6.2, SD = 1.4). Supplementation of fats containing 12 and 14 carbon chain fatty acids (C12, C14) decreased both DMI and ttNDFd. Oil decreased ttNDFd by 2.1 percentage units, but the DMI decrease was not significant. No other type of fat consistently decreased ttNDFd, though calcium salts (n = 13 of palm oil, n = 6 of unsaturated C18) increased ttNDFd by 2.2 percentage units and decreased DMI by 1.45 kg/day. A regression analysis across all types of fat, using Proc Glm in SAS 9.4, of the difference in ttNDFd versus the difference in DMI within study shows a positive relationship. This suggests that the increase in ttNDFd for calcium salts is not explained through the decrease in DMI. Overall, these results indicate that the addition of a fat supplement, in which the fatty acids are C16 or greater in length, has minimal effects on ttNDFd. Feeding calcium salts in place of free oils would likely increase ttNDFd, but decrease DMI. Table 1. Effects of fat supplemented diets compared with control diets by fat type

Fat type	No. of studies	ΔttNDFd percentage units	SE	P-value	ΔDMI, kg/d	SE	P-value	ΔFA content of diet %DM	SE
C12/C14	6	−11.32	1.67	0.0001	−4.04	0.66	0.0001	3.8	0.72
Oil	11	−2.13	0.97	0.04	−0.59	0.39	0.13	2.9	0.41
Animal vegetable blend	8	−1.15	1.36	0.41	−0.64	0.54	0.18	3.8	0.58
High C16	5	−0.31	1.43	0.83	−0.31	0.57	0.34	2.9	0.72
Tallow	22	−0.27	1.17	0.82	−0.88	0.47	0.07	2.5	0.62
Hydrogenated fat	14	0.42	1.36	0.76	0.42	0.54	0.37	3.4	0.48
Calcium salts	19	2.2	0.83	0.01	−1.48	0.33	0.0001	3.2	0.37