RESUMEN
The objective was to assess the in vitro rumen fermentation characteristics, methane production, and biohydrogenation of unsaturated fatty acids of diets with two protected fat (PF) sources from soybean or linseed oil, two levels of PF (0 and 6%) and two forage sources (canola silage (CS) or alfalfa hay (AH)) in a factorial 2x2x2 completely randomised design. Only fatty acids content at final incubation was affected (P<0.05) by triple interaction, where C18:2 was highest with AH plus 6% soybean PF (4.41mg/g DM), while C18:3 was with CS plus 6% linseed oil protected (1.98mg/g DM). C18:2 cis-9 trans-11 had high concentration (308 mg/g DM; P<0.05) with AH plus 6% PF regardless PF type, and C18:1 trans-11 was higher with 6% PF than without PF (13.41 vs 7.89 mg/g DM). Cumulative methane production was not affected by treatments (0.9973 ± 0.1549 mmol/g DM; P>0.05). Gas production and in vitro NDF digestibility were lower with 6% PF of linseed than soybean (160.88 vs 150.97 ml; and 69.28vs 62.89 %, respectively P<0.05). With linseed PF the NH3-N concentration was highest in CS than AH (41.27 vs 27.95 mg/dL; P<0.05) but IVDMD had the opposite result (78.54 vs 85.04). In conclusion, although methane production was not affected and in vitro digestibility and gas production were reduced with linseed PF, the concentration of C18:3 and C18:1 trans-11 was increased, which could improve the lipid profile of milk. The negative effects on digestibility were less with AH than of CS regardless of PF type and level.