Effects of various sources of unsaturated oil on ruminal fermentation characteristics, nutrient digestion, enteric methane emissions, nitrogen partitioning, and milk production in dairy cows

The objective of this study was to investigate the effects of dietary inclusion of different sources of unsaturated vegetable oil on ruminal fermentation characteristics, nutrient digestion, enteric CH₄ emissions, nitrogen partitioning, milk production, and milk fatty acid composition. Eight multiparous ruminally cannulated Holstein cows were used in a duplicated 4 × 4 Latin square design (28-d periods) balanced for residual effects. Treatments consisted of a TMR based on alfalfa silage and corn silage (forage/concentrate ratio 52:48; DM basis) either not supplemented (CTL), or supplemented (4% of DM) with either sunflower (SUN, 67.5% cis-9 18:1), safflower (SAF, 74.6% cis-9, cis-12 18:2), or linseed (LSO, 54.9% cis-9,cis-12,cis-15 18:3) oil. The inclusion of the vegetable oil was performed primarily at the expense of another energy source (i.e., corn grain). Feeding the oil-supplemented diets decreased DMI (-1.2 kg/d, on average) and increased apparent total-tract digestibility of CP, compared with the CTL. However, no changes were noted in digestibility of DM, OM, NDF, gross energy, or the effective ruminal degradability of DM (i.e., in situ technique). Milk yield decreased only when feeding SUN (-3.3 kg/d) or SAF (-4.1 kg/d), compared with the CTL. Milk fat yield decreased when using SAF (-0.44 kg/d, -38%), SUN (-0.32 kg/d, -28%), and LSO (-0.21 kg/d, -18%), compared with the CTL. Milk protein yield was reduced with SUN (-9%) and SAF (-7%) compared with the CTL but was unaffected by LSO. The content of MUN decreased with oil supplementation by 28%, 20%, and 10% for LSO, SAF, and SUN, respectively. In contrast, the efficiency of dietary N use for milk N secretion was not affected when cows were fed the oil-supplemented diets. Relative to N intake, fecal N decreased, whereas urinary N increased with oil-supplemented diets. Oil supplementation did not alter enteric daily CH₄ production (SF₆ tracer technique) or yield. In the present study, the oils differing in unsaturation level did not change overall ruminal fermentation characteristics, OM digestibility, or CH₄ production. Feeding the oil-based diets had no effect on N balance but shifted N excretion from feces to urine. Cows consuming the oil-supplemented diets exhibited reduced milk fat yield, depending on the unsaturation level, with a more pronounced decrease with SAF (i.e., a source of cis-9,cis-12 18:2), followed by SUN (i.e., a source of cis-9 18:1) and LSO (i.e., a source of cis-9,cis-12,cis-15 18:3).