Investigating interactions between grass silage- or corn silage-based diets and 3-nitrooxypropanol on fermentation and methane emission dynamics and rumen microbiome in dairy cattle.

The objective of this study was to investigate mechanisms underlying the relationship between basal diet (BD) composition (grass silage- vs. corn silage-based) and 3-nitrooxypropanol (3-NOP) supplementation, focusing on the rumen microbiome, and gaseous emission and ruminal fermentation dynamics. Eight rumen-fistulated, multiparous Holstein-Friesian dairy cows producing 29.3 ± 8.17 kg/d of milk at 159 ± 82.8 DIM (mean ± SD) at the beginning of the experiment were blocked according to parity, DIM, and milk production, and randomly assigned to a double 4 × 4 Latin square design with 4 treatments. The 4 dietary treatments were arranged according to a 2 × 2 factorial design, with 2 BD and 2 dosages of 3-NOP. The grass silage-based diet (GS) consisted of 33.5% concentrate and 66.5% grass silage, whereas the corn silage-based diet (CS) consisted of 33.1% concentrate, 12.7% grass silage, and 54.2% corn silage (all on DM basis). Both BD were supplemented with either 0 or 80 mg 3-NOP/kg DM. Treatment periods lasted 17 d and consisted of a 14-d adaptation period in a freestall barn followed by a 3-d measurement period in climate respiration chambers. No significant interaction between BD and 3-NOP on CH₄ emission was observed, although numerically CH₄ emissions were more strongly reduced, and H₂ emissions more strongly increased, upon 3-NOP supplementation with the CS diet (-25.9% for CH₄ yield [g/kg DMI], -26.9% for CH₄ intensity [g/kg ECM], +9.2-fold for H₂ yield [g/kg DMI], and + 8.9-fold for H₂ intensity [g/kg ECM]) compared with the GS diet (-12.9% for CH₄ yield [g/kg DMI], -14.8% for CH₄ intensity [g/kg ECM], 6.9-fold for H₂ yield [g/kg DMI], and 6.7-fold for H₂ intensity [g/kg ECM]). An interaction between BD and 3-NOP was observed for the apparent total-tract digestibility (ATTD) of DM, OM, and gross energy (GE), which were lower for CS compared with GS, but only with 0 mg 3-NOP/kg DM. The ATTD of CP was greater for CS compared with GS, but only with 80 mg 3-NOP/kg DM. Replacing grass silage with corn silage increased DMI and milk protein content, whereas milk fat content decreased, and milk production remained unaffected, resulting in a lower feed efficiency. No effect of BD on ruminal pH, ruminal VFA (except for molar proportion of isobutyrate), or CH₄ emissions was observed. Although DMI decreased, none of the lactation characteristics were affected by 3-NOP. Methane yield and intensity decreased by 18.4% and 19.4%, respectively, and H₂ yield and intensity increased by 7.6-fold and 7.8-fold, respectively, with 3-NOP. A shift in ruminal VFA was observed from acetate to propionate and butyrate. In line with this shift, 3-NOP decreased methanogenesis and the relative abundance of methanogens, whereas the Wood-Ljungdahl pathway increased, suggesting enhanced acetogenesis.