Investigating interactions between grass silage- or corn silage-based diets and 3-nitrooxypropanol on fermentation...

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 dynamics of gaseous emissions and ruminal fermentation parameters. Eight rumen-fistulated, multiparous Holstein-Friesian dairy cows producing 29.3 ± 8.17 kg/d of milk at 246 ± 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.