Methane mitigation potential of 3-nitrooxypropanol in lactating cows is influenced by basal diet composition

The objective of this study was to investigate whether the CH₄ mitigation potential of 3-nitrooxypropanol (3-NOP) in dairy cattle was affected by basal diet (BD) composition. The experiment involved 64 Holstein-Friesian dairy cows (146 ± 45 d in milk at the start of trial; mean ± SD) in 2 overlapping crossover trials, each consisting of 2 measurement periods. Cows were blocked according to parity, d in milk, and milk yield, and randomly allocated to 1 of 3 diets: a grass silage-based diet (GS) consisting of 30% concentrates and 70% grass silage (DM basis), a grass silage- and corn silage-mixed diet (GSCS) consisting of 30% concentrates, 42% grass silage, and 28% corn silage (DM basis), or a corn silage-based diet (CS) consisting of 30% concentrates, 14% grass silage, and 56% corn silage (DM basis). Two types of concentrates were formulated, viz. a concentrate for the GS diet and a concentrate for the CS diet, to meet the energy and protein requirements for maintenance and milk production. The concentrate for the GSCS diet consisted of a 50:50 mixture of both concentrates. Subsequently, the cows within each type of BD received 2 treatments in a crossover design: either 60 mg of 3-NOP/kg of DM (NOP60) and a placebo with 0 mg of 3-NOP/kg of DM (NOP0) in one crossover or 80 mg of 3-NOP/kg of DM (NOP80) and NOP0 in the other crossover. Diets were provided as total mixed ration in feed bins, which automatically recorded feed intake. Additional concentrate was fed in the GreenFeed system that was used to measure emissions of CH₄ and H₂. The CS diets resulted in a reduced CH₄ yield (g/kg DMI) and CH₄ intensity (g/kg milk). Feeding 3-NOP resulted in a decreased DMI. Milk production and composition did not differ between NOP60 and NOP0, whereas milk yield and the yield of major components decreased for NOP80 compared with NOP0. Feed efficiency was not affected by feeding 3-NOP. Interactions between BD and supplementation of 3-NOP were observed for the production (g/d) and yield (g/kg DMI) of both CH₄ and H₂, indicating that the mitigating effect of 3-NOP depended on the composition of the BD. Emissions of CH₄ decreased upon 3-NOP supplementation for all BD, but the decrease in CH₄ emissions was smaller for GS (−26.2% for NOP60 and −28.4% for NOP80 in CH₄ yield) compared with both GSCS (−35.1% for NOP60 and −37.9% for NOP80 for CH₄ yield) and CS (−34.8% for NOP60 and −41.6% for NOP80 for CH₄ yield), with no difference between the latter 2 BD. Emissions of H₂ increased upon 3-NOP supplementation for all BD, but the H₂ yield (g/kg DMI) increased 3.16 and 3.30-fold, respectively, when NOP60 and NOP80 were supplemented to GS, and 4.70 and 4.96 fold, respectively, when NOP60 and NOP80 were supplemented to CS. In conclusion, 3-NOP can effectively decrease CH₄ emissions in dairy cows across diets, but the level of CH₄ mitigation is greater when supplemented in a corn silage-based diet compared with a grass silage-based diet.