Effect of 3-nitrooxypropanol supplementation combined with 6-hour grazing on enteric methane emissions and milk production characteristics

Various studies have shown that 3-nitrooxypropanol (3-NOP) reduces methane emissions in dairy cows. Providing 3-NOP in a partial mixed ration or TMR is recommended to continuously expose the rumen to the additive. During grazing, there is temporarily no exposure to 3-NOP. Therefore, to use 3-NOP as a methane reduction measure in a grazing system, the effect of providing 3-NOP through a mixed ration provided inside the barn only on methane emissions needs further research. Therefore, an incomplete Latin square (balanced crossover) design with 40 lactating Holstein-Friesian cows (average milk yield of 39.8 ± 7.0 kg/d per cow, mean BW 691 ± 63.6 kg, average lactation stage of 122 ± 56 DIM, and a mean parity of 2.6 ± 1.2) was set up including 3 treatment periods and 4 treatments. The treatments consisted of full-time indoor housing with a placebo additive (A), full-time indoor housing with 80 mg 3-NOP/kg DMI (B), 6-h grazing with a placebo additive (C), and 6-h grazing with 80 mg 3-NOP/kg DMI (D). The 24 possible treatment sequences were randomized over the cows in a balanced way based on individual cow information including parity, BW, lactation stage, fat- and protein-corrected milk yield, roughage feed intake, and concentrate intake. The actual 3-NOP concentration in the total diet was 77 mg/kg in treatment B and 80 mg/kg in treatment D. Animal performance, fresh grass intake, methane emissions, and nutritional value of the feed were monitored throughout the experiment. Results showed that DMI was not different between treatments with the same housing conditions, but was significantly lower (-1.4 kg DM/d) for 6-h grazing compared with full-time indoor housing. The milk yield of grazing treatments was significantly lower (-1.6 kg/d) than full-time housed cows receiving the placebo, whereas the milk yield of full-time housed cows receiving 3-NOP was not different from the full-time housed placebo treatment, nor the grazing placebo treatment. However, fat- and protein-corrected milk yield was significantly lower in grazing treatments compared with full-time housed treatments, with -2.57 kg/d for 3-NOP treated cows and -2.03 kg/d for placebo treated cows. The absolute CH₄ emission was lower in grazing treatments (-7.2% for placebo and -12.3% for 3-NOP) compared with full-time housed treatments with similar additive treatments, but clearly lower in 3-NOP treatments versus placebo treatments (-25.1% in no-grazing and -29.2% in 6-h grazing treatments).