Effects of 3-nitrooxypropanol and varying concentrate feed proportions in the ration on methane emission, rumen fermentation and performance of periparturient dairy cows.

The climate-relevant enteric methane (CH₄) formation represents a loss of feed energy that is potentially meaningful for energetically undersupplied peripartal dairy cows. Higher concentrate feed proportions (CFP) are known to reduce CH₄ emissions in cows. The same applies to the feed additive 3-nitrooxypropanol (3-NOP), albeit through different mechanisms. It was hypothesised that the hydrogen not utilised for CH₄ formation through the inhibition by 3-NOP would be sequestered by propionate formation triggered by higher CFP so that it could thereby give rise to a synergistically reduced CH₄ emission. In a 2 × 2-factorial design, low (LC) or high (HC) CFP were either tested without supplements (CON_LC, CON_HC) or combined with 3-NOP (NOP_LC, 48.4 mg/kg dry matter (DM); NOP_HC, 51.2 mg 3-NOP/kg DM). These four rations were fed to a total of 55 Holstein cows from d 28 ante partum until d 120 post partum. DM intake (DMI) was not affected by 3-NOP but increased with CFP (CFP; p < 0.001). CH₄/DMI and CH₄/energy-corrected milk (ECM) were mitigated by 3-NOP (23% NOP_LC, 33% NOP_HC) (p < 0.001) and high CFP (12% CON, 22% 3-NOP groups) (CFP × TIME p < 0.001). Under the conditions of the present experiment, the CH₄ emissions of NOP_LC increased to the level of the CON groups from week 8 until the end of trial (3-NOP × CFP × TIME; p < 0.01). CO₂ yield decreased by 3-NOP and high CFP (3-NOP × CFP; p < 0.001). The reduced body weight loss and feed efficiency in HC groups paralleled a more positive energy balance being most obvious in NOP_HC (3-NOP × CFP; p < 0.001). ECM was lower for NOP_HC compared to CON_HC (3-NOP × CFP; p < 0.05), whereas LC groups did not differ. A decreased fat to protein ratio was observed in HC groups and, until week 6 post partum, in NOP_LC. Milk lactose and urea increased by 3-NOP (3-NOP; p < 0.05). 3-NOP and high CFP changed rumen fermentation to a more propionic-metabolic profile (3-NOP; CFP; p < 0.01) but did not affect rumen pH. In conclusion, CH₄ emission was synergistically reduced when high CFP was combined with 3-NOP while the CH₄ mitigating 3-NOP effect decreased with progressing time when the supplement was added to the high-forage ration. The nature of these interactions needs to be clarified.