Gas exchange, rumen hydrogen sinks, and nutrient digestibility and metabolism in lactating dairy cows fed 3-nitrooxypropanol and cracked rapeseed.

Enteric methane production needs to be reduced in livestock, also in dairy cows. Methanogenesis by methanogens in the rumen is responsible as their fermentation results in hydrogen accumulation which is usually picked up into the CH₄ molecule as one of the end products. The compound 3-nitrooxypropanol (3-NOP), marketed as Bovaer, has been shown to inhibit the enzyme methyl-coenzyme M reductase, which is responsible for the last step in methanogenesis, and which in studies have shown a substantial reduction in methane yield. Adding extra fat to the diet has been reported to also reduce methane yield, but in a different mode of action as it dilutes fermentable organic matter, inhibits fibrolytic bacteria and methanogens, and enhances biohydrogenation. The effect of added fat on methane yield is not as pronounced as in the case of 3-NOP. To establish whether 3-NOP and fat supplementation could be additive in reducing methanogenesis, the study cited was conducted.

Fat in the form of cracked rapeseed and 3-NOP were fed alone or in combination to four Holstein multi-cannulated dairy cows, with the objective to investigate effects on gas exchange, dry matter intake (DMI), nutrient digestion, and metabolism. The study design was a 4 x 4 Latin square with a 2 x 2 factorial treatment arrangement with two levels of fat supplementation: 33g fat/kg DM or 64g fat/kg DM for low and high fat diets, respectively, and two levels of 3-NOP: 0mg/kg DM or 80mg/kg DM. In total, four diets were formulated: low fat (LF), High fat (HF), 3-NOP and low fat (3LF) and 3-NOP and high fat (3HF). The cows were fed ad libitum and milked twice daily. The adaptation period lasted eleven days, followed by twelve days with twelve diurnal sampling times of digesta and ruminal fluid. Thereafter, gas exchange was measured for five days in respiration chambers. Chromic oxide and titanium dioxide were used as external flow markers to determine intestinal nutrient flow for digestibility measurements.

No interactions between fat supplementation and 3-NOP were observed for methane yield, total-tract digestibility of nutrients or total volatile fatty acid (VFA) concentrations in the rumen. Methane yield (g/kg DMI) was decreased by 24% when cows were fed 3-NOP. In addition, 3-NOP increased carbon dioxide (CO₂) and hydrogen yield (g/kg DM). However, CO₂ production was decreased when expressed on a daily basis. Fat supplementation did not affect methane yield but tended to reduce methane as percentage of gross energy intake. An 11% decrease in DMI was observed when cows were fed 3-NOP, resulting in a 9.3% reduction in milk yield and a 6.6% reduction in energy-corrected milk; the latter because 3-NOP increased milk fat and lactose contents. The lower DMI mediated a lower passage rate causing a tendency for higher rumen and total-tract NDF digestibility, when cows were fed 3-NOP. Total VFA concentrations in the rumen were negatively affected by both 3-NOP and fat supplementation. Furthermore, 3-NOP caused a shift in the VFA fermentation profile, with decreased acetate proportion and increased butyrate proportion, whereas the propionate proportion was unaffected. There was also an increase in the alcohols in the rumen fluid, indicating together with the shift in VFA concentrations a re-direction of hydrogen into other hydrogen sinks, thereby the observed decrease in methanogenesis when cows were fed 3-NOP.

In conclusion, fat supplementation did not reduce methane yield, whereas 3-NOP reduced methane yield, irrespective of fat level. The reduction in DMI and as a result milk yield is however cause for concern in the economic context, and therefore the concentration of supplementary 3-NOP must be considered. It is also likely that dietary composition will yield different mitigation results.