Effects of ruminal digesta retention time on methane emissions: a modelling approach

The reasons for among-animal variations in methane (CH4) emissions are not fully understood. There is experimental evidence that ruminal digesta mean retention time (MRT) can affect CH4 emissions. The objective of the present study was to evaluate the contribution of among-animal variations in MRT on CH4 emissions and nutrient supply for dairy cow (default MRT = 34 h) and sheep (default MRT = 41 h), using the mechanistic Nordic dairy cow model Karoline. The simulations (n = 100) were made for a cow (bodyweight 600 kg) and for a sheep (bodyweight 60 kg) eating 20 kg and 1.0 kg DM/day, respectively. The diet for the dairy cow consisted of grass silage, barley and rapeseed meal (60:30:10 on a DM basis; crude protein 156 g/kg DM, neutral detergent fibre 450 g/kg DM) and the sheep diet was grass alone. Normal distribution of MRT values was assumed. Variability (coefficient of variation (CV) = 0.086) on default MRT was introduced by random-number generator of Excel. Intake, diet composition and digestion kinetic parameters were constant in all simulations, only ruminal MRT variables were changed in each simulation. Predicted CH4 emission increased with increased MRT for dairy cow (range from 407 to 488 g/day) and sheep (from 25.0 to 29.2 g/day). Increases in predicted CH4 emissions were partly associated with enhanced organic matter (OM) digestibility in dairy cow (from 0.715 to 0.758) and sheep (from 0.731 to 0.773). Greater CH4 emissions per kilogram digested OM with increased MRT were mainly related to reduced efficiency of microbial cell synthesis in the rumen both for dairy cows (22.8 +/- 0.91 g N/kg OM truly digested; CV = 0.040) and for sheep (20.7 +/- 0.92 g N/kg OM truly digested; CV = 0.044). Predicted CH4 yield was 20% and 17% greater in dairy cow and sheep, respectively, with the short (n = 10) compared with the long (n = 10) ruminal digesta MRT. Linear regression indicated that CH4 emissions increased by 0.37 (dairy cow) and 0.33 (sheep) g/kg DM intake per 1 h increase in ruminal digesta MRT. It is concluded that among-animal variation in MRT can markedly contribute to among-animal variation in CH4 emissions from ruminants.