Modelling methane emissions from ruminant diets with variable forage-to-concentrate ratios and retention times – An in vitro evaluation

This study aimed to improve the predictive accuracy of in vitro models for estimating in vivo methane (CH4) emissions in Nordic dairy systems by evaluating five forage-to-concentrate (F:C) ratios and incorporating a modelling approach based on ruminal mean retention time (MRT). The tested ratios included 100:0 (100 F), 80:20 (80 F), 60:40 (60 F), 40:60 (40 F), and 20:80 (20 F), where 100 F consisted solely of grass silage, and the remaining diets incorporated barley grain and rapeseed meal as concentrate. All diets were balanced for crude protein (20 % DM), but ether extract and neutral detergent fiber content decreased as concentrate levels increased. To improve the biological relevance of in vitro results, CH4 production was corrected using a ruminal MRT model to better simulate in vivo conditions. Higher concentrate inclusion linearly increased (P < 0.001) total gas and predicted in vivo CH4 production. However, after applying MRT adjustments, the modified model reduced the variation in CH4 predictions across F:C ratios, resulting in values that more closely reflected expected in vivo emissions. The pH declined (P < 0.001) at lower F:C ratios. Organic matter degradability (OMD) followed a quadratic pattern (P < 0.001), peaking in 60 F and 40 F diets and decreasing in 100 F and 20 F. While total volatile fatty acid concentrations were unaffected by F:C ratio, acetate proportion declined linearly (P < 0.001) as concentrate increased, whereas isobutyric and butyric acid proportions rose. Overall, these findings support the application of MRT-adjusted models to enhance the alignment between in vitro predictions and in vivo CH4 emissions.