Ranking lambs by feed efficiency – are there changes in ruminal fermentation parameters and enteric methane emissions?

Abstract This study investigated the influence of feed efficiency on ruminal fermentation parameters and methane (CH 4 ) emissions in Santa Inês lambs. Forty intact male lambs were classified into three feed efficiency groups based on residual feed intake (RFI) and weight gain (RIG): low (RFI + and RIG-), medium (RFI ± and RIG±), and high (RFI- and RIG+). The experimental trials were conducted in two phases with controlled feeding and CH 4 emissions measured in individual chambers. Ruminal fermentation was assessed through ruminal fluid sampling and protozoa counting. Data were analyzed using repeated measures mixed models. No significant differences ( P > 0.05) were observed among groups for dry matter intake, body weight, and CH 4 emissions. Numerically, high-efficiency lambs (RIG+) emitted 15.2% less CH 4 per kg of dry matter intake, highlighting the relationship between lower emissions and higher efficiency. The lack of variation in fermentative parameters suggests that a uniform diet may have limited significant differences among groups with varying efficiency. In addition, these results indicate that RFI and RIG classifications are sensitive to the animals’ life stage and may show limited consistency across evaluation periods.