Effects of feeding level on enteric methane emissions and utilisation of energy and nitrogen in dry ewes of two genotypes offered fresh ryegrass

The objective of the present study was to evaluate the effect of feeding levels on enteric methane (CH₄) emissions and nitrogen (N) and energy utilisation efficiencies of ewe sheep offered fresh ryegrass. Twenty-four dry ewes (16 mo old and 47.6 ± 5.1 kg body weight) were used in a factorial design study, with 2 genotypes (12 ewes/genotype, Belclare vs. Lleyn) × 3 feeding levels (maintenance level vs. ad libitum vs. intermediate level (targeted level = 1.5 maintenance)). All sheep were offered fresh grass harvested daily in the morning from the second regrowth sward. All animals were housed in individual pens for 19 d and then in individual respiration chambers for further 5 d with feed intake, faeces and urine outputs and CH₄ emissions measured during the final 4 d. There was no significant interaction between genotype and feeding level on any variable evaluated, except for CH₄ emissions with which the interaction was significant (P < 0.01). Sheep offered diets at the maintenance level had significantly higher digestibilities of dry matter (DM), organic matter (OM), neutral detergent fibre, acid detergent fibre and energy than those given at intermediate level or ad libitum (P < 0.001). However, increasing feeding level significantly decreased CH₄ emissions as a proportion of nutrient intake (DM, OM, digestible DM and digestible OM) or energy intake (gross energy (GE), digestible (DE) and metabolisable energy (ME)), and N loss in urine and manure as a proportion of N intake (P < 0.01). Belclare ewes had greater ME/GE and ME/DE and lower CH₄ energy/ME intake, urine N/N intake and manure N/N intake than Lleyn ewes (P < 0.05). The present data were also used to develop a range of prediction equations for CH₄ (R² = 0.274 to 0.800, P < 0.001) and manure N (R² = 0.410 to 0.915, P < 0.001) emissions using feed intake. The present result indicates that optimizing grazed grass intake is an effective mitigation strategy to reduce the environmental footprint of grazing sheep production in term of per unit of feed intake.