Partition of dietary energy by sheep fed fresh ryegrass (Lolium perenne) with a wide-ranging composition and quality

Context Perennial ryegrass (Lolium perenne L.) is an important forage in temperate ruminant agricultural systems, but its quality and composition is highly variable and can become dominated by senescent material during summer and dry periods. Nutritive value is often predicted on the basis of chemical composition and calculated metabolisable energy (ME), which is rarely measured. Aim To measure the partition of energy from ryegrasses with widely varying proportions of leaf and senescent material, among faeces, urine and methane (CH4) in sheep. Methods Three experiments were undertaken with two groups of young wether sheep, fed seven diets of freshly cut ryegrass, at maintenance levels of intake to measure the partition of dietary energy among faeces, urine and CH4, enabling ME to be calculated. Across the seven diets, green leaf accounted for 16.0-87.6% of feed offered, whereas senescent material ranged from 2.8% to 65.0% and nitrogen (N) ranged between 8.6 and 31.2 g/kg DM. Measured ME was compared with predicted values based on digestible energy. Methane emissions were determined by both open-circuit respiration chambers and sulfur hexafluoride marker dilution. Key results Apparent digestibility (g/100 g) of DM ranged from 51.8 to 75.3 and N from 26.7 to 73.9. The percentage of the gross energy of feeds partitioned to CH4 (chamber) was 6.01-7.42, and 1.77-5.69 to urine. Effects of digestibility on CH4 yields (g/kg DM intake) were minor and the sum of energy losses to CH4 and urine were 13.5-17.6% of digestible energy (DE) intake, suggesting an underestimation of ME when predicted from DE. Use of sulfur hexafluoride substantially overestimated CH4 emissions when the permeation tubes had been in the sheep for 80 days. Conclusions A high proportion of senescent material in ryegrass had little effect on CH4 yields from sheep and energy losses to urine were mostly less than 3% of energy intake and were related to dietary N concentration. Implications Dietary ME calculated from DE is likely to be underestimated when ryegrass contains a high proportion of senescent material, due to reduced urinary energy losses, as CH4 yields are largely unaffected by maturity or senescence.