Rapeseed cake or oil use in a mixed forage and concentrate diet for growing cattle: manure biomethane yield, digestibility, nitrogen balance, and rumen/faecal microbiota

Rapeseed cake and oil elicit anti-methanogenic effects when included in ruminant diets. However, there is a paucity of information on their effect on diet digestibility and the potential methane ( CH 4 ) yield from manure. Analysis of the rumen and faecal microbiota helps identify microbes of interest to gaseous emissions reductions. The present work consists of two studies. Study one: Total tract digestibility, nitrogen balance and faeces biomethane potential of Charolais-cross growing cattle supplemented with rapeseed cake or oil in a forage and concentrate diet. Study two: The rumen and faecal microbiota of cattle fed rapeseed oil compared to unsupplemented contemporaries. Study one: twelve 15-month-old Charolais-cross steers with an initial BW of 503 kg (SD 28) were offered a 40:60 forage-to-concentrate diet on a DM basis with concentrates containing an unsupplemented control ( CON ), rapeseed cake ( RSC ), or rapeseed oil ( RSO ). Grass silage was offered ad libitum . Rapeseed cake and oil constituted 18.5% and 3.2% of DM intake. Cattle were housed in metabolism stalls over 10 days where nutrient intake, faecal and urine measurements, and faeces biomethane potential ( BMP ) were recorded. Study two: Metataxonomic analyses of the prokaryotic and eukaryotic components of rumen digesta and faecal samples collected on day 71 of a preceding study from the CON and RSO groups (n = 12 per treatment) were performed. Study one: There were no treatment differences for DM, organic matter, NDF, ADF or CP digestibility ( P > 0.05). RSC and RSO groups had greater ether extract digestibility than CON ( P < 0.001). There was no treatment differences in nitrogen use efficiency ( P = 0.21). Faeces from RSC and RSO groups had a 53% higher BMP than CON ( P = 0.01). Study two: The rumen and faecal bacterial composition of RSO-fed cattle was different to CON ( P < 0.05). The relative abundance ( RA ) of four bacterial amplicon sequence variants ( ASV ) (mostly fibrolytic) in a mixture of solid and liquid rumen digesta and one fibrolytic ASV in the faeces were lower in RSO (q value; adjusted ( adj. ) P < 0.05). Two rumen (amylolytic and fatty acid regulation) and two faecal bacterial ASVs (fibrolytic and fatty acid regulation) were higher in RSO (adj. P < 0.05). Three rumen archaea ASVs had lower RA in RSO (adj. P < 0.05). Treatment had no effects on fungi and protozoal communities. Faeces from rapeseed-supplemented cattle could increase CH 4 emissions from stored manure but could be an enhanced anaerobic digestion feedstock.