Lupinus angustifolius seed meal supplemented to dairy cow diet improves fatty acid composition in milk and mitigates methane production

•L. angustifolius (lupine) seed meal (LSM) was fed to dairy cows at 2 kg/day/cow.•Dietary LSM did not affect milk yield, but increased beneficial fatty acid content.•LSM lowered ruminal total volatile fatty acid concentration and acetate proportion.•Dietary LSM decreased rumen archaeal population and methane production.•Feeding of LSM has economical, environmental and nutritional benefits in dairy cows. Recently, the narrow-leafed, blue lupine (Lupinus angustifolius) seeds, including the Tango variety, have been used as a potential feed of livestock animals. However, there is still a limited knowledge of possible action of the Tango lupine seed meal (LSM) on ruminal fermentation, enteric methane production, milk yield and composition in dairy cattle. Thus, the present study aimed to investigate the effect of LSM supplementation to dairy cows on ruminal enteric methane production, microbial population, ruminal biohydrogenation and fatty acid (FA) proportion in the ruminal fluid and milk. An in vitro trial comprising of control and five experimental groups (supplementation of LSM at 20, 40, 60, 80, and 100 g/kg diet) was conducted to determine an effective dose level for an in vivo experiment. The in vitro trial was followed by an in vivo experiment conducted on 60 high-yielding Polish Holstein Friesian dairy cows assigned into two groups: the control (n = 30) and the experimental LSM group (n = 30; 2 kg LSM/day/cow). Supplementation of LSM decreased methane emission and archaeal abundances in vitro and in vivo and increased ammonia level in vivo. Ruminal total volatile fatty acid concentration and acetate proportion were reduced by LSM supplementation both in vitro and in vivo. Milk yield and its basic composition were not influenced by the LSM. However, the LSM increased the unsaturated FA content and decreased n6 to n3 FA ratio in milk. Supplementation of LSM upregulated the mRNA expressions of FASN and ELOVL5 genes and downregulated the LPL gene expression in milk somatic cells. The daily cost of LSM diet and the cost of 1 kg of milk production were lower compared with the control. It is concluded that LSM supplementation may improve milk quality without affecting milk yield and has the potential to decrease methane production in dairy cows.