Early life treatment with probiotic <i>Lacticaseibacillus rhamnosus</i> strains drives sustained reductions in enteric methane emissions in growing dairy heifers

<p dir="ltr"><b>Background</b>. Methane (CH₄) is the world’s second most abundant greenhouse gas, with 27% of anthropogenic CH₄ emissions resulting from enteric fermentation in ruminant livestock. Lactic acid bacteria (LAB) are natural inhabitants of the mammalian gastrointestinal tract and have been used in livestock as direct-fed microbial products to promote gut health, but their ability to influence CH4 emissions from ruminant animals is poorly understood.</p><p dir="ltr"><b>Methods</b>. More than 1700 LAB were screened against the rumen methanogens <i>Methanobrevibacter boviskoreani</i> JH1, <i>Methanosphaera</i> sp. WGK6, <i>Mbb. ruminantium</i> M1 and <i>Mbb. gottschalkii</i> D5 in high-throughput, pure culture assays and rumen in vitro incubations to assess their anti-methanogenic potential. Freeze-dried preparations of the best performing LAB strains were fed daily to Friesian heifer calves from birth to 14 weeks and their CH₄ emission were monitored in respiration chambers up to 12 months of age.</p><p dir="ltr"><b>Results</b>. The pure culture methanogen and rumen in vitro screens identified <i>Lacticaseibacillus rhamnosus</i> FNZ118 (Kowbucha™ FNZ118) and <i>L. rhamnosus</i> FNZ142 (Kowbucha™ FNZ142), as capable of reducing CH4 production. FNZ118 or FNZ142 strains fed daily to Friesian heifer calves for the first 14 weeks of life significantly lowered CH₄ production at 6 weeks, 14 weeks, 9 months and 12 months of age compared to control animals. The lower CH₄ production was driven by decreased dry matter intake and reduced ruminal metabolite concentrations, but without affecting live weight, suggesting an improvement in feed conversion efficiency. The observed effects did not cause major changes in the structure of the rumen microbiome.</p><p dir="ltr"><b>Conclusions</b>. These findings demonstrate that early life provision of specific <i>L. rhamnosus</i> strains lower CH₄ production and have potential to mitigate enteric greenhouse gas emissions from growing dairy cattle.</p>