Lactobacillus spp. supplementation under grazing conditions: Methane and milk production responses during early...

Both in vitro and in vivo studies have shown the potential of Lactobacillus spp. to mitigate enteric methane. However, their mitigation impact on early-lactation dairy cows grazing high-pasture diets has not been studied. This study evaluated the effect of supplementing freeze-dried or liquid Lactobacillus spp. products on enteric methane emission from early-lactation dairy cows offered a high-forage diet. Forty spring-calving Holstein Friesian dairy cows, grazing a perennial ryegrass-based pasture (PRG; Lolium perenne L.), milked twice daily, were randomly allocated to 1 of 3 treatment diets offered for 35 d: (1) CON, control, a basal diet (n = 14); (2) FLA, basal diet plus freeze-dried Lactobacillus spp. (n = 13) delivering 5.0 × 10¹⁰ cfu/milking; or (3) LLA, basal diet plus liquid Lactobacillus spp. (n = 13) delivering 5.75 × 10¹⁰ cfu/milking. The basal diet comprised 6.9 kg DM per day of a grain mix plus PRG pasture at an allowance of ∼25 kg DM/cow per day. Covariate observations were made for the first 5 d of the 40-d experiment, before commencing treatments. During the last 5 d of the experiment, individual forage DMI for each cow was estimated using the n-alkane technique, and methane emissions from individual cows were measured using the modified sulfur hexafluoride (SF₆) tracer technique. Data were analyzed using a completely randomized design-based analysis of covariance. Methane production (g/d), yield (g/kg DMI), and intensity (g/kg ECM) were not affected by the inclusion of either form of Lactobacillus spp. in the diet, although a numerical reduction of 6% in methane yield was observed in the FLA group compared with the CON group. Methane emissions from the CON cows were ∼38% lower than those estimated using inventory factors. This is likely due to the low fiber concentration and low DM concentration of the PRG offered, likely resulting in a high ruminal passage rate, reducing ruminal retention time and extent of fermentation. The high passage rate could also have restricted the colonization potential of supplemented Lactobacillus spp. This, combined with the large rumen volume of dairy cows and low dose rate, may partly explain the limited methane mitigation. Notably, production responses to supplementation with Lactobacillus spp. have been more evident in small ruminants, suggesting species-specific differences. These findings highlight the need for tailored probiotic strategies, particularly in pasture-based dairy systems.