288 Fueling the microbiome: How dietary energy and protein influence rumen and milk microbial communities in transition dairy cattle.

Abstract Our research aimed to optimize rumen stability and support milk performance in transition dairy cows by evaluating dietary strategies that could enhance rumen fermentation without causing ruminal dysbiosis. Specifically, we investigated the effects of permeate supplementation, as a low oxidative potential energy source, and reduced metabolizable protein (MP) intake during the transition period on both rumen and milk microbiomes, and lactation performance. Two experiments were conducting using pregnant cannulated Holsteins, with treatment diets provided from two weeks pre-calving to four weeks post-calving. In the energy trial, during the entire experimental period, treatment cows received either milk permeate (n = 6) or water (n = 6) ad libitum, with liquid intake measured daily. In the reduced MP trial, cows received either 80% of recommended MP (n = 5) or 100% MP (n = 6), with all cows being returned to 100% MP from weeks 2 to 4 post partum. In both trials, rumen content and milk samples were collected at weeks 1 and 4 post-partum for microbiome analysis using metagenomic techniques. Dry matter intake (DMI), and milk production were recorded daily and composition was analyzed for each week post-partum. Diets were iso-nitrogenous in the energy trial and iso-energetic in the protein trial. Permeate-fed cows had lower DMI and higher liquid intake, but milk yield was unaffected. Rumen microbial diversity (Shannon; p ≤ 0.0001) and richness (Chao1; p ≤ 0.0001) were reduced in permeate-fed cows, with increased Lachnospiraceae and decreased Prevotella and Ruminobacter abundances. The milk microbiome was largely unchanged, except for a fivefold increase in Bifidobacterium at week 4. Milk urea levels were significantly lower in permeate-fed cows (p = 0.001), with no other compositional changes. In the reduced MP trial, DMI remained stable, but milk production tended to decrease during the treatment period (p = 0.09). The rumen microbiome was unaffected, while Micrococcaceae were differentially abundant in milk (FDR = 2.181e-1) in the short term. After two weeks of 100% MP restoration, no differences were observed in milk production or microbial composition. Overall, milk permeate supplementation altered the rumen microbiome without reducing milk yield, while reduced MP intake temporarily decreased milk production but had no lasting effects on milk performance. These findings suggest that nutrient management strategies during the transition period can modulate rumen microbial communities to support a stable gastrointestinal ecosystem without compromising lactation performance. These strategies offer potential benefits for improving digestive health of transition cattle in Canada without compromising productivity.