Hepatic metabolites indicate differences during late mid-lactation in Holstein cows with different levels of pasture inclusion

The objective of this study was to explore hepatic metabolic adaptations in mid-lactation Holstein cows managed under feeding strategies with different levels of pasture inclusion. Sixteen multiparous North American Holstein cows were assigned from calving to 180 days in milk (DIM) to either a fixed pasture strategy (FixP; n = 8), in which grazed pasture represented approximately one-third of dry matter intake and the remainder was provided as total mixed ration (TMR), or a maximum pasture strategy (MaxP; n = 8), in which pasture intake was maximized and cows were supplemented with concentrate and conserved forage according to pasture availability. At 180 ± 20 DIM, plasma samples and liver biopsies were collected for biochemical analyses, quantitative PCR, and targeted liver metabolomics by gas chromatography time-of-flight mass spectrometry. Milk yield, milk components, and body condition score did not differ between feeding strategies. Plasma urea nitrogen was greater in MaxP than FixP cows (6.64 vs. 4.96 mmol/L; P = 0.01). In liver, FixP cows showed greater abundance of metabolites related to carbohydrate metabolism, including phosphoenolpyruvate, fructose-6-phosphate, glucose-6-phosphate, and sucrose, together with greater expression of genes related to the pentose phosphate pathway and fatty acid synthesis, including glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, ribose-5-phosphate isomerase A, acetyl-CoA carboxylase alpha and fatty acid synthase (p ≤ 0.10). In contrast, MaxP cows showed greater abundance of metabolites associated with nitrogen metabolism, including citrulline, ornithine, glutamine, and creatinine. These results indicate that, during mid-lactation, greater pasture inclusion is associated with enhanced hepatic nitrogen metabolism, whereas partial replacement of pasture with TMR is associated with greater hepatic carbohydrate-related metabolism and fatty acid synthesis.