1653 Metabolomics analysis reveals effect of corn silage levels on ruminal metabolic profiles in Holstein heifers.

Controlling DMI could be one of the strategies to reduce feed cost and to increase efficiency in dairy heifer growth, whereas the metabolic mechanisms involved in control feeding have not been well examined. The objective of this study was to determine the effect of differing forage-to-concentrate ratios on the ruminal metabolite profiles in heifers under restricted feeding. Twenty-four Holstein heifers (8–10 mo and 253 ± 29 kg of BW) with similar body condition were randomly assigned into four groups and fed diets containing 20% (F20), 40% (F40), 60% (F60), and 80% (F80) of corn silage. All diets were isonitrogenous and isocaloric and provided equal amount of nutrients, and allowed for 800 g/d of ADG. The amount of feed offered was adjusted weekly based on BW. Total tract apparent digestibility of nutrients was determined with acid-insoluble ash as an internal digestibility marker with the TMR and fecal samples. Rumen fluid was sampled from each cow through a stomach tube 4 h after morning feeding at d 30 and analyzed using gas chromatography-time-of-flight/mass spectrometry. The digestibilities of DM (80.9% for F20, 79.4% for F40, 77.2% for F60, and 73.1% for F80, P < 0.001) and OM (84.3%, 82.3%, 80.1%, and 76.1%, respectively, P < 0.001) decreased linearly with increased corn silage. In total, 247 metabolites were identified from all four groups. The principal component analysis of the relative concentration of mutual metabolites revealed four separated metabolite profile clusters of four groups. The clusters derived from the F40, F60, and F80 partly overlapped with each other, whereas the cluster from the F20 was separated from the other three groups. When the mutual metabolites were used for pathway analysis, the impact values of the pathway were 0.32, 0.23 and 0.21 for pyruvate metabolism, citrate cycle and lysine degradation, respectively. These three pathways may play important roles in improvement of nutrition degradation and utilization. These indicated that lower forage level could promote rumen fermentation and provide more available nutrition for heifers.