Abstract
The effect of an ionophore on the ruminal metabolome of temperate and tropically-adapted cattle was evaluated. For 21 d, five Angus, 5 Brahman, and 5 BrahmanXAngus F1 steers (264 kg BW) with 5 steers per pen within breedtype, were fed Tifton bermudagrass hay ad libitum, at the conclusion of which rumen fluid (100-to-250 mL) was collected and stored at -80°C (PRE). During a subsequent 21-d period, all steers were fed Tifton hay ad libitum plus a supplement (908 g corn, 217 g soybean meal, 60 g dried molasses) with monensin (220 mg·steer(−1)·d(−1)) individually, at the conclusion of which rumen fluid (100-to-250 mL) was collected and stored at -80°C (POST). Rumen fluid samples were processed for an untargeted analysis by LC/MS on Compound Discoverer v.3.1. Approximately 2,043 compounds were detected with 825 being annotated by ChemSpider and other databases, of which several were associated with four neurotransmitters; acetylcholine (ACh), dopamine, γ-aminobutyric acid (GABA), and serotonin. After monensin, several ACh-related molecules increased (P < 0.003) in Angus (POST:PRE ratio=7.906) but ACh decreased (P < 0.043) in the F1 (ratio= 0.391) steers. Dopamine and dopamine-related molecules decreased (P < 0.01) in Angus (ratio= 0.149) and F1 steers (ratio= 0.206) after monensin. No changes for rumen presence of ACh or dopamine occurred in Brahman steers after monensin. Multiple molecules related to GABA increased in Angus (P < 0.03) and Brahman (P < 0.02) steers. Serotonin-related molecules such as the metabolite 5-hydroxyindoleacetic acid (5-HIAA; ratio= 4.389) increased (P < 0.005) only in Brahman steers. The addition of monensin to the diet differentially altered the presence of neurotransmitter-related molecules in the rumen fluid of temperate and tropically adapted beef cattle. Further research regarding physiological roles of ruminal neurotransmitters and modulation by ionophores that affect ruminal microbiota is warranted. Support: USDA FAH