Tannin Supplementation Modulates the Grazing Behavior and Metabolomic Profiles of Beef Cattle

Because plant tannins can modulate rumen function and herbivore behavior, we tested whether supplemental tannins impact the foraging strategies and metabolic signatures of beef cattle grazing smooth bromegrass pastures. Twenty-four Angus cow-calf pairs were allocated to six 3.6-ha paddocks (four pairs/paddock), randomly assigned to Control (Ctrl; n = 3) or Tannin treatment (TT; n = 3). Animals received 1 kg/cow/day of dried distillers' grains with solubles (DDGS), with TT receiving an additional 0.4% tannins (2:1 condensed: hydrolizable). Grazing occurred during four 15-day periods (July-September) across two years using the same 24 cows. Foraging behavior classified using hierarchical Hidden Markov Models (HMM), and, along with habitat-resource use and targeted UPLC-MS/MS metabolomic data, were analyzed using mixed-effects models. Grazing behaviors classified using hierarchical HMM achieved 97% accuracy for state and 86.2% for behavioral-state classification. Tannin supplementation did not affect diel or collar-derived thermal exposure patterns (p > 0.05) but increased walking-to-grazing transitions (p = 0.04), enhancing foraging continuity as grazing peaked from dusk, while resting and rumination remained stable. TT cattle moved more toward water and supplement, used more NDVI-rich patches, even within steeper slopes (differences = 0.33, 1.10, and 0.41 ± 0.005; p < 0.04, respectively). Although no overall treatment differences were detected in metabolite concentrations after Benjamini-Hochberg FDR adjustment (FD p > 0.05), Random Forest permutation-importance ranking (RFPI) identified hydroxytyrosol 4-O-glucoside, DIMBOA, 7,8-dihydroxyflavone, and catechin gallate as the strongest exploratory predictors of tannin exposure (RFPI scores > 0.05). These metabolites are associated with antioxidant activity, microbial transformation, and nitrogen-related metabolism, suggesting potential metabolic responses to protein-tannin supplementation. This adaptation improved nutrient use, selective grazing, and terrain exploration, linking metabolism and foraging behavior to promote beef cattle performance and rangeland sustainability.