Birdsfoot trefoil (Lotus corniculatus L.), a deep-rooted perennial legume, is productive and persistent under irrigation in the slightly alkaline soils and cool, dry climate of the northern Mountain West. The tannins in birdsfoot trefoil prevent bloat and improve the efficiency of protein use and productivity of ruminants. Previous reports of seasonal fluctuation in the concentration of tannins in birdsfoot trefoil foliage were ambiguous. This study was carried out near Logan, Utah and was used to determine the herbage tannin concentration of three birdsfoot trefoil cultivars (Norcen, NC83-HT, and Bokor) that differed in tannin concentration; one alfalfa cultivar (Spredor 4) was used as a control. Herbage from three replications was sampled at 2-week intervals from mid-May until late August. All plots were managed for hay production, and were clipped at 6-week intervals, in early June, mid-July and late August. Samples were frozen in the field, freeze-dried, ground to 0.5 mm, and analyzed for extractable tannin using a purified birdsfoot trefoil tannin standard. Tannin concentrations differed by cultivar and by season. The tannin concentration of all birdsfoot trefoil cultivars increased between early June and mid-July, and reached their highest concentration in mid-July. Tannin concentrations in the first (early June to mid-July) and second (mid-July to late August) regrowth cycles were higher than in spring growth (mid-May to early June). Lignin concentration of the three birdsfoot trefoil cultivars ranked in the same order as tannin concentration, and was higher than for alfalfa. The NDF digestibility of BFT was lower than for alfalfa, and NDF digestibility was negatively correlated with tannin concentration. The tannin concentration of Norcen birdsfoot trefoil also grown near Logan, Utah and sampled before grazing in rotationally stocked pastures did not increase in mid-summer but remained low throughout the season. These tannin concentration results will inform the management of birdsfoot trefoil for grazing and hay production, particularly where the goal is to maximize tannin intake by ruminants.