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Plant secondary compounds (PSC) are increasingly recognized as important in animal health, welfare and nutrition, yet little is known about possible complementarities among forages that contain PSC. This study explored how sheep modify their foraging behaviour in pastures containing four forages with different PSC when challenged with intraruminal administration of a single PSC. Six pairs of lambs received a dose of a single PSC in three successive periods (1 – Condensed tannins, 2 – Saponins, and 3 – Ergotamine) of 5 d each. Six other pairs of lambs received just the vehicle. After the early-morning dosing, lambs were allowed to graze a choice of: 1 –Lotus corniculatus [birdsfoot trefoil (BFT)], 2 –Medicago sativa [alfalfa (AA)], 3 –Festuca arundinacea [endophyte-infected tall fescue (TF)], high in tannins, saponins and alkaloids, respectively, and 4 –Dactylis glomerata [orchard grass (OG)]. The foraging behaviour of lambs was recorded at 1-min intervals (scans). Lambs dosed with tannins had more scans than Controls on BFT (P = 0·002) and TF (P = 0·004). Lambs dosed with saponins had more scans than Controls on TF (P = 0·01), and animals treated with ergotamine had more scans than Controls on BFT (P = 0·03). Lambs showed the lowest proportion of scans on AA after a dose of saponin (P ? 0·09) and on TF after a dose of ergotamine (P ? 0·009). Thus, lambs minimized the negative impacts of PSC through changes in their foraging behaviour by avoiding over-ingesting any one PSC and by selectively increasing the preference for complementary PSC-containing forages.