BACKGROUND: We hypothesized that eating a food containing saponins (SAP), or tannins (TAN) prior to foods containing the alkaloids gramine (GRA) or 5-methoxy-N,N-dimethyltryptamine (TRP) would provide benefits not possible when the alkaloid-containing foods were eaten alone. METHODS: In Trial 1, four groups of five lambs were first offered food with SAP for 30 min followed by food with either GRA or TRP for 3.5 h in a 2 × 2 factorial arrangement of a completely randomized design that included alkaloid (GRA or TRP) with or without SAP. In Trial 2 TAN replaced SAP. All foods were isocaloric (3.3 Mcal kg−1) and isonitrogenous (14% crude protein). Foods, fecal and urine samples were collected and analyzed for dry matter intake and apparent digestibility of dry matter, energy (in megajoules, MJ), nitrogen (N), and neutral detergent fiber. RESULTS: Supplemental SAP did not affect digestibility of the parameters tested (P > 0.10). Supplemental TAN increased digestibility of N (g kg−1, P = 0.04), N retained (g day−1, P = 0.07), N digested (g day−1, P = 0.06), and N retained/N consumed (g kg−1, P = 0.07). However, digestibilities of dry matter (g kg−1, P = 0.0026), energy (MJ 1000 MJ−1, P = 0.003), neutral detergent fiber (g kg−1, P = 0.008), and digested N retained (g kg−1, P = 0.07) were lower for lambs fed TAN than for unsupplemented animals. CONCLUSIONS: Tannin supplementation can improve retention of nitrogen in animals fed alkaloid-containing grasses such as reed canarygrass and tall fescue. Combinations of forages with complementary primary and secondary compounds enable animals to maintain intake and improve nutrient utilization. Copyright © 2012 Society of Chemical Industry