Nutritional state and secondary compounds influence trembling aspen ( Populus tremuloides ) intake by lambs

The foraging ecology of mammalian herbivores is determined by plant secondary compounds (PSC) that defend plants against herbivory, by nutrients that are required for maintenance, growth, and reproduction, and by the interaction between these variables. In this study, we explored the influence of an herbivore’s nutritional state and experience on intake of aspen (Populus tremuloides) leaves with contrasting concentrations of a PSC, phenolic glycosides (PG). Thirty-two lambs were randomly assigned to two groups (N = 16). Each group received aspen leaves with either higher (21.2%) or lower (16.3%) concentrations of PG throughout the study. In Experiment 1, half of the lambs within each group received rations with a high (22.2% crude protein [CP]; 3.13 Mcals/kg) or low (11.9% CP; 3.13 Mcals/kg) concentration of CP. During Experiment 2, half of the lambs within each group received rations with a high (3.45 Mcals/kg; 16.1% CP) or low (2.52 Mcals/kg; 18.5% CP) concentration of energy. During Experiment 3, all lambs received a simultaneous offer of aspen leaves containing higher (20.6%) and lower (14.8%) concentrations of PG. Aspen intake was greater when lambs received the high-protein or the low-energy rations (P