Coping with shrub secondary metabolites by ruminants

Rangelands throughout the world contain varying but often substantial proportions of shrubs. Shrubs are generally heavily chemically defended, and herbivores must either contend with their plant secondary metabolites (PSM) or avoid a significant component of the available forage. Browsing ruminants are exposed to thousands of chemicals in infinite combinations and concentrations that are constantly changing both temporally and spatially. The success with which a herbivore navigates this complex environment is in part attributed to its ability to cope with PSM. Plant secondary metabolites can affect a number of physiological andmetabolic processes (e.g., alteredmicrobial activity, reduced digestion, compromisedacid/basebalance, toxicity), althoughnegative consequences to theherbivore range fromharmless to lethal, depending factors suchasdose, animal species, planeofnutrition, and physiological state. Herbivores have a variety of intertwinedmechanisms to cope with consumption of PSM, ranging fromphysiological (e.g., salivary proteins, detoxification pathways) to behavioral (e.g., avoidance, regulation of intake below critical threshold, cautious sampling, altering size andpatternof feedingbouts, diet switching, consumingdiverse and/or complementary diets). Secondary compoundsmay affect requirements for nutrients (e.g., protein, minerals, and glucose) and water, and may alter basal metabolic rate. Energy requirementsmay also increase to accommodate increased travel towater and supplementation sites to counter these negative effects, particularly on arid rangelands. A number of management strategies exist for minimizing the negative effects of PSM consumption on livestock. Supplementation to replace nutrients depleted during detoxification (e.g., amino acids and glucose), additives such as PEG and charcoal to reduce absorption and increase excretion of PSM, maintaining animals in good body condition, and behavioral modifications (e.g., diet training) are among the potential management options for enhancing the ability of ruminants to cope with PSM when browsing shrubby ecosystems.