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 and metabolic processes (e.g., altered microbial activity, reduced digestion, compromised acid/base balance, toxicity), although negative consequences to the herbivore range from harmless to lethal, depending factors such as dose, animal species, plane of nutrition, and physiological state. Herbivores have a variety of intertwined mechanisms to cope with consumption of PSM, ranging from physiological (e.g., salivary proteins, detoxification pathways) to behavioral (e.g., avoidance, regulation of intake below critical threshold, cautious sampling, altering size and pattern of feeding bouts, diet switching, consuming diverse and/or complementary diets). Secondary compounds may affect requirements for nutrients (e.g., protein, minerals, and glucose) and water, and may alter basal metabolic rate. Energy requirements may also increase to accommodate increased travel to water 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.