Abstract :
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.
