Submitted to: Small Ruminant Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/23/2010
Publication Date: 10/18/2010
Citation: Estell, R.E. 2010. Coping with shrub secondary metabolites by ruminants. Small Ruminant Research. 94:1-9.
Interpretive Summary: Shrub encroachment into rangelands can have serious implications for forage and livestock production. Shrubs often contain defense compounds called plant secondary metabolites (PSM) that can limit the ability of animals to eat them. These compounds can negatively affect ruminants by decreasing palatability, altering rumen microbial populations, and can cause toxicity in extreme cases. Ruminants utilize a combination of behavioral and physiological mechanisms to cope with PSM. Avoidance, changing meal size and frequency to keep PSM intake below a critical level, and switching to different forages are behavioral responses to PSM. Herbivores may be able to increase PSM intake by eating a more diverse diet because a variety of metabolic pathways are used to detoxify PSM rather than overwhelming a single system. Shrub consumption may affect nutrient requirements because protein and glucose are used during PSM detoxification. Secondary compounds may change metabolic rates or have diuretic effects that could alter water and energy requirements.
Technical 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.