Plant Secondary Metabolites

Other Plant Secondary Metabolites

• Plants produce secondary metabolites as defenses against microbes, insects, and animal predation
• These metabolites can have both adverse and beneficial effects on animal health
• The effects depend on concentrations

Bio-activity

• Plant secondary metabolites interact with various molecular and cellular targets due to their structures
• They can interact with enzymes, hormone receptors, neurotransmitter receptors, and transmembrane transporters
• These metabolites can mimic responses at the molecular level

After Ingestion

• Plant secondary metabolites can pass through animals unchanged
• They can combine with bile salts and be excreted
• They can interact with tissues in the gastrointestinal tract (GIT)
• They can be absorbed directly
• They may undergo further transformation
• They can be transformed within the GIT and then absorbed, often increasing hydrophilicity
• They are finally excreted via the kidneys in urine

Tannins

• These are divided into condensed tannins (proanthocyanidins) and hydrolyzable tannins
• Condensed tannins are polymers of flavan-3-ols
• Condensed tannins are not toxic per se but can have adverse or beneficial effects
• Hydrolyzable tannins consist of a carbohydrate core, often glucose
• This core is esterified with gallic acid or ellagic acid and are toxic
• Other tannins do not fit into the above two classes

Condensed Tannins

• Condensed tannins interact with digestive processes by interfering with digestive enzymes
• They bind to feed components which include proteins or minerals
• They can be found in aquaculture

Condensed Tannins Qualities

• They are widespread and typical of plant tannins
• They are commonly found in forage legumes
• Predominantly in dicotyledons, especially in the Leguminosae family
• They occur mainly in cell vacuoles
• They can be in free or cell-wall-bound form with carbohydrate or protein

Effects of Tannins on Animal Nutrition

• Astringency from tannins can decrease palatability and thus feed intake
• Tannins precipitate proteins, interfering with the function and availability of important dietary and endogenous proteins, including enzymes
• The pH dependency of the tannin-protein complex shifts protein digestion from the rumen to the small intestine
• They may create rumen undegradable protein (RUP) or an indigestible tannin-protein complex

Effects of Tannins on Production

• Tannins can decrease protein degradation
• In some cases, tannins may increase the efficiency of urea recycled to the rumen
• They lower ruminal [NH3] due to decreased rate protein degradation
• Tannins can increase glycoprotein content and excretion of saliva
• Excessive in-rumen [NH3] is detrimental to microbial growth
• Effects can be positive or negative depending on [CT]

Condensed Tannins- Ruminants

• High dietary [CT] at >50 g/kg DM leads to VFI and digestive efficiency
• Moderate [CT] intake from 20-40 g/kg DM has beneficial effects on protein metabolism
• Moderate [CT] intake leads to an increase in protein outflow from the rumen
• Moderate [CT] leads to lact", wool growth, reproductive performance, LWG
• Condensed Tannins have demonstrated decreased effects of internal parasites in sheep
• Tannins have also been shown to have a decreases the risk of bloat in cattle

Monogastric Animals and Tannins

• Diets containing less than 5% tannins result in a decreased growth rate, lower protein utilization, damage to the digestive tract lining, altered excretion of cations, and increased excretion of proteins & EAA
• In poultry, small quantities of tannins can cause adverse effects
• For poultry diets of 0.5-2.0%, there will be decreased growth and decreased egg production
• Poultry diets consisting of 3-7%, lead to death
• Pigs have similar harmful effects to tannins

Hydrolyzable Tannins and Toxicity

• Hydrolyzable Tannins are often toxic to ruminants and can occur in animal feeds
• Commonly occur in Oak plants
• Several tropical tree legumes are known to create the issue
• Occurance is know in Yellow wood (Terminalia oblongata) in Australia, which occurs widely in the eastern Qld, occurring mainly along Burdekin and Fitzroy rivers and their tributaries

Hydrolyzable Tannins and Injuries

• Hydrolyzable Tannins are known to injure species who consume them
• Cattle: liver and kidney injury with acute intoxicat
• Sheep: only liver injury has been observed

Post Mortem Diagnosis - Hydrolyzable Tannins

• Uraemic-smelling carcass
• Acorns or oak leaves in the rumen/reticulum
• Hemorrhagic gastroenteritis
• Necrosis of liver
• Kidney damage with proximal tubular necrosis
• Subcutaneous hemorrhages & edema, particularly abomasum & perineum
• High mortality & morbidity in sheep & cattle fed oaks & other tree species with > 20% HT

Lectins

• Abundant in plants, mostly in the legume family
• Mostly found in seeds
• Can be present in tubers & plant saps

Lectins Effects

• Undigested lectins lead to: binding to gut cells, interaction with enzymes and non-available amino acids
• All resulting in increases in cell turnover, poor protein digestibility, negative n-balance
• Then resulting in : alterations in metabolism, draining of energy, impaired immunological functions
• Eventually death

Lectins Mode of Action

• Binding to gut epithelium disrupts brush borders, leading to reduction in absorptive surface area and absorption of nutrients
• Liver enlargement may be due to disturbance of the general metabolism
• They have antimicrobial effects against bacteria, fungi & protozoa and may be useful alternatives to antibiotics

Protease Inhibitors

• Abundant in raw cereals & legumes, particularly soybeans, chickpea, cowpea, faba beans, field peas, mung beans, and pigeon pea
• These can be easily denatured & inactivated by heat
• There may be several types within a plant, differing in molecular weight, structure (AA sequence) & specific action
• Potency depends on their origin & the target enzyme
• They can alter digestive processes when present in high concentration
• Adverse effects are observed primarily in animals with a high weight of the pancreas, such as chickens
• No pancreatic enlargement is observed in large animals with small pancreases, such as dogs, pigs, and calves

Saponins

• Present in > 100 plant families
• Commonly found in Legumes
• Ranging between 18-41 mg/kg in various legume seeds
• At 67 mg/kg in defatted roasted soybean flour

Saponin Effects

• Saponin-Containing Forages used as Livestock Feed
• Bitter taste occurs
• Lowers uptake of certain nutrients including monosaccharide and cholesterol
• Inhibition of metabolic & digestive enzymes
• Lyse cells in intestinal mucosa

Saponin Toxicity

• Clinical signs of toxicity include:
• Anorexia
• Listlessness
• Gastroenteritis
• Weight loss