L20: Voluntary Feed Intake Flashcards
Why do we need to know VFI?
What % is intake generally?
- to determine the nutrient concentration needed in the feed
- intake (on DM basis) is generally b/w 2-2.5% body weight
What is VFI controlled by?
CNS
Describe short term regulation of VFI
• Hunger vs satiety
• Chemostatic regulation
- Absorption of nutrients signals to CNS
- Glucose and insulin levels influence intake
- Peptide hormone cholecystokinin released from gut when digestive products reach duodenum
• Thermostatic theory?
- Do animals eat to keep warm? Unlikely.
Describe long term regulation of VFI
• Lipostatic theory (fat level)
- Preservation of constant bodyweight
- Supported by studies in chooks
- Pig studies different
- Selection for fattening?
• Leptin – secreted by white adipose tissue
- Suppresses intake
- Obese mice lack the gene
• Peptide YY (PYY) - Released from enteroendocrine cells when colon is full to suppress intake
List and describe some other factors affecting VFI
• Palatability: sensory appeal of food
- Animals show distinct preferences for certain
tastes/smells
• Physiological factors
- Rats adjust intake to keep energy intake constant
- Varies with metabolic LW
- Alters with pregnancy, lactation and exercise
• Nutrient deficiencies
- *• Choice feeding**
- Nutritional wisdom
- Self regulation of intake
What constraints intake in grazing animals?
- Pasture intake limits production
- except on very high quality pastures
- ruminants need high feed intakes
- *Constraints to intake:**
- *• animal’s capacity to use energy**
• properties of the pasture
• environmental factors
• pasture distribution
What is the equation for: Ingestion rate (IR)?
Intake?
- IR = (bites/unit time) x (intake/bite)
- Intake = ingestion rate x time spent grazing
Constraints to intake for grazing animals
Describe constraint on capacity to use energy
Energy demand
• maintenance, growth, lactation, pregnancy, work
Change in capacity to use energy (e.g. change in physiological state)
- changes energy deficit and hence hunger signals
- Calving/lambing and lactation (body fat mobilisation?)
- Sheep can change grazing behaviour when capacity to use energy is elevated
- E.g. twin vs single lactating ewes • graze for longer, have greater intake rates
• genotype differences
Constraints to intake in grazing animals
- why does rumen constrain intake?
• Rumen constrains intake
• less than 8% fibre digested/hour
• Good relationship between intake and rumen digesta clearance rate
What factors affect the clearance of digesta from rumen?
Clearance rate = outflow rate (L/hr)/ rumen digesta size
• Outflow rate
- Rate of digestion
- Rate of onward passage
• Changes in rumen volume
-E.g. late pregnancy
Constraints to intake in grazing animals
describe constraint due to feed physical properties
-chewing
• Particle size influences onward passage
- Ingestive chewing
- aimed at bolus formation
- particle size reduction less with more fibrous feeds and rapid grazing
- Chewing or rumination related to forage ‘toughness’ and plant anatomy
- Chewing also an energy expense
Constraints to intake in grazing animals
describe constraint due to feed physical properties
-digestion
Immature forages rapidly digested
- Less structural CHO
• More fibrous forages slower digestion but softens when chewed
- Digestion impaired
- More time chewing
- More energy cost
• Decreased particle size increases digestion rate
- Greater surface area
- >1mm to leave rumen
• Chemical structure and lignin content can decrease digestion
How can feed nutrient composition constrain intake?
Nutrient deficiencies can constrain intake by reducing:
- digestion in the rumen
- rate of microbial synthesis (e.g. S)
- capacity to dispose of nutrients
- Facilitated transport mechanisms
- Leading to accumulation which may be toxic
- Especial micronutrients
- maintenance of rumen microbes • Cellular repair
How can nitrogen be a intake constraint?
Decreased nitrogen impairs microbial synthesis and hence intake
- with forage diets physical factors generally a greater constraint
- with low quality pastures less than 100g protein per kg digestible organic matter constrains intake
- water soluble carbohydrate content may influence microbial protein production
List environmental constraints on VFI
- Temperature
- cold increases VFI to increase heat production
- heat decreases VFI to decrease heat production
- these effects are acute, animals will acclimayise
- Photoperiod
- Distance to water
