L20: Voluntary Feed Intake Flashcards Preview

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Flashcards in L20: Voluntary Feed Intake Deck (16)

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?



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

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)?


  • 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


• 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


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


List behavioural contraints on VFI

  • Neophobia
    -fear of anything new
    -problem when introducing new feed
  • Feed aversion
    -associates food w/ sickness, illness, poison, toxicity etc
    -bad experience