Week 7: Voluntary Feed Intake Flashcards
Why do we need to know the VFI?
To determine the nutrient concentration needed in the feed
On a dry matter basis, what percentage is the intake usually?
Between 2 - 2.5% of bodyweight
Intake by monogastric animals is controlled by what?
CNS
Short and long term regulatory mechanisms
What is short term regulation?
Hunger vs satiety
- chemostatic regulation
- thermostatic theory
What is 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
What is long term regulation involve?
- Lipostatic theory (fat level)
- Leptin
What is the lipostatic theory?
-Preservation of constant bodyweight
What is leptin?
Secreted by white adipose tissue Suppresses intake Peptide YY (PYY) is released from enteroendocrine cells when colon is full to suppress intake
What are some constraints to intake?
- ANimals capacity to use energy
- Properties of the pasture
- ENvironmental factors
- Pasture distribution
What does pasture intake limit?
Production, except on very high quality pastures
What is the Ingestion rate?
(IR) = (Bites/unit time) x (Intake/bite)
What is Intake?
Intake = Ingestion rate x time spent grazing
Why is capacity to use energy a constraint to intake?
- Energy demand (maintenance, growth, lactation, pregnancy, work)
- CHange in capacity to use energy (changes energy deficit and hence hunger signals)
- Calving/lambing and lactation
What is the clearance rate?
outflow rate (L/hr) / rumen digesta size
What is the outflow rate?
- rate of digestion
- rate of onward passage
What is ingestive chewing?
- aimed at bolous formation
- particle size reduction less with more fibrous feeds and rapid grazing
Why are immature forages rapidly digested?
Less structural CHO
What can decrease digestion?
Chemical structure and lignin
Nutrient deficiencies can constrain intake by reducing what?
- digestion in the rumen
- rate of microbial synthesis
- capacity to dispose of nutrients
- maintenance of rumen microbes
Why does decreased nitrogen do?
Impairs microbial synthesis and hence intake.
What other factors can constrain intake?
- Mineral deficiencies
- When pasture dry matter less than 20%
- Palatability
- Physical properties of feed
How can temperature affect intake?
Cold: increase VFI to increase heat production
Heat: decreases VFI to decrease heat production
What are other enviro factors influencing intake?
Photoperiod: larger meals and more often during the day, seasonality
DIstance to water: longer distance: VFI decreases, energy expense, more energy for maintenence, less energy for production
Behavioural factors influencing intake?
Neophobia: fear of something new
Feed aversion: associate food with sickness, illness etc , bad experience
