Week 7: Voluntary Feed Intake

Question 1

Why do we need to know the VFI?

Answer 1

To determine the nutrient concentration needed in the feed

Question 2

On a dry matter basis, what percentage is the intake usually?

Answer 2

Between 2 - 2.5% of bodyweight

Question 3

Intake by monogastric animals is controlled by what?

Answer 3

CNS

Short and long term regulatory mechanisms

Question 4

What is short term regulation?

Answer 4

Hunger vs satiety

• chemostatic regulation
• thermostatic theory

Question 5

What is chemostatic regulation?

Answer 5

Absorption of nutrients signals to CNS
Glucose and insulin levels influence intake
peptide hormone cholecystokinin released from gut when digestive products reach duodenum

Question 6

What is long term regulation involve?

Answer 6

• Lipostatic theory (fat level)

- Leptin

Question 7

What is the lipostatic theory?

Answer 7

-Preservation of constant bodyweight

Question 8

What is leptin?

Answer 8

Secreted by white adipose tissue
Suppresses intake 
Peptide YY (PYY) is released from enteroendocrine cells when colon is full to suppress intake

Question 9

What are some constraints to intake?

Answer 9

• ANimals capacity to use energy
• Properties of the pasture
• ENvironmental factors
• Pasture distribution

Question 10

What does pasture intake limit?

Answer 10

Production, except on very high quality pastures

Question 11

What is the Ingestion rate?

Answer 11

(IR) = (Bites/unit time) x (Intake/bite)

Question 12

What is Intake?

Answer 12

Intake = Ingestion rate x time spent grazing

Question 13

Why is capacity to use energy a constraint to intake?

Answer 13

• Energy demand (maintenance, growth, lactation, pregnancy, work)
• CHange in capacity to use energy (changes energy deficit and hence hunger signals)
• Calving/lambing and lactation

Question 14

What is the clearance rate?

Answer 14

outflow rate (L/hr) / rumen digesta size

Question 15

What is the outflow rate?

Answer 15

• rate of digestion

- rate of onward passage

Question 16

What is ingestive chewing?

Answer 16

• aimed at bolous formation

- particle size reduction less with more fibrous feeds and rapid grazing

Question 17

Why are immature forages rapidly digested?

Answer 17

Less structural CHO

Question 18

What can decrease digestion?

Answer 18

Chemical structure and lignin

Question 19

Nutrient deficiencies can constrain intake by reducing what?

Answer 19

• digestion in the rumen
• rate of microbial synthesis
• capacity to dispose of nutrients
• maintenance of rumen microbes

Question 20

Why does decreased nitrogen do?

Answer 20

Impairs microbial synthesis and hence intake.

Question 21

What other factors can constrain intake?

Answer 21

• Mineral deficiencies
• When pasture dry matter less than 20%
• Palatability
• Physical properties of feed

Question 22

How can temperature affect intake?

Answer 22

Cold: increase VFI to increase heat production
Heat: decreases VFI to decrease heat production

Question 23

What are other enviro factors influencing intake?

Answer 23

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

Question 24

Behavioural factors influencing intake?

Answer 24

Neophobia: fear of something new

Feed aversion: associate food with sickness, illness etc , bad experience