Voluntary feed intake

Question 1

Describe voluntary feed intake…?

Answer 1

• need to know to determine nutrient concentration
• intake (dry matter basis) ~2-2.5%bw
• affected by many factors differing between monogastrics & ruminants

Question 2

Intake by monogastrics controlled by..?

Answer 2

CNS & short & long term regulatory mechanisms

Question 3

Describe short term regulation…? hormone involved?

Answer 3

hunger vs satiety
chemostatic regulation:
- absorption of nutrients -> signal CNS
- glucose & insulin levels influence intake
- peptide hormone (cholecystokinin) released from gut when products reach duodenum

Question 4

Modulating signals of appetite regulation…?

Answer 4

glucose, CCK, GIT filling, smell, vision, body fat reserves

Question 5

Desrcibe long term regulation…? Including lipostatic theory.

Answer 5

lipostatic theory:
- preservation of constant body weight
- studies in chooks support theory (pig studies differ)
Leptin secretion by adipose tissue -> supress intake via peptide YY. (obese mice lack leptin gene)

Question 6

Other factors affecting feed intake…?

Answer 6

• ‘palatibility’ - sensory appeal of food ie. distinct preferences for certain tastes/smells
  physiological factors:
• varies with metabolic LW
• alters with preg., lactation & exercise
  nutrient deficiencies
  choice feeding (nutritional wisdom)
  eg. reducing energy content of chick diets -> long term regulation to maintain constant energy intake

Question 7

Intake by grazing animals depends on…? Limits of pasture intake?

Answer 7

pasture intake limits production (except on very high quality pastures; ruminants need high feed intakes)
constraints to intake:
- animals capacity to use energy
- properties of the pasture
- environmental factors
- pasture distribution

Question 8

Voluntary feed intake (VFI) equations…?

Answer 8

ingestion rate (IR) = (bites/unit time) x (intake/bite)
intake = ingestion rate x time spent grazing

Question 9

Constraints to intake - ‘Capacity to use energy’ characteristics…?

Answer 9

energy demand - maintenance, growth, lactation, preg., work
change in capacity to use energy (change in physiological state) - changes energy deficit -> hunger signals
calving/lambing & lactation

Question 10

What behaviours do sheep adopt to account for changing energy demands?

Answer 10

can change grazing behaviour when capacity to use energy is elevated (eg. lactating ewe graze for longer & have greater intake rates)
genotype differences

Question 11

Constraints to intake - ‘Feed physical properties’ characteristics…?

Answer 11

rumen constrains intake (<8% fibre digested/hour

good relationship between intake & rumen digesta clearance rate

Question 12

What are the factors affecting clearance of digesta from rumen…? Formula…?

Answer 12

clearance rate = outflow rate (L/hr) / rumen digesta size
outflow rate (rate of digestion; rate of onward passage)
changes in rumen volume

Question 13

How does chewing affect onward passage…?

Answer 13

by influencing particle size
ingestive chewing - aimed at bolus formation; particle reduction is less with more fibrous feeds & rapid grazing
chewing or rumination related to forage ‘toughness’ & plant anatomy

Question 14

Relationship between chewing time & forage energy?

Answer 14

Positive linear relationship ie. as chewing time increases -> forage energy increases

Question 15

What factors affect digestion…?

Answer 15

maturity of forages (immature forages -> rapidly digested)
more fibrous forages -> slower digestion but softens for chewing
digestion impared (more time chewing)
decreased particle size increases digestion rate
chemical structure & lingin can decrease

Question 16

How does form of roughage affect DMI?

Answer 16

pelleted roughage -> greater intake than long (unprocessed), but lower digestibility
overall better DMI & digestibility than ‘long’ roughage

Question 17

How does feed nutrient composition constrain intake…?

Answer 17

nutrient deficiencies constrains intake by:

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

Question 18

How does N impact intake?

Answer 18

decreased N impairs microbial synthesis & hence intake:

• on forage diets physical factors > constraint
• low quality pastures ( constrains intake
• water soluble CHOs influences microbial protein production

Question 19

How do minerals constrain intake?

Answer 19

Na, Mg, Cu, Co, I, S deficiencies (among others) constrains intake
particularly Co & I -> no plant requirement
Mg & S deficiency -> impaired digestion

Question 20

How does herbage water affect intake?

Answer 20

when pasture DM < 20% -> possible intake depression

intake varies throughout day due to water content & water-soluble CHO content

Question 21

Define palatibility & list the effect palatability has on intake…?

Answer 21

attractiveness of food in terms of sight, smell & taste
physical properties (abrasions etc possible > effect on young)
short term preference ie. hunger lowers sensory thresholds (overcome aversion to maintain intake); huge variation between animals

Question 22

Different receptors involved in palatibility…?

Answer 22

Flavour receptors - odor, taste, texture
Body receptors - liver, GIT, CNS
Chemo-/osmo-/mechanoreceptors

Question 23

Constraints to intake - environmental factors…? Describe how environmental temperature can affect feed intake PAST EXAM QUESTION

Answer 23

temperature
cold: increase VFI -> increase heat heat prod. via shearing
heat: decrease VFI -> decrease heat prod. (effects acute; animals acclimatise; night feeding; concentrates vs roughages)
Basically, as temp. increases -> DM intake decreases in different spp. (eg. layers, growing pigs, cattle, dairy cows)

Question 24

photoperiod & distance to water…?

Answer 24

photoperiod - larger meals & more often during day
distance to water:
- longer distance -> number of drinks/day decreases & VFI decreases