lecture quiz 12: biological evaluation

Question 1

feed to gain ratio

Answer 1

• aka feed conversion ratio (FCR)
• FCR = (feed consumption)/(body weight gain)
• lower # is better ➔ produce more with less feed
• for every pound of weight gain/milk/eggs prod animal consumed x pounds of food
• well balanced diet imrpoves FCR
• industries selected chx w/ lower FCR
• FCR measures feed efficiency for meat animals
• higher for ruminants than chx/pigs b/c fed lower quality diet
  
  • can only compare using human-edible food

Question 2

digestibility

Answer 2

proportion of feed that is not excreted & assumed to be absorbed by amimal (%)

• looking for ways to improve digestibility of feedstuffs (e.g. processing/grinding)
• can be calculated for DM, organic matter, & various nutrients (& energy)

Question 3

apparent digestibility

Answer 3

= (amount consumed − amount feces)/amount consumed x 100

• assumes all DM in feces is from undigested feed
• there are substances in feces not arising directly from feed = metabolic fecal products (MFP)
  
  1. intestinal cells & mucosa
  2. enzymes secreted in GI
  3. microbial mass
• MFP mainly important for proteins, but have lipids (e.g. cell membranes)
• underestimates digestibility (always lower than true digestibility)

Question 4

true digestibility

Answer 4

= [amount consumed − (amount feces − MFP)]/amount consumed

• accounts for metabolic fecal products
• MFP can be quantified by measuring fecal output of fasted animals (only for non-ruminants)
• always higher than apparent digestibility
• for fiber: apparent always = true (animals cannot make)

Question 5

feed energy evaluation systems

Answer 5

• gross energy has little direct value ∴ energy systems evaluate “available” energy content
• 2 energy systems:
  
  1. total digestible nutrients (TDN)
  2. caloric system

Question 6

caloric system

Answer 6

• uses gross energy as a start to get energy values of feedstuffs using bomb calorimetry
• helps convert gross energy system to usable energy values to formulate diet
• accounts for energy losses
• if you don’t meet energy requirement animal loses weight
  
  • surplus energy ➔ weight gain

1. digestible energy (DE)
2. metabolizable energy (ME)
3. net energy (NE)
4. maintenance energy (NEm)
5. productive or recovery energy (NEp)

Question 7

digestible energy

Answer 7

DE = GE − FE

• FE = fecal energy
• accounts for energy lost in feces
• represents energy absorbed by animal but not necessarily available for usage
• some energy lost along metabolic processes
• burn feces in bomb calorimeter

Question 8

metabolizable energy

Answer 8

ME = DE − UE (− MtE)

• UE = energy lost in urine
  
  • bomb calorimetry (must be freeze-dried first)
• MtE = energy lost in gases
  
  • account for in ruminants (non-ruminants also have but negligible)
  • anything you can burn has energy
  • don’t account for CO2 b/c non-combustible
  • measure methane production in respiration chamber
  • apply heat of combustion (~212.5 kcal/mol)
• represents portion of ingested gross energy left over after accounting for losses in feces, urine, & methane gas
• still does not represent total energy available
• used for energy evaluation of horses
• ME available for most species

Question 9

main indicator of whether or not you’re meeting energy requirements is…

Answer 9

body weight gain or body weight loss

Question 10

net energy (NE)

Answer 10

NE = = GE - UE - MtE - HI

• accounts for heat loss via heat increment (HI)
• most accurate energy value you can get
• portion of GE in diet that animal can readily use for maintenance, growth, lactation, & production
• not available for all feeds & species
  
  • used for beef cattle & dairy cows ➔ more energy losses for ruminants

Question 11

heat increment (HI)

Answer 11

1. heat of nutrient metabolism
2. heat of fermentation (from the rumen, cecum, & colon)

• useful in winter for animals raised in a non-confinement system
• could be harmful in summer under hot envir conditions
• dietary protein > dietary carbs > dietary fats
• forages > grains
• dietitians can alter diet to modify HI depending on envir conditions
• hard to measure
  
  1. estimate values using equations
  2. bottom-up approach = adding maintenance energy + productive energy

Question 12

bottom up approach to net energy

Answer 12

NE = heat energy (from maintenance) + NEp (productive energy retained in tissues, milk, conceptus, etc.)

• whenever energy is used for maintenance there is heat generated
• no heat associated w/ production/weight gain
• heat energy (HE) can be directly measured by isothermal calorimetry
  - during fasting: ant heat generated must be from maintenance
  - during fasting: HI = 0

Question 13

maintenance energy (NEm)

Answer 13

energy required for:

1. basal metabolism
2. voluntary activity
3. thermal regulation

Question 14

productive or recovered energy (NEp)

Answer 14

energy used for:

1. gain / growth (muscle, fat)
2. lactation
3. egg production
4. conceptus

Question 15

energy values

Answer 15

• DE, ME, NE
• species-dependent
• too laborious to measure routinely
• estimations come from chemical composition

Question 16

net energy values efficiency

Answer 16

• not all NE is used w/ same efficiency for every purpose
• most efficiently used for maintenance
• least efficiently used for weight gain
• efficiency of net energy for lactation is relatively high (almost as efficient as maintenance)
• must meet energy requirement for lactation, gain, etc.

Question 17

physiological fuel values

Answer 17

• calories listen on human label
• not GE
• equivalent of ME
• calculated by subtracting energy lost in excreta (feces & urine) from GE