The Science Behind the Grade
The Science 

Behind the Grade

How biology and physiology impact production, 

performance, and carcass quality and acceptance. 

Dr. Jennifer N. Martin

Assistant Professor

Center for Meat Safety & Quality 
Envision a high-quality beef experience
Beef Quality 

Intrinsic Traits

• Palatability Traits (tenderness, juiciness, flavor)

• Safety

• Nutrition 

• Appearance 

Extrinsic Traits 

• Price

• Production strategy

• Socio-cultural values (welfare, sustainability, etc.)
Focus on Palatability 

Palatability Traits

• Tenderness

• Flavor

• Juiciness

What do we know about palatability? 

• Tenderness is essential for consumer acceptance

– >90% of steaks are considered tender or very tender

• Flavor may be equally important to tenderness

– 58% of consumers say flavor is more important than 

tenderness

– 43% of consumers say tenderness is more important

than juiciness

National Beef Quality Audit Summaries

FLAVOR

TENDERNESS

JUICINESS
Focus on Palatability 
If marbling is highly associated with a positive 

eating experience...

...let’s just produce all Prime beef
National Beef Quality Audit Summaries

Prime & Choice %

May 2019: 

8% Prime

May 2020: 

11-12% Prime
Producing High Quality Beef is an Art...

...and a balance

National Beef Quality Audit Summaries

Trait 1991 1995 2000 2005 2011 2016

USDA YG 3.2 2.8 3.0 2.9 2.9 3.1

USDA QG 686 679 685 690 693 696

Fat thickness, in 0.59 0.47 0.47 0.51 0.51 0.56

HCW, lbs 760.6 747.8 786.8 793.4 824.5 860.5

LM area 12.9 12.8 13.1 13.4 13.8 13.9

Trend of greater high-quality carcasses has 

been met with trend of increasing Yield Grade 

and HCW 2.90%

6.70%

14.55%

22.10% 22.60%

16.60%

8.60%

3.40% 2.50%

0.00%

5.00%

10.00%

15.00%

20.00%

25.00%

<1.49 1.5-1.99 2-2.49 2.5-2.99 3-3.49 3.5-3.99 4-4.49 4.5-4.99 >5

YG Frequency Distribution

Yield Grade

NBQA, 2016
Balancing Quality with Yield 

YG-1

+$3.79/cwt

QG %

Prime 0.07

Choice 4.06

Select 4.79

Other 0.55

+$1.86/cwt

YG-2

QG %

Prime 0.94

Choice 23.61

Select 10.90

Other 1.05

-$11.64/cwt

YG-4

QG %

Prime 0.97

Choice 9.31

Select 1.40

Other 0.40

-$17.21/cwt

YG-5

QG %

Prime 0.22

Choice 1.86

Select 0.33

Other 0.12

Par

YG-3

QG %

Prime 1.78

Choice 29.94

Select 6.20

Other 1.49

NBQA, 2016
In order to balance eating quality and yield, we must understand...

• The science of muscle, fat, and skeletal development

• The variables influencing carcass performance and eating quality

Producing High Quality Beef is an Art...

...and a science
Building High 

Quality Beef

Starts at the Cell 

Ends at the Rail
From the Cell to the Rail

Myocytes

Adipocytes

Fibroblasts

Mesenchymal Stem Cells
Science of Skeletal Development

High quality cattle begin with good skeletal structure 

• Similar to the foundation of a house

• Bones serve as levers for skeletal muscles

Skeletal structures form prenatally 

• Bone formed from mesoderm layer

• At birth, bone content is high (2:1)  decreases as animal grows

• Nutrition, use, hormones, and management can influence post-natal 

skeletal development and bone growth

• Calcium and functional use influence bone growth 

• Castration and estrogen exposure inhibit bone growth 
Science of Skeletal Development

Post-Natal Skeletal Changes

• Bone growth occurs longitudinally 

• Longitudinal growth occurs at the epiphyseal plate--ossification

• Amount of bone growth depends on: 

• Rate of new cell production

• Size of cells before ossification

Why Does Skeletal Development Impact Beef Quality? 

• Skeletal system creates foundation for muscle development

• As bones grow longitudinally muscles grow

• Large frame size usually attributed with faster rate of lean meat 

growth (tend to be later maturing)

• Large-framed cattle produce learner beef
Science of Muscle Development

Muscle development begins in early gestation 

• 1st two months  primary muscle cells develop

• 2 through 8 months  secondary muscle cells majority of 

skeletal muscle

• Number of skeletal muscles cells is largely set by month 8 of 

pregnancy

• Skeletal muscle isn’t priority for energy partitioning (brain, heart)

• After birth, muscle growth is hypertrophic (growth in size)

What does this mean? 

• Prenatal factors influence skeletal muscle development

• Management of cows mid-gestation correlated with muscling

• Poor nutrition during gestation = light muscling, low yield
Science of Muscle Development

Postnatal Muscle Changes

• At birth, focus is on small muscles associated with the skeleton

• Necessary for biological functionality 

• After birth, substantial changes in muscles not associated with the skeleton 

• Reflection of use, location, nutrition, etc. 

Differences in Muscle Fiber Type 

• Primarily Type I fibers at birth

• Transition to glycolytic metabolism

• Changes influenced by: 

• Use

• Breed-type

• Nutritional plane

Trait Type I Type IIA Type IIX

Metabolism Oxidative Oxidative & Glycolytic Glycolytic

Speed Slow Medium Fast

O2 Requirement Aerobic Aerobic Anaerobic 

Size Small Small Large

Lipid Content High High Low

Myoglobin High High Low

Fatigue Resistance High High Low

Mitochondrial Activity High High Low

Color Red Intermediate White

Example Diaphragm Psoas major Chuck/Round
Science of Muscle Development

Muscle fiber type and metabolism impact a variety of beef quality factors:

• Ultimate pH Decline

• Marbling (Type I Fibers ↑ Marbling) 

• Tenderness (cross sectional area, use)

• Water Holding Capacity

• Color
Science of Fat Development

Lipid cells develop in late gestation and early postnatal period

• Overlap between adipogenesis and secondary myogenesis 

• Lipid cell development follows a sequential order: 

• Visceral  subcutaneous  intermuscular  intramuscular 

Adipose cells are clustered in connective tissue or within/between muscle bundles

• Intramuscular adipocytes exist between primary/secondary muscles bundles (perimysium) 

• Lipid-storing ability of intramuscular adipocytes lower than subcutaneous adipocytes 

Given the value of marbling, significant efforts to understand intramuscular fat 

• Fibro-adipogenic progenitors  the foundation for intramuscular fat development 
Science of Fat Development

Li et al., 2020 
Science of Fat Development

Zhao et al., 2019 

Intramuscular Fat

• Develops later

• Less mature

• Glucose required

Subcutaneous Fat

• Develops early

• More mature

• Acetate Required
Zhao et al., 2019 

Building High 

Quality Beef

Starts at the Cell 

Ends at the Rail

Zhao et al., 2019 
Balancing Postnatal Changes

As animal matures, muscle 

growth slows and is replaced by 

fat deposition

Fat deposition is energy 

inefficient 

• Especially IMF
Producing High Quality Beef is an Art...

...a Science

...a Balance
Managing Variables that Impact Quality

Pre-Harvest Factors

• Breed or breed-type (Bos indicus vs. Bos Taurus)

• Prenatal Nutrition

• Age of animal

• Stress prior to harvest

• Production systems

• Performance technologies (i.e. Beta-agonists, implants, etc.)

Post-Harvest Factors

• In-plant practices (chilling, temperature, etc.)

• Aging method (wet vs. dry aging)

• Aging length 

• Packaging

• Cooking/Preparation 

Days postmortem

3 7 14 21

WBSF,kg 2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

Treatment x age P = 0.002

Control Optaflexx, 200 mg/hd/day Zilmax, 7.5 ppm
Producing High Quality Beef is Essential

Consumers demand a high-quality eating experience. 
As price increases, demand for high-quality 

and consistency will also increase...

...so what?
Building High 

Quality Beef

Starts at the Cell 

Ends at the Rail

Understand the Science

Respect the Art

Perfect the Balance
The Science Behind 

the Grade

How science affects the grade

Dr. Bucky Gwartney

International Marketing Specialist

USDA – Agricultural Marketing Service
Science and the Grade Application

• Quality grade—

characteristics of the 

meat that predict the 

palatability of the lean

• Yield grade—indicates the 

yield of closely trimmed, 

boneless retail cuts 

expected to be derived from 

the major wholesale cuts 

(round, sirloin, short loin, 

rib, and chuck)
USDA Quality Grade

% Desirable % Undesirable

Prime

Choice

Select

Standard

Adapted from Smith et al., 1987. Journal of Food Quality 10:269-286
Quality Grade Determination

• Sex Classification

• Maturity evaluation based on evidences of skeletal maturity and color 

and texture of ribeye muscle (between 12th & 13th rib). Only if over 30 

months. 

• Marbling and Firmness of Ribeye Muscle (between 12th & 13th rib)
Quality Grade Determination

• Sex Classification – Bulls vs Steers vs Heifers

• Intact males (Bulls) deposit less marbling but are higher muscled

• Castrated males (steers) deposit more marbling but are less muscular

• Females (heifers) tend to be a little fatter and lighter than steer carcasses 
Quality Grade Determination

• Sex Classification

• Maturity evaluation based on evidences of skeletal maturity and color 

and texture of ribeye muscle (between 12th & 13th rib). Only if over 30 

months. 

• Marbling and Firmness of Ribeye Muscle (between 12th & 13th rib)
Skeletal Maturity

• Sacral Vertebrae • Lumbar Vertebrae • Thoracic Vertebrae • Rib bones • Chine Bones • Skeletal ossification occurs 

from the posterior end (rear) 

to the anterior end (head) of 

the carcass

CARCASSMATURITY
Factors 

Potentially 

Influencing 

Physiological 

Maturity

Gender

Sex condition

Breed type

Implant program

Diet
Skeletal

Maturity

Physiological maturity determined by 

evaluating the size, shape and ossification of 

the bones and cartilages, the color & texture of 

the lean of the ribeye.

All maturity indicating factors are 

considered – composite 

evaluation

Factors seldom develop to the same 

degree – limitless number of 

potential combinations to consider
Quality Grade Determination

• Sex Classification

• Maturity evaluation based on evidences of skeletal maturity and color 

and texture of ribeye muscle (between 12th & 13th rib). Only if over 30 

months. 

• Marbling and Firmness of Ribeye Muscle (between 12th & 13th rib)
Marbling 

• Fat within the muscle

• Intramuscular fat

• Evaluated on the ribeye between 

the 12th & 13th ribs
Marbling

• Influences

• Chill

• Bloom

• Lighting
Conditions or Defects Preventing or Lowering 

a Grade

• Blood Splash

• When a carcass is not dressed and 

prepared properly at harvest, 

blood pressure within the carcass 

can spike and capillaries in the 

muscle tissue can rupture.

• Carcasses can have a “small” 

amount of blood splash & still 

grade. The “small” amount is 

based on the “Slight” marbling 

card. 
Conditions or Defects Preventing or Lowering 

a Grade

• Calloused

• Result of steatosis; fatty tissues 

spreads into areas of the muscle 

creating a callous section; strenuous 

muscle exertion.

• The maximum amount of calloused to 

still grade is a small amount (less than 

1⁄2 inch)
Conditions or Defects Preventing or Lowering 

a Grade

• Dark Cutters

• Dark Cutting characteristics are determined in percentage, from 10 to 100%.

• Dark Cutting beef affects overall quality grade

• Each percentage dark relates to the discount amount you will assign to the 

carcass. 

• Up to 1% of beef carcasses are dark cutters 
What is Dark Cutting Beef?

• Dark, firm, and dry is the name of the condition

• Dark in color because of light absorption

• Firm because muscle proteins are pulled together more tightly

• Dry in appearance due to more water held in the muscle

• Caused by a variety of factors including:

• Aggressive activity

• Estrus

• Stress – long term

• Growth Promotants

• Weather

• Most of the sugars in the muscle are used up by the live animal during stress

• Minimal sugars left for postmortem muscle metabolism

Figure Source: Principles of Meat Science, 4th Edition
USDA Yield Grade
Yield Grade 

Factors

Thickness of fat over the ribeye 

(backfat)

Ribeye area

Percentage of kidney, pelvic, heart fat 

(KPH)

Hot Carcass Weight
Yield Grade Factors

Thickness of fat over the ribeye (backfat)

Remember this is the second area 

where fat is deposited
Yield Grade Factors

Ribeye Area

This is a measurement of the 

muscle growth and mass
Yield Grade Factors

Hot Carcass Weight

This is where we relate how well a 

carcass is muscle in relation to its 

overall weight
Yield Grade Factors

Percentage of kidney, pelvic, heart fat 

(KPH)

This is a measurement of the 

internal fat deposits, one of the first 

areas for fat development
Percentages of Retail cuts for the various YG

Yield 

Grade

Percent Boneless, Closely Trimmed Retail 

Cuts from the Round, Loin, Rib & Chuck

1.0 > 52.3%

2.0 52.3-55.0%

3.0 50.0-47.7%

4.0 47.7-45.4%

5.0 <45.4%
FY2020 Beef Grading Summary

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00

Prime

Choice

Select

Standard

Percentage

Quality Grade

FY 2020 Quality Grade Percentages
0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00

YG 1

YG 2

YG 3

YG 4

YG 5

Percentage

Yield Grade

FY 2020 Yield Grade Percentages

FY2020 Beef Grading Summary
Thank you for Participating

For more information on this webinar series and the 

USDA Cattle and Carcass Training Centers, visit: 

www.ams.usda.gov/grades-standards/beef/cattle-carcass-training-centers