GROWTH & DEVELOPMENT

B. Hypertrophy (increase in cell size)

1. muscle cells Have all your muscle &

2. nerve cells nerve cells at birth.

3. most adipose cells -- have most at birth

IV. Prenatal growth

V. Bone growth

All meat-producing animals follow a common pattern of bone development.

A. Structure and areas of growth

Epiphysis

Epiphyseal plate....(cartilage)

New cartilage is formed as long as bone growth continues. The cartilage is gradually ossified to form bone. When the cartilage in the epiphyseal plate ossifies and no new cartilage is formed, bone growth ceases

Diaphysis

(diameter

growth) Growth in diameter occurs by the periosteum depositing new bone cells (osteocytes). As new bone is deposited, deeper, inner bone is removed causing an increase in the size of the marrow cavity. Inner bone material that is removed is used by the periosteum to make new bone.

SHEEP: Break joint = lamb; Spool joint = mutton

B. Amount of cartilage (or lack of it) is used to estimate maturity in carcasses. Maturity is a primary factor in determining carcass quality grade.

(Choice vs commercial)

Maturity is the best single indicator of meat tenderness. As muscle gets older, there is increased connective tissue.

C. Frame size measurements of beef cattle

Frame size is determined by 2 factors: 1) height at the hooks (hips)

2) age

Hip height is used because hip height maturity is reached several months before mature height at the shoulders.

Frame size is of great importance in identifying the physiological maturity pattern of cattle on the growth curve. If you know the frame size of a bull, you have a good estimate of his eventual mature size.

If you know the frame size of feeder cattle, you have a good idea what their slaughter weight will be:

Slaughter weights of large-, medium-, and small-frame

slaughter cattle at 0.50 inches of backfat

	Slaughter Weight
	Steers (lb)	Heifers (lb)
Frame size
Large	>1,200	>1,000
Medium	1,000-1,200	850-1,000
Small	1,000	<850

VI. Fat Development and Deposition

A. Embryonic development

1. Adipose development begins during mid-to-late stages of fetal development.

2. Differentiation ®® round-type cell

Small fat globules start to enter the cell

B. Postnatal growth

1. At birth animals have very little fat

Example: Pig = ~2% fat at birth

2. As animal grows, fat deposition occurs by enlargement of individual fat cells, and for a short period of time new fat cells develop.

Enlargement accounts for most of fat development.

3. Fat is deposited at specific locations, and as the animal grows, it develops further from these locations.

1. Kidney, pelvic, and heart fat (KPH)

a. Makes up 3.5% of carcass weight (USDA Choice)

b. Fat surrounds kidney even in young calves

c. Increases as weight increases

d. Dairy breeds have higher kidney fat than beef breeds even with less body fat.

Genetic difference

2. Subcutaneous fat (under skin)

a. Largest quantity of fat in the body of hogs

b. Is deposited in layers and connective tissue separates these fat layers

Outer layer - laid down relatively early in life

Increases very little as animal grows to mature wt.

Middle layer - most development during growth to "normal" market weights

Inner layer - (directly over longissimus muscle)

develops much later

Fat-type pig - evident at 120 lb.

Meat-type pig - very small at 230 lb.

In selection for meat-type pig, we have reduced the middle and inner layers of fat, but the outer layer changed very little.

850 lb greatest deposition in rump, hind flank, lower loin, brisket, and center of shoulder

1000 lb deposition increases; extends from top of rump to lower flank and joins with center portion of loin and chuck area.

3. Intermuscular fat (called "seam fat" in the trade industry)

a. located between the muscles

b. largest quantity of fat in cattle

c. I.M. fat often surrounds moving muscle parts;

fills spaces between bone & points of muscle attachment

d. Large deposits of I.M. fat often causes problems for the packer and retailer in the merchandising of cuts. The shoulder (lamb and pork) and chuck (beef) are particularly bad.

4. Intramuscular fat (marbling)

a. Located between muscle fibers and within muscle fibers

b. Called marbling -- indicator of palatability

Remember: Maturity (age) and marbling are 2 main factors in carcass quality grades.

c. Most deposition is in the later stages of growth

(Some is even present at birth.)

d. First to leave if animal is starved

e. Marketing live animals on full feed:

0.4 to 0.5 inches B.F. = 60-70% Choice (depending on breed)

Measured between the 12th/13th ribs, ~5 inches of the top midline

C. Relationship between sex of animal and fat deposition

1. Young animals grow rapidly and more muscle protein than fat is deposited at this early stage.

As the animal matures, a greater portion of the gain is fat.

2. Sex of animal makes a big difference in rate of fat deposition as animal matures:

Intact (noncastrated) males are normally the leanest

Cattle: steer normally leaner than heifer

Sheep: wether normally leaner than ewe

Hogs: gilts have more muscle (less fat) than barrows at market weight.

3. Hormones play a major role is sex differences in fat deposition

a. Testosterone

Stimulates muscle development and has an inhibitory influence on fat deposition.

Bulls and rams have more efficient weight gains plus more muscular, trimmer carcasses

ALSO - bulls and rams won't grade at typical market weights. Testosterone also inhibits intramuscular fat. Animals have to be fed to heavier weights.

b. Estrogens

Increases rate of closure of epiphyseal plate; therefore, slowing growth. (Earlier closure than males)

Heifers ready for market ~150 lb. lighter than steers.

c. Why are gilts leaner than barrows?

Don't know answer.

(Gilts do have a male pattern of growth hormone secretion)

VII. Muscle Growth and Development

A. Muscle structure

1. Contraction is major function

In general:

Loin and postural muscles most tender

Sirloin

Round

Chuck least tender

Selected muscles within each of the less tender areas have acceptable tenderness.

B. Prenatal development

1. Total muscle-cell numbers are genetically established before birth

2. Postnatal development is an increase in size.

C. Postnatal development

1. Muscle fibers grow in diameter in length

2. Tenderness decreases as the animal ages because of connective tissue. There becomes more connective tissue and more cross-linking of the connective tissue.

3. Pigs....

at 80 days of age: ~50% of muscle-fiber growth achieved Intramuscular fat starts to increase Water content decreases
At 120 days of age: Muscle-fiber growth ~75% complete Fat is increasing rapidly.
At 150 days of age: 95% of maximum fiber diameter is attained. Ready for market Additional weight gain is fat.

VIII. Body Composition

A. Growth curve

1. Sigmoidal or S-shaped

Holds for all species: mice to elephants yeast, bacteria, flies, pumpkin.......

2. Events during growth (phases of the growth curve)

a. Relatively little growth before birth and initial increase following birth is slow.

b. Early -- rapid growth of the essential organs, skin, and bone. Head and legs are proportionally greater than the trunk.

c. Have further development of the organs, skin, and bone plus very rapid muscle growth. Some fat is starting to be deposited.

Body becomes longer but still lacks depth

Weight gains are rapid and efficient, mostly due to muscle and bone growth.

Bone growth is nearly complete by the end of the phase

Some fat is being deposited, even during this period of lean growth.

d. Last phase -- increase in growth is nearly all fat.

C. Growth and development of bone, muscle, and fat considered separately

1. Bone is earliest maturing -- becomes rather constant

2. Muscle is next

3. Rapid increase in fat occurs when muscle begins slowing down.

Composition of a typical steer		Composition of an over-finished steer
YG3, Choice steer (approx.)		YG 5 steer (text)
Muscle	60%	Muscle	43%
Fat	25%	Fat	44%
Bone	15%	Bone	13%