Growth and body composition

What is the difference between growth, development and maturity?

• Growth: increase in body weight from birth towards mature size.

• Development: changes in body proportions, conformation, chemical composition and physiological function.

• Maturity: when muscle mass reaches its maximum, described in the lecture as the lean target.

What are hyperplasia, hypertrophy, isometric growth and allometric growth?

• Hyperplasia: increase in cell number.

• Hypertrophy: increase in cell size.

• Isometric growth: body parts grow at approximately the same rate.

• Allometric growth: different body parts grow at different rates, changing body proportions.

What are the three stages of prenatal growth, and what happens during each?

• Zygote phase: approximately the first 11 days; cell division occurs without an increase in overall size.

• Embryo phase: approximately days 12–25; major tissues differentiate and the three germ layers develop.

• Foetal phase: rapid increase in body weight and development. The lecture notes that the foetus can reach around 60–70% of birth weight during this phase.

What are the three embryonic tissue layers, and what do they form?

Answer:

• Ectoderm: skin, nervous system and related structures.

• Mesoderm: muscles, skeleton, circulatory and reproductive systems, and parts of the urinary system.

• Endoderm: digestive system, lungs, liver, bladder and related structures

What factors influence growth and development, and why is maternal nutrition important?

Genetics, individual variation, nutrition, age, sex and breed all influence growth. Maternal nutrition is especially important during the last third of pregnancy, when foetal growth is rapid. Poor nutrition can restrict foetal growth and affect development after birth.

Which hormones regulate growth, and what do they do?

• Growth hormone (somatotropin): supports muscle and bone growth.

• Insulin: major anabolic hormone that supports nutrient use and tissue growth.

• Androgens: promote muscle and bone growth; help explain faster growth in males.

• Oestrogens: contribute to growth in females.

• Glucocorticoids: can reduce growth potential by withdrawing nutrients from the blood.

How is growth measured, and what are the limitations of liveweight?

Growth is commonly measured by changes in liveweight over time. However, liveweight alone cannot distinguish gains in muscle, fat, organs, bone or gut contents. The lecture identifies serial slaughter as the best method for assessing changes in body composition.

What does the sigmoid growth curve show?

The cumulative growth curve is S-shaped:

• Early phase: growth accelerates and weight gain increases.

• Inflection point: the growth rate reaches its maximum.

• Later phase: growth decelerates as the animal approaches maturity.

What are ADG and relative weight gain, and how do they differ from total growth?

• Total growth: cumulative increase in liveweight over time.

• Average daily gain (ADG): weight gained per day.

• Relative weight gain: weight gain expressed as a percentage of previous bodyweight.

ADG measures the amount gained each day, whereas relative gain accounts for the animal’s existing size.

In what order do tissues and organs develop as an animal grows?

Organs essential for life develop early. The lecture highlights the brain, eyes, kidneys, heart and liver early in foetal development, with digestive, skeletal and muscular systems developing later. After birth, the skeleton develops ahead of muscle. Bone grows first in length and then in thickness; fat deposition increases as the animal matures.

What is the order of fat deposition in the animal?

• Mesenteric fat

• Peri-renal fat

• Intermuscular fat

• Subcutaneous fat

• Intramuscular fat

How does nutrition affect tissue growth and nutrient allocation?

Nutrients are partitioned between tissues and body functions rather than distributed equally. Nutrition level affects growth rate, tissue development and body composition. Restricted nutrition can limit growth, while the tissues that develop earliest are prioritised according to the animal’s developmental stage.

How do age, sex and breed influence growth and maturity?

Growth rate depends on age, mature size and sex. Different breeds have different mature weights. Males generally grow faster, eat more and are heavier than females at the same age; the lecture notes males weigh approximately 5–7% more at birth. Comparisons between breeds should consider physiological age, not just chronological age.

Is faster growth the same as earlier maturity?

No. Animals can grow faster but still take longer to reach mature size because their mature weight is larger. Growth rate and maturity are different concepts, so comparisons should consider both the animal’s age and its mature size.