Growth and Animal Production | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

Introduction

• The term "growth" refers to the organized coordination of diverse processes that guide the zygote's transformation into a reproductive-capable adult animal. This progression, from a single cell to a multicellular adult, involves the generation of new biochemical components, such as protoplasm. Throughout animal growth, cells both increase in number and size, and extracellular substances accumulate. 
• The cleavage of the embryo, the augmentation of red blood cells, monocytes, hair follicles, and ectodermal cells contribute to cell multiplication. Neurons and skeletal muscles experience cell hypertrophy during their growth, while structures like fat, plasma, and cartilage expand through the inclusion of external or extracellular substances. Consequently, growth is intricately linked to various cell constituents, including nuclei, nucleoli, chromosomes, centrioles, mitochondria, cytoplasmic organelles, enzymes, and cell membranes.

Growth can be classified into two main categories

• Prenatal Growth: Primarily developmental in nature, prenatal growth involves the transformation of a single-celled zygote into a fully formed organism. This intricate process encompasses cleavage, differentiation, and organogenesis.
• Cleavage: Cleavage, a distinct form of mitotic cell division, does not involve cell growth. Instead, it results in the formation of two-celled and four-celled embryos. During cleavage, blastomeres, the cells formed, decrease in size without any growth. The accumulation of fluid between blastomeres pushes the cells to one side, and at this stage, the embryo is termed a blastocyst.

Differentiation entails the progressive specialization of cells, both structurally and functionally. Blastomeres are initially totipotent, but as the embryo progresses, their fate becomes predetermined. The blastocyst stage is marked by evident differentiation, with trophoblast cells forming fetal membranes and ICM cells developing into the fetus. The ICM further differentiates into three germ layers: ectoderm (responsible for epidermis, hair, hooves, and the nervous system), mesoderm (contributing to the circulatory system), and endoderm (developing into the inner lining of the digestive system and most glands).

• Organogenesis encompasses the development of various body organs through the interaction of ectoderm, endoderm, and mesoderm. This interaction involves processes such as invagination, evagination, budding, and hollowing. Simultaneously, histogenesis, the development of specific cellular characteristics in different organs, takes place. For instance, muscle cells develop contractile elements during this process.
• In cattle, somites (body segments) along the spinal cord start to differentiate around the 19th day of pregnancy, with the number gradually increasing. Although most organs are non-functional at their initial appearance in the embryo, the second to sixth weeks of gestation in cattle witness the earliest formation of major organs and body parts. Following organ initiation, there is a rapid growth process involving both cell proliferation and an increase in cell size.
• Differential growth: Approximately 60% of the total weight of the conceptus corresponds to the birth weight of the fetus, with the remaining 40% attributed to fetal membranes and fluids. Organs within the fetus experience varied and non-constant growth rates, with tissues like nucle, bone, or fat growing at different rates in different parts of the body and at different gestational stages. Early in gestation, the cephalic region undergoes rapid growth, resulting in a disproportionately large head. Due to the diverse growth rates of fetal organs, the overall conformation of the fetus undergoes constant changes.
• Distinct growth gradients define the fetus, with the head, limbs, and forequarters being relatively more developed at birth than the muscles. In early prenatal growth, muscle cells increase in number, while in later prenatal and postnatal life, muscle cells increase in size. The maximum adult size is determined early in the fetal stage, and breed differences arise from variations in muscle cell numbers rather than muscle cell size. While the skeleton's growth is relatively constant, certain dimensions increase more rapidly than others, leading to changes in body proportions.
• Postnatal Growth: Postnatal growth encompasses an individual's growth after birth, and this increase in body mass is not limitless. Typically, there is an initial rapid increase that gradually slows down until the organism reaches a steady state or adulthood. Growth is a complex process involving changes in chemical composition, water content, protein, the ratio between proteins and fat, and alterations in body shape. Animal growth can be viewed as the concentration of anabolism (building up of materials) and catabolism (breaking down of materials). Growth persists as long as anabolism predominates over catabolism, and a steady state is achieved when both processes are in equilibrium
• Brody (1945) defined growth as a relatively irreversible time change in measured dimensions of the body, considering both size and weight while excluding fluctuations due to factors like feed availability, gestation, or disease. Some differentiate fattening from growth, arguing that fat serves as a reserve food source and may not represent true growth. However, distinguishing fat with functional significance from fat solely acting as a reserve energy source poses challenges, making the inclusion of fattening within the concept of growth justifiable.
• Maturation or Puberty: Puberty is the postnatal stage when gonads secrete sex hormones in quantities sufficient to trigger accelerated growth of genital organs and secondary sex characteristics. In animals, the onset of puberty is often influenced more by the attainment of a specific body size (two-thirds of adult size) than by age, and this size is genetically determined. Bulls, for example, initiate sperm production at around 47 weeks of age and a body weight of 250-300 kg, but these figures may vary in tropical cattle due to genetic factors.

Puberty in Animals

Male Puberty:

• Development of Sperm: Before male animals like bulls, rams, stallions, boars, and bucks start producing sperm, there's an increase in hormones like androgens.
• Early Signs: Rams reach puberty at about 200 days of age and 32 kg of body weight, stallions at 16-17 months, boars between 4-7 months, and bucks between 4-5 months.
• Hormonal Changes: Unlike females, males show a steady or constant level of hormones (FSH and LH) during puberty. This triggers the development of reproductive organs and the production of gametes (sperm).
• Weight Variation: The age and body weight at which males reach puberty can vary widely based on breed and environment. Nutrition plays a crucial role, with underfeeding delaying puberty and overfeeding hastening it.

Female Puberty:

• Cyclical Ovulation: In females, puberty is marked by the start of cyclical ovulation, indicating the readiness for reproduction.
• Importance of Timing: Delayed onset of puberty in females can impact reproductive efficiency, especially in seasonal breeders, where missing a breeding season can mean a significant loss.
• Factors Influencing Onset: For ewes and heifers, body size is crucial for puberty onset, while body weight also matters in seasonal breeders. Most gilts reach puberty around 200 days of age.
• Nutritional Role: Nutrition, especially energy intake, is vital for female puberty. Increased body weight contributes to the growth of reproductive organs, essential for reproductive efficiency.

Nervous System's Role

Inhibitory Influence: Lesions in the hypothalamus can induce early puberty, suggesting that the hypothalamus normally inhibits puberty onset. Even tiny amounts of sex hormones in infancy can suppress the release of reproductive hormones.

• Understanding puberty in animals is important for managing their reproductive health, and factors like nutrition and hormonal balance play key roles in this natural process.

Puberty and Hormones:

• Initiation of Puberty: Hormones like GnRH, FSH, and LH are released from the anterior pituitary when the sensitivity of the hypothalamus decreases.
• Gonadal Activities: These hormones stimulate gametogenic (formation of reproductive cells) and steroidogenic (production of sex hormones) activities in the gonads.
• Changes in Animals: Puberty brings about characteristic physiological and behavioral changes in animals.
• Hypothalamus Control: Contrary to previous beliefs, it's now known that the hypothalamus has ultimate control over puberty onset, with gonads and pituitary playing supporting roles.
• Environmental Factors: Light and temperature can influence puberty onset through the hypothalamus.

Growth Curves

• Measuring Fetal Size: Foetal size is assessed by total foetal length, crown-rump length, curved crown-rump length, or foetal weight.
• Understanding Pre-natal Growth: Growth during pregnancy is often studied using growth curves or mathematical formulas.

Types of Growth Curves

• Absolute Growth: Change in foetal volume, crown-rump length, or weight over time.
• Relative Growth: Absolute growth per initial dimension.
• Specific Growth: Logarithm of size plotted against age.
• Foetal Growth Patterns: Absolute growth is not linear but increases exponentially until birth. In cattle, over half of foetal growth happens in the last two months of gestation.
• Regulation of Growth: Changes in the rate of pre-natal growth suggest that foetal size plays a role in self-regulating growth.
• Organ Growth: Mathematical relationships among the growth rates of different foetal organs influence the overall shape and form of the foetus.

Measuring Pre-natal Growth

Crown-Rump Length (CR)

• Measured from the highest point on the skull to the rump.
• Determines the length of the foetus, indicating its growth.

Curved Crown-Rump Length (CVR)

• Measurement of the curved length from the highest point on the skull to the rump.
• Provides an alternative perspective on foetal length, considering natural curves.

Vertebral Column Length (VR)

• Measures the length of the foetus's vertebral column.
• Evaluates the growth and development of the backbone.

BlVRT (BLVRT)

• Stands for "Blade Vertebral Column Tail Length."
• Focuses on the length of the vertebral column and tail, providing specific details about these regions.

Total Foetal Length (VRT)

• Measures the overall length of the foetus.
• Gives a comprehensive indication of the foetus's size and growth.

Vertebral Column Tail Length (VRT)

• Specifically measures the length of the vertebral column tail.
• Offers insights into the development of the tail region.

Fig: The pre-natal growth curves of farm animals

(Top) Absolute live weight change; adult weight is considered at 100. (Middle) Weight gam per unit time; maximum gain per unit time is considered as 100. Bettum) Weight change expressed as a percentage of the previous weight

Fig: Growth of the chick during embryonic and post-hatching periods

Fig: Changes in the crown-rump length and weight of bovine fottuses during the-frst two months of life (left) and during mid- and late gestation (right). The change in length is nearly constant and linear with time.

Measuring Growth in Simple Terms

Foetal Weight as an Indicator:

• Challenges: Difficult to accurately measure intra-uterine foetal weight.
• Solution: Foetal length can be easily measured using X-ray or ultrasound scan.

Common Measure: Weight Gain:

• Represented as: Average growth rate, Relative growth rate, Cumulative growth rate, Instantaneous growth rate.
• Average Growth Rate: Absolute weight gain per unit time (W2-W1/t2-t1).
• Relative Growth Rate: Percentage weight gain in relation to initial weight (Minots equation).
• Cumulative Growth Rate: Average relative growth rate over a specific time interval.
• Instantaneous Growth Rate: Theoretical concept, practically impossible to measure directly.

Importance of Growth Rates

• General Principle: The speed at which an animal grows is more crucial than its final weight.
• Desirability: Rapid growth rates are desirable for breeding stock, dairy and meat animals, layer birds, and broilers.
• Consideration: Not all animals are allowed to reach mature weight; growth rates should be optimized at different stages.
• Challenges: Maximum growth rates are not always desirable at all ages due to changes in body conformation.