Postnatal Growth

When raised under ideal conditions, domestic animals exhibit a sigmoidal pattern of weight gain. Rate of gain and final mature weight are influenced by a multitude of factors, such as species, breed, genotype, gender, or the use of exogenous growth promotants. When compared at the same body weight, animals that are heavier at maturity (e.g., large frame vs. small frame or males vs. females) generally grow faster (Fig. 1), contain more bone and protein, and have less fat than animals that mature at a smaller size.[1] The amount of energy needed to maintain an animal body increases with the size of the animal. The inflection point of the growth curve, where growth begins to slow, reflects a decreasing proportion of the nutrients consumed that are used for growth.[2] This typically coincides with rapid accumulation of fat in animals maintained on a high level of nutrition.

Increases in body weight reflect different patterns of growth of each organ. Most internal organs approach their mature weight long before final body weight is reached, and are considered early maturing.[1] Of the other major body tissues, bone is earlier developing than muscle, and muscle develops prior to extensive fat deposition (Fig. 2). Growth of a tissue is influenced by functional demands, e.g., the extent of bone mineralization depends on both body weight and functional demand placed on the bone. Similarly, bone growth in length determines growth in length of associated muscles, but muscle diameter is associated with the force that is demanded of a muscle. Muscles used for locomotion, such as the hindlimb biceps femoris, have larger-diameter muscle fibers and more connective tissue than postural muscles, e.g., the psoas major. The following discussion will highlight the cellular aspects of postnatal bone, skeletal muscle, and adipose tissue accretion. These tissues compose the majority of body mass in all domestic animal species.

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