Factors that alter the growth and development of ruminants.

Growth is defined as an increase in tissue mass. Mass increases by hyperplasia early in life and hypertrophy later in life, although hyperplasia of adipose tissue continues throughout life. The growth curve, being mass or cumulative weight plotted against age, is sigmoid, consisting of a prepubertal accelerating phase plus a postpubertal decelerating phase. Mathematically, this curve can be described as a function of mature mass, fractional growth rate, and age. At a specific fraction of mature mass, body composition seems to be constant, but the degree to which nutrition can alter mature mass is not certain. If mature mass is altered, body composition at any given mass will be altered. Mature mass can be decreased by starvation or protein deficiency early in life. Alternatively, retarding the deposition of fat or the administration of estrogenic compounds may increase mature protein mass. Many of the advances in rate and efficiency of growth and in reduced fat of meat cuts can be explained by increased mature protein mass of ruminants. Animals with higher mature weight require more energy for maintenance and reach puberty later in life, so a larger mature mass is not desirable for the breeding herd. Indeed, smaller replacement heifers would prove economical if reproduction were not decreased. A period of restricted growth and fat deposition (as on pasture) can increase the slaughter weight of small cattle into a more desirable range, presumably through increasing mature protein mass. However, calves with retarded growth often make less efficient feedlot gains than do calves finished immediately after being weaned. For growing large-framed heifers, pasture alone often provides an inadequate energy supply for early puberty, but excessive amounts of supplemental feed can enhance fat deposition in the udder, which subsequently decreases milk production. By manipulating the supply of specific nutrients and hormones, it may prove feasible in the future to reduce fat deposition in specific tissues and to alter mature body protein mass.