FACTORS THAT ALTER THE GROWTH AND DEVELOPMENT OF RUMINANTS

Growth is defined as an increase in tissue mass. Mass increases by hyp erplasia early in life and hypertrophy later in life, although hyperpl asia of adipose tissue continues throughout life. The growth curve, be ing mass or cumulative weight plotted against age, is sigmoid, consist ing of a prepubertal accelerating phase plus a postpubertal decelerati ng 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 mas s is altered, body composition at any given mass will be altered. Matu re mass can be decreased by starvation or protein deficiency early in life. Alternatively, retarding the deposition of fat or the administra tion of estrogenic compounds may increase mature protein mass. Many of the advances in rate and efficiency of growth and in reduced fat of m eat cuts can be explained by increased mature protein mass of ruminant s. Animals with higher mature weight require more energy for maintenan ce and reach puberty later in life, so a larger mature mass is not des irable for the breeding herd. Indeed, smaller replacement heifers woul d prove economical if reproduction were not decreased. A period of res tricted growth and fat deposition (as on pasture) can increase the sla ughter 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, pas ture alone often provides an inadequate energy supply for early pubert y, but excessive amounts of supplemental feed can enhance fat depositi on in the udder, which subsequently decreases milk production. By mani pulating the supply of specific nutrients and hormones, it may prove f easible in the future to reduce fat deposition in specific tissues and to alter mature body protein mass.