The quality and value of the carcass in domestic meat animals are reflected in its protein and fat content. Preadipocytes and adipocytes are important in establishing the overall fatness of a carcass, as well as being the main contributors to the marbling component needed for consumer preference of meat products. Although some fat accumulation is essential, any excess fat that is deposited into adipose depots other than the marbling fraction is energetically unfavorable and reduces efficiency of production. Hence, this review is focused on current knowledge about the biology and regulation of the important cells of adipose tissue: preadipocytes and adipocytes.

Beef steers were fed in two phases 1) to determine the relative importance of changes in DMI, gastrointestinal tract fill, energy expenditures, and composition of gain in the compensatory growth phenomenon, 2) to compare the effects of growth restriction due to ad libitum consumption of a low-energy (low-concentrate) diet to those of limited intake of a high-energy (high-concentrate) feed, and 3) to examine changes in carcass composition and quality resulting from different types of growth restriction. During the growing phase (237 to 327 kg), steers were fed either a high- (C) of low- (F) concentrate diet. Diet F was available for ad libitum consumption (FA) and diet C was available either for ad libitum consumption (CA) or on a limited basis (CL) to match the live weight gains by the FA group. During the finishing phase (327 to 481 kg), all steers received diet C, either for ad libitum consumption (CA) or restricted (CL) to 70% of the intake by the corresponding CA steers. Backfat thickness was markedly reduced (P < .001) by final feed restriction (7.4 and 6.9 mm for CL-CL and FA-CL respectively), compared with CA-CA (12.6 mm). Backfat also was lower in CL-CA (11.6 mm, P < .10) and FA-CA (9.9 mm, P < .05) than in CA-CA steers. Conversely, marbling scores were similar among groups, except for the FA-CL steers, which had lower marbling scores than FA-CA and CL-CA steers (P < .05). Higher DMI following growth restriction were accompanied by increased rates of live weight (+54 and +27%) and empty body weight (EBW; +57 and +43%) gain for CL-CA and FA-CA steers, respectively, compared with CA-CA steers. Gain:feed (EBW basis) were improved in some restricted/refed groups (+30, +13, and +10%, for Cl-CA, CL-CM respectively CA-CA. Increased DMI played a major role in the compensatory gain response in both CL-CA and FA-CA groups. Maintenance requirement was reduced (-17%) in CL-CA and increased in the FA-CA group (+21%); both changes affected the magnitude of compensatory gain in those animals. In contrast, composition of gain had little or no effect on the compensatory gain response. Programmed feeding can be used to manipulate carcass quality, but low-concentrate feeding during the growing phase may impair overall feedlot performance.

No abstract

No abstract

Beef steers were fed in two phases to compare the effects of diet and intake on growth and cellularity of visceral organs. During the growing phase (237 to 327 kg), steers were fed either a high (C) or low (F) concentrate diet. Diet F was available ad libitum (FA), whereas diet C was available either ad libitum (CA) or on a limited basis (CL) to match live weight gains of the FA group. During the finishing phase (327 to 481 kg), all steers received diet C either ad libitum (CA-CA, CL-CA, and FA-CA) or restricted (CL-CL and FA-CL) to 70% of the intakes of corresponding CA steers. Marked nutritional effects on liver growth (e.g., -25 and -15% in CL and FA, respectively, relative to CA) were due mainly to changes in cell size (i.e., protein:DNA), with smaller changes in cell numbers (i.e., DNA). Hyperplasia and hypertrophy played a role in growth of the forestomachs, although cell numbers and sizes tended to change in opposite directions, limiting magnitudes of changes in organ mass. Protein synthetic capacity (i.e., RNA) varied as well, often in parallel with cell number. This result differed from that observed in intestines, which maintained constant cell sizes but underwent marked changes in cell number. For liver, amounts of absorbed nutrients seemed to be the main factor driving hypertrophy. The organs of the gastrointestinal tract responded to physical and chemical signals, as shown by the effects of dietary fiber on growth of the forestomachs and intestines. Forestomachs responded mainly to diet fiber content, whereas the intestines responded to diet type and nutrient supply. Feeding programs for beef animals often include changes in diet type and periods of feed limitation, and these in turn affect visceral organ growth and metabolism. Because visceral organs are a major contributor to whole-body energy expenditures, factors affecting these tissues must be understood. This study supports the concept that workload determines organ size, but dietary factors influencing workload clearly vary for each organ.