VISCERAL ORGAN MASS AND CELLULARITY IN GROWTH-RESTRICTED AND REFED BEEF STEERS

Beef steers were fed in two phases to compare the effects of diet and intake on growth and cellularity of visceral organs. During the growin g 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 ma tch 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-C A, 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 du e mainly to changes in cell size (i.e., protein:DNA), with smaller cha nges in cell numbers (i.e., DNA). Hyperplasia and hypertrophy played a role in growth of the forestomachs, although cell numbers and sizes t ended 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 ob served in intestines, which maintained constant cell sizes but underwe nt marked changes in cell number. For liver, amounts of absorbed nutri ents seemed to be the main factor driving hypertrophy. The organs of t he 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. Fee ding programs for beef animals often include changes in diet type and periods of feed limitation, and these in turn affect visceral organ gr owth and metabolism. Because visceral organs are a major contributor t o whole-body energy expenditures, factors affecting these tissues must be understood. This study supports the concept that workload determin es organ size, but dietary factors influencing workload clearly vary f or each organ.