Within-herd variation in energy utilization for maintenance and gain in beef cows.

: Fourteen mature, nonpregnant, nonlactating Angus cows (498 kg) were individually fed through two consecutive phases (maintenance [M], 80 d and ad libitum [A], 70 to 79 d) to estimate within-herd variation in individual cow ME requirements for maintenance (MEm) and to identify factors contributing to this variation. Body composition was determined at initiation of phase M, at termination of phase M (also initiation of phase A) and at the end of phase A by a two-pool D2O dilution technique. Daily MEm averaged 156.7 kcal/kg BW.75 (SD = 18.4 kcal/kg BW.75) and efficiency of ME use for tissue gain or loss averaged 76% (SD = 30%). Estimates of ME intake to maintain 1 kg of protein or 1 kg of fat were 192.9 (SE = 24.8) or 20.7 (SE = 21.5) kcal. These data indicate that among cows of similar fat masses, those with larger protein masses had higher energy requirements for maintenance. Daily MEm was positively correlated (P less than .16) with liver weight (r = .40) and relative proportions of liver (r = .44; P less than .16) and heart (r = .48; P less than .10) in the empty body. Also, daily MEm was correlated negatively (P less than .05) with weight (r = -.71) and relative proportion of omental and mesenteric fat (r = -.78). Estimates of ME required for deposition of 1 kcal of protein or fat were 5.56 (SE = 1.01) or 1.26 (SE = .09) kcal. Weight of liver and the sum of liver, spleen, kidney and heart weights increased 1.58 (R2 = .47) and 1.95 kg (R2 = .52) per kilogram of daily weight gain during phase A. These results indicate that increased performance caused increased organ mass (liver).