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).

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Dry matter intake and BW data from 14 mature, nonpregnant, nonlactating Angus cows that were individually fed through two consecutive 70- to 80-d periods (maintenance and ad libitum) were used to predict ADG (ADG = -.512 + .213 DMI - .0017 BW, R2 = .95). This equation then was used to identify feed efficiency types among these cows. Cows were identified as average type (A) if ADG was within one SE of predicted ADG, and as efficient (E) or inefficient types (I) if ADG exceeded one SE above or below, respectively, its predicted ADG. Four, four, and six cows were identified and grouped as I, A, and E types, respectively. During the maintenance period, DM and ME intake and ADG were similar (P greater than .10) across all three efficiency types. But during the ad libitum period, voluntary DM and ME intakes of I cows were greater (P less than .05) than those of A or E cows. Average daily gains of I cows during ad libitum feeding were greater (P less than .10) than those of A cows. Daily ME required for maintenance of I cows was highest, that of A cows was intermediate, and that of E cows was lowest (180.2, 154.6, and 135.1 kcal/kg BW.75, respectively). Inefficient cows tended (P greater than .10) to have less fat and deposited more protein (P less than .05) than A and E cows (137.9 vs 77.2 and 46.2 protein g/d, respectively). Concurrent with higher protein accretion rates, liver weights of I cows were heavier (P less than .05) than those of A and E cows (8.58 vs 7.79 and 7.68 kg, respectively). Inefficient cows were characterized by higher energy requirements for maintenance. Their high protein accretion may partially explain their higher maintenance requirements.