Published online by Cambridge University Press: 27 March 2009
A study was made of the effect on body composition of growing Angus steers from 300 to 440 kg at three different rates. The rates were: High (H) 0·8 kg/day, Low (L) 0·4 kg/day and High-Maintenance (HM) 0·8 kg/day followed by a period during which body weight was maintained constant.
The animals were individually penned and the different growth rates were achieved by controlling intakes of a pelleted feed. Two animals (part of H treatment) were killed at 300 kg and the remaining 27 animals (nine in each treatment), were killed at common body weights of 330, 363, 400 and 440 kg.
Analyses of covariance were used to compare linear regression equations representing results from each treatment. In order to do this, the logarithmic transformation of the allometric equation, y = axb, was used. As a proportion of empty body weight (EBW), hot carcass weight (HCW) was greater in both the HM and L groups than in the H group, indicating a greater offal component of EBW in the H animals. The loss in weight of the dressed carcass during storage at 2 °C for 24 h was similar in all three groups and was 0·98% of HCW. The proportions of HCW in the fore- and hind-quarters were similar in each group.
At the same dissected side weight (DSW), the weight of bone was significantly greater in both the HM and L groups than in the H group indicating that bone development was related to both age and carcass weight. There were no significant differences between the groups in the proportions of either muscle or total fat. However, the proportion of kidney and channel fat in the H group was greater than in the HM and L groups and the proportion of subcutaneous fat was also greater in the H than in the L group. The amount of connective tissue in the three groups followed, in general, group differences in bone. Analyses of the dissected components of the separate fore- and hind-quarters showed that the difference between the H and L groups in the proportion of subcutaneous fat in the DSW was due to a relatively greater development of this fat in the fore-quarter of the H animals.
No differences were found between treatments in the proportion of ‘expensive muscles’ relative to total side muscle (TSM). However, there were treatment effects on the proportions of TSM formed by certain groups of muscles; two examples are: the proportion of group 4 muscles (abdominal muscles) was higher and the proportion of group 6 muscles (distal muscles of the fore-leg) was lower in the H than in the L treatments. Bone weight distribution was similar in all treatments.
There were differences between the H and HM groups in the relative growth ratios for all fat tissues (subcutaneous, intermuscular and kidney and channel fat) compared with total side fat. However, in the H and L groups, the relative growth ratios for corresponding fat categories were similar. The weight of kidney and channel fat in the left side of the carcass was significantly greater than in the right side of the carcass in all treatments.