A NOTE ON THE COMPOSITION OF LIVE-WEIGHT GAINS
ESTIMATED BY REGRESSION ANALYSIS
I. MCDONALD AND M. KAY
The Rowett Research Institute, Bucksbum, Aberdeen
IN experiments involving measurements on beef carcasses, the animals can
be slaughtered at a given degree of fatness, at a prescribed live weight or at a
prescribed age. The ultimate choice depends upon the purpose of the experiment but where comparisons are being made between different breeds
receiving the same diet it is usual to slaughter animals at the same degree of
fatness. This subjective assessment of the time at which the animals are to be
slaughtered results in widely different slaughter weights and ages. In order to
aid the interpretation of results, Pomeroy (1965) suggested that animals should
be killed at at least three different live weights so that an estimate of the rate
of change of various parameters can be derived.
TABLE 1
Increase in the weights of joints of cattle carcasses per 100 kg increase in live
weight at slaughter (estimated as regression coefficients)
Joint
Friesian
bulls
Friesianf
steers
100
11-1
5-6
26-7
6-9
9-9
0-3
17-1
Hereford Hereford
x Friesian x Friesian
bulls
steers
Pooled
values and
SE
High-price cuts
Silverside plus round
Rump plus loin
Sixth to twelfth ribs
Total
13-9
101
4-0
28-1
13-2
13-4
5-7
32-3
12-3±0-9
ll-7±0-8
51 ±0-7
29-1 ±1-7
Low-price cuts
21-7
24-4
23-4
22-4
Forequarter
23-2±l-3
Flank
12-7
ll-9±0-9
6-3
12-8
11-2
Hind shank
1-2
l-7±0-3
00
1-5
2-2
38-2
28-1
37-7
35-8
36-7+1-8
Total
t The estimates for the Friesian steers are included for completeness but, because of the
narrow range of slaughter weights, they are exceedingly imprecise.
A total of 64 Friesian and Hereford x Friesian bulls and steers were
individually tethered on slats and were fed ad libitum on a diet of bruised
barley with a protein-mineral-vitamin supplement. It was planned to
slaughter the animals at different live weights from 325 to 500 kg for each of the
four groups. Analysis of growth rates, feed conversion rates and carcass
dissection measurements was intended to provide data for use in the estimation of economically optimal slaughter weights. Unfortunately, many of the
animals suffered from arthritis and other troubles attributed to the close
tethering. It was decided that dissection of the carcasses of such animals
would serve no useful purpose, and dissection data were finally obtained from
35 out of the original 64. Table 2 gives the numbers of animals within each
sub-group and the mean slaughter weights.
The jointing procedure shown in Figure 1 was carried out on the right side
of eac% carcass twenty-four hours after slaughter, and each joint weighed.
The 9th-llth rib cut was dissected into muscle, fat, and bone.
2M
553
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554
MCDONALD AND KAY
The mean rates of live weight gain for 52 healthy animals over the interval
from 150, kg to 300 kg live weight were 1-32 kg/day for the bulls and 1-22
kg/day for the steers, the difference being significant (P<005). There was
no breed difference in rate of gain. Mean rates of gain for the 35, out of these
52, that were dissected were 1 -35 kg/day for the bulls and 1 -24 kg/day for the
steers.
TABLE 2
Estimated mean carcass composition of cattle killed at 400 kg live weight with
SE of adjusted mean values and of differences between means of groups
Friesiaii
bulls
No. of animals
Actual weight at slaughter
Mean (kg)
SD
High-price cuts
Silverside+round (kg)
Rump+loin (kg)
Sixth to twelfth ribs (kg)
Total (kg)
Low-price cuts
Forequarter (kg)
Flank (kg)
Hind shank (kg)
Total (kg)
Ratio of high-price to
low-price cuts
Composition of ninth-eleventh
rib cut
Bone (kg)
Muscle (kg)
Fat (kg)
Bone(%)
Muscle (%)
Fat (%)
Hereford Hereford Approximate
Overall
Friesian X Friesian x Friesian
SEof
significance
bulls
steers
steers
differences
of differences
6
7
10
12
383
±60
356
±19
397
±44
355
±50
54-2
±0-96
38-6
±0-84
' 21-7
±0-74
114-5
54-6
±0 ; 96
39-6
±0-84
20-6
±0-74
114-8
±1-8
53-8
±0-73
37-2
±0-64
19-6
±0-56
110-7
±1-4
88-6
±1-42
30-4
±0-90
11-7
±0-32
130-7
±1-8
90-8
±1-43
±0-90
11-5
±0-32
133-3
±1-8
89-2
±109
28-9
±0-69
±0-24
129-2
±1-4
88-2
±116
32-5
±0-74
10-7
±0-26
131-4
±1-5
0-885
±0016
0-865
±0016
0-856
±0012
±18
310
110
±1-2
P<005
±11
P<005
±0-9
N.S.
±2-3
N.S.
±1-8
N.S.
±i-i
P<005
±0-40
N.S.
±2-3
N.S.
0-853
±0013
±0020
N.S.
±010
N.S.
±0-24
N.S.
±0-18
P<0001
±1-1
P<005
51-5
±0-78
39-9
±0-69
20-6
±0-60
1121
±1-5
116
103
114
106
±008
4-37
±019
1-67
±0-14
16-5
±0-9
. 61-6
±2-0
21-9
±2-1
±008
4-31
±019
±006
4-15
±015
1-57
±0;ll
.16-8
±0-7
60-5
±1-5
22-8
±1-6
±006
4-19
±015
2-30
±011
14-0
±0-7
55-8.
±1-6
30-2
±1-7
211
±0-14
13-6
±0-9
57-8
±2-0
28-6
±2-1
:
+2 5
N.S.
±2-7
P<001
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COMPOSITION OF LIVE WEIGHT GAIN
555
Separate regression analyses of carcass dissection measurements on live
weight at slaughter were done for each of the four groups of animals.
Regression coefficients did not differ significantly between groups and were
therefore pooled to give the estimates shown in Table 1. This indicates that
of the increase in live weight, within the range 300-450 kg considered here,
an average of 29-1% is attributable to increase in weight of 'high-price'
joints, and 36-7% to increase in 'low-price' joints, leaving 34-2% to be
Silverside
+ Round
Hind Shank
6-12th
Ribs
Forequarter
FIG. 1. Jointing procedure.
attributed to increases in weight of intestinal tract and contents, other
internal organs, and head, feet and tail.
A comparison is made in Table 2 between the estimated carcass compositions of the four groups of animals at a common slaughter weight of
400 kg. The mean slaughter weights varied from group to group and the
measurements have been adjusted to what would have been expected at 400
kg by using the pooled regression coefficients. It will be seen that there are
no major differences in weights of the various joints between bulls and steers
or between breeds. As would be expected there is a large difference between
bulls and steers in the weight and the percentage of fat in the 9th-l lth rib cut.
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556
MCDONALD AND KAY
ACKNOWLEDGEMENT
We wish to express our gratitude to Messrs Robert Lawson & Sons (Dyce) Ltd for their
generous help with this experiment, both in the provision of facilities and in carrying out
most of the work on the carcasses.
REFERENCES
POMEROY, R. W., 1965. Carcass evaluation. Wld Rev. Anim. Prod., 1: 83-88.
{Received 25.ii.67)
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