1. The growth of muscle and fat in the bovine carcase was investigated in one hundred and sixty-four cattle calves and foetuses over the carcase weight range 1.10 kg to 389.52 kg. Total dissected fat varied from 1.3% to 35.1%. The cattle were of four basic breeds (Friesian, Aberdeen Angus, Hereford, Australian Illawarra Shorthorn (A.I.S.)) and their crosses. The genetic homogeneity required in certain sections of the study was provided by 63 Friesians, 44 of which were half-sibs.
2. A pre-natal growth phase, from 150 days gestation to birth was investigated.
3. Growth within the musculature was examined on a gross anatomical and microanatomical basis. Six muscles, m. extensor carpi radialis, m. extensor digiti tertii, m. biceps brachii, m. longissimus, m. semitendinosus and m. rectus abdominis were used in a detailed study of muscular growth.
4. The growth of fat tissue was examined by studying five fat depots, subcutaneous, intermuscular, kidney, channel and intramuscular. Intramuscular fat was determined by an ether extraction technique.
5. A number of preliminary experiments were conducted during the establishment of microanatomical technique. Results are reported.
6. Various methods of measuring growth were used. The growth coefficient, determined from the allometric equation, was commonly employed.
7. The retardation of growth in weight of individual muscles, on an age basis, was not apparent when growth of these muscles was assessed relative to total muscle weight.
8. Differential growth was demonstrated in six muscles relative to total muscle weight, from 150 days gestation to 630 days old. An abrupt change in growth impetus in four of the muscles, at birth, was evident. These muscles and their pre-natal and postnatal growth impetuses were
m. extensor carpi radialis high-low
m. extensor digiti tertii high-low
m. longissimus low-high
m. rectus abdominis low-high
The possibility of these muscle growth patterns being a result of the new demands thrust upon the musculature of the newborn calf are discussed.
9. A study of the distribution of the weight of the nine "standard muscle groups" (Butterfield, 1963c), over five age phases, from 150 days gestation to 630 days old, confirmed that differential growth patterns occurred within the musculature. These patterns (or growth impetuses) were described as
Standard muscle group / growth impetus
1 / high-average
2 / high-low
3 / average-high
4 / low-high
5 / average or low-average
6 / low
7 / low-average-high or low-high
8 / average
9 / low-average
10. Dietary restriction in Friesian bull calves, sufficient to produce an empty live weight of 44.45 kg compared to their adequately fed half-sibs* empty live weight of 95.03 kg, was associated with marked changes in muscle growth. Total muscle weight and individual muscle weight were restricted by over 50.0% and a differential influence upon individual muscle weight growth patterns was observed. Those muscles with a high post-natal growth impetus, m. longissimus, m. rectus abdominis and m. semitendinosus were retarded most, and muscles with a low post-natal growth impetus were retarded least.
11. A study of muscle-weight-distribution and growth coefficients of six muscles in Friesian and A.l.S. bull calves showed no significant differences between breeds, except for the proportion of m. extensor carpi radialis whose weight relative to total muscle weight was significantly greater (P<0.01) in A.l.S. calves. In older cattle, the growth coefficient of m. longissimus was significantly greater (P<0.01) in a group of Friesian steers compared to that in a group of Angus steers. A study of the muscle-weight-distribution of these steers, following adjustment by covariance of the total dissected fat percentage, showed that significant differences in the distribution of the “standard muscle groups“ remained.
12. The influence of sex on the growth coefficients of individual muscles and “standard muscle groups" was examined over wide total muscle weight ranges. M. extensor carpi radialis had a significantly greater growth coefficient (P<0.05) in females compared to that of steers. All other comparisons showed no significant differences between sexes.
13. Growth in weight of muscles was associated with large increases in muscle fibre diameter and muscle length.
14. Changes in muscle fibre diameter associated with muscular growth were investigated by examination of the regression coefficients of muscle fibre diameter on individual muscle weight. Significant differences occurred in the three muscles with a high growth impetus (m. longissimus, m. semitendinosus and m. rectus abdominis). This was due, almost solely, to the large growth coefficients obtained over the 85-160 days growth phase.
15. Restricted nutrition caused approximately 30.0% retardation in individual muscle weight, in Friesian bull calves at 84 days old, compared to muscle weights in a group of adequately-fed half-sibs at the same age. This was associated with muscle fibre diameters in the restricted calves that were about 8.0 µ less than in the muscles of their adequately-fed half-sibs. On a muscle fibre diameter basis the 84-day-old calves, on restricted nutrition, were comparable with a group of adequately-fed half-sibs of 19 days mean age.
16. A comparison of the muscle fibre diameters between bull and heifer half-sib Friesian calves, at 84 days old, following the adjustment by covariance of individual muscle weight, revealed no significant differences.
17. When regression coefficients for cross-sectional area over individual muscle weight were examined, significant differences between age phases occurred. With the majority of regression coefficients the standard errors were very large. Cross-sectional area did not appear to be closely associated with individual muscle weight. The possibility of this finding being a result of technique or being associated with the structure of the muscle, is discussed.
18. An attempt to establish whether post-natal hyperplasia occurred in bovine muscle, produced an inconclusive result. The reason for the great variability in the regression coefficients of muscle fibre number over individual muscle weight, and the high standard errors, is discussed. It may be associated with the difficulty of extrapolating muscle fibre counts from small sample areas to the complete cross-sectional areas of large beef muscles.
19. Detailed examination of the musculature of a Friesian steer carcase (2 3 .9% total dissected fat) showed that the intramuscular fat content of individual muscles or small groups of anatomically similar muscles, varied from 1.5% (m. pectineus) to 20.8% (mm. intercostales).
20. The partition of fat between five fat depots in a group of bovine carcases, was examined. Differential changes occurred in the proportions of the fat depots with increasing side weight. Intermuscular and subcutaneous depots increased in proportion, the latter more slowly in relation to increasing side weight. Kidney, channel and intramuscular depots showed a progressive decline in their proportions with increasing side weight.
21. Regression equations were developed to express changes in weights of the five fat depots relative to total side fat weight They were
Fat depot / Regression equation
intermuscular / y = -196.50 + 0.507X
subcutaneous / y = - 36.20 + 0.286x
intramuscular / y = 109.14 + 0.110x
kidney / y = 91 .83 + 0.082x
channel / y = 32.23 + 0.016x
All regressions were significant (P<0.01).
22. An examination of the intramuscular fat distribution throughout the ''standard muscle groups" in fattened (26.8% total dissected fat) and unfattened (14.0%) steers suggested a differential growth of this depot. The probability that the growth of intra muscular fat was not greatly influenced by patterns of growth withi the musculature, is discussed.
23 When the partition of intramuscular fat between the "standard muscle groups" of fattened and unfattened steers was examined, three groups, 3, 5 and 6 were found to vary significantly (P<0.05). The possibility that these changes were related to muscle growth patterns, is discussed.
24. The effects of intramuscular fat weight on three carcase parameters, total dissected muscle weight, "standard muscle group" weight and muscle-bone ratio, were examined. Regression coefficients, and growth coefficients involving these parameters were developed from muscle weights, as dissected, and muscle weights corrected for intramuscular fat weight. In all cases, when respective pairs of coefficients were compared, no significant differences were found.