T GROWTH OF MUSCLE AND FAT IN BEEF STEERS FROM 6 TO 36 MONTHS OF AGE 1 HE continuing demand for an increase in the quantity and quality of lean from beef steers emphasizes the importance of learning more about the growth and development of meat animals as they are affected by breeding, feeding and management. The factor of quantity is measured by comparisons of rate of gain, feed conversion, cutability, conformation, age, thickness of fat covering and size of rib eye. Quality is a subjective attribute of fresh meat that includes color, texture, marbling, fat distribution and potential or apparent eating satisfaction. These quality factors are influenced by age, weight, breed, nutrition and sex as well as some other variables. This research was conducted to study the changes in rate and nature of growth of selected muscles, total separable lean and separable fat in beef steers grown under different feeding regimens. All of the steers received the same feed mixture, but it varied in amounts to control rate of gain. Data were obtained at specific chronological ages of 6, 12, 18, 24, 30 and 36 months. Materials and Methods Fifty-one Angus steer calves, averaging 45 kg, were obtained 3 to 10 days after birth and allowed free-cholce access to the experimental diet at 15 days of age. All calves received an equal amount of whole milk until weaning at 60 days. The groups consisted of three feeding regimens: group 1, full-fed; group 2, restricted to 0.34 to 0.50 kg of gain per day and group 3, fed the same as group 2 until 180 days (6 months) prior to slaughter then full-fed. All of the steers received the same 70:30 concentrate roughage diet; the amount fed per day controlled the weight gains. All of the animals were individually fed from day one and were turned out in a dry lot for approximately 4 hr. during the day. Six animals were slaughtered at 6 1 Conducted in cooperation with Fieid Crops and Animal Products Research Branch, Market Quality Research Division. 2 Animal Science Research Division, A.R.S., Beltsville, Maryland 20705. R. L. HINER AND J. BOND U. S. Department o] Agriculture 2 225 months of age, three from group 1 and three from group 2. There were no group 3 calves slaughtered at 6 months as none were available. At the remaining slaughter periods of 12, 18, 24, 30 and 36 months, nine animals were slaughtered, three from each group. The animals were slaughtered within plus or minus 14 days of the age designated. Feed was withheld 24 hr. prior to slaughter, but the animals had free access to fresh water at all times. All animals were weighed and slaughtered in accordance with approved methods. The slaughter data obtained for each animal included weights of the various organs, glands and hot dressed carcass. Fresh lean samples were removed from five muscles within 45 rain. after bleeding from the right side for detailed biochemical study of the protein, fatty acids and enzyme systems. The five muscles sampled were the longissimus, semitendin'osus, biceps ]emoris, obliques abdominis internus and diaphragma. The dressed carcasses were shrouded and allowed to chill for 48 hr. at 1 to 2 C, and the chilled carcasses were weighed. On the sixth day after slaughter, the carcasses were graded and measured according to procedures outlined in "Techniques and Procedures in Animal Production Research" (1959). The following muscles were carefully removed in their entirety from the intact left half of the carcass: lot~gissimus, semimembranosus, semitendinosus, biceps ]emoris, rectus femoris, adductor, psoas major and triceps brachii. After removal of the sixth to the 12th wholesale rib, the remainder of the right side including the sixth, seventh, eighth and 12th rib was carefully separated int O lean, fat and bone and each component weighed. Likewise, the same separation was made on the remaining left side. The right 9-10-11th rib section of the prime rib was separated into eye muscle, remaining lean, fat, bone and tendon and each of the five components was weighed. As soon as the eight muscles were removed, each was trimmed free of all fat covering and connective tissue cover and weighed. Both longitudinal and JOURNAL OF ANIMAL SCIENCE vol. 32, no. 2
226 HINER AND BOND COMPOSITION AND CHEMICAL ANALYSIS OF THE EXPERIMENTAL DIET Ingredients % Cracked corn 50 Alfalfa hay '~ 15 Timothy hay ~ 15 Molasses 9 Soybean meal 10 Trace mineral salt 1 Vitamin A b _[_ Chemical analysis c Dry matter 90.7 Crude protein 14.5 Gross energy 4193.6 a Acid detergent fiber 13.3 ~ Ground through a 19.0 mm screen. b 2,992 UI units of vitamin A/kg feed. e Dry basis. a Calories/g. cross-sectional tracings were made, the latter across the center of the muscle except for the longissimus which was made at a previously marked position between the 12th and 13th rib. The areas of the intact muscles and cross-sectional tracings were made with a compensating planimeter and expressed in square centimeters. Results and Discussion The slaughter weight, chilled carcass weight and dressing percent for the three groups of steers at the six age intervals are shown in table 1. There were significant or highly significant differences in slaughter and chilled carcass weights due to feeding regimens between animals in each of the age groups except those slaughtered at 36 months of age. The differences in dressing percents, based on slaughter and chilled carcass weights at 6, 30 and 36 months, were significantly different due to feeding regimen. The group 1 animals were much higher in dressing percent at each 6-month age interval than either groups 2 or 3, an indication that the group 1 steers were depositing more of their weight in the form of fat as shown in table 2. The increase in dressing percent for group 3 steers from 12 to 36 months varied from 58.50 at 12 months to 60.81 at 36 months of age or 2.31%; whereas, those in group 1 increased from 60.49 to 65.80 or 5.31% and group 2 from 56.30 to 61.37 or 5.07%. The average weights of the eight muscles studied from the full-fed animals at 6 months of age were all heavier than the average of those from group 2 animals (table 2). However, only four muscles from the group 1 animals, the longissimus, semimembranosus, rectus ]emoris and psoas major, were significantly heavier in weight than those from group 2. The total weight of separable lean in the group 1 animals also was significantly heavier at 6 months of age. During each of the succeeding 6-month growth intervals, the increases in weight of the muscles and separable lean from the three groups of animals varied considerably between groups and from one growth period to another. The weights of the muscles from group 1 at each 6-month TABLE 1. SLAUGHTER WEIGHT, ~ CHILLED CARCASS WEIGHT AND DRESSING PERCENT OF THREE GROUPS OF STEERS BY SLAUGHTER AGE AND FEEDING REGIMEN Age in months 6 12 18 24 30 36 Slaughter weight Group 1 ~ kg 170.6" 365.1"* 398.8** 505.8",* 530.7* 556.0 Group 2 e kg 109.2 233.2 271.6 348.4 427.9 533.3 Group 3 '1 kg... 267.6 376.9 413.4 460.3 524.2 Chilled carcass Group 1 kg 89.8* 221.2" 236.8** 319.8" 342.3* 376.2 Group 2 kg 53.1 131.7 152.6 204.3 247.7 327.0 Group 3 kg... 156.6 224.1 249.5 277.4 318.9 Dressing percent Group 1 % 52.14" 60.49 59.36 63.17 64.52* 65.80* Group 2 % 48.86 56.30 56.18 58.75 57.88 61.37 Group 3 %... 58.50 59.44 60.43 60.34 60.81 ~ Average of three animals. b Group 1 full-fed--70:30 concentrate:roughage. e Group 2 restricted to gain 0.34 to 0.50 kg per day; same ration as group I. a Group 3 restricted as in group 2 until 180 days prior to slaughter, then full-fed as in group 1. P~.05 within slaughter age. ~ P~.01 within slaughter age.
GROWTH OF MUSCLE AND FAT 227 TABLE 2. WEIGHT OF individual MUSCLES FROM THE LEFT SIDE, TOTAL SEPARABLE LEAN AND SEPARABLE FAT OF THE CARCASS BY SLAUGHTER AGE AND FEEDING REGIMEN Age in months 6 12 18 24 30 36 Longissimus Group 1 " g 1784" 3481 3535 4230 4325 4827 Group 2 g 1027 2581 2786 3721 4287 4920 Group 3 g... 2890 3758 4303 4301 4679 Semimenbranosus Group 1 g 1512" 2588 2777 3097 3527 3948 Group 2 g 905 1918 2303 2997 3414 3468 Group 3 g... 2158 3085 3193 3367 3625 Rectus fernor~ Group 1 g 548** 851 957 823 1367 1386 Group 2 g 324 755 815 927 1304 1347 Group 3 g... 790 917 990 1283 1387 Biceps temor~ Group 1 g 1548 2644 3442 4022 4328 5153 Group 2 g 930 2225 2814 3848 4051 4837 Group 3 g.,.. 2738 3176 3948 4457 4661 Semitend~os:us Group 1 g 672 1220 1378 1442 1737 2111 Group 2 ~ 321 954 1052 1545 1618 1913 Group 3 g... 1064 1441 1539 1666 1713 Psoasmajor Group 1 g 459** 778 1240 1331 1482 1721 Group 2 g 280 633 971 1304 1459 1532 Group 3 g... 674 1192 1375 1377 1547 Adductor Group 1 g 496 888 1130" 1236 1143 1120 Group 2 g 371 652 860 1149 1119 1047 Group 3 g... 769 1056 1134 986 1126 Triceps brachii Group 1 g 783 1648" 1776 2020 2337 2728 Group 2 g 448 1179 1344 1831 2050 2550 Group 3 g... 1287 1714 1857 2069 2395 Separable lean Group 1 kg 60.3* 121.7 137.5" 156.1 179.4 190.8 Group 2 kg 37.2 90.7 102.5 134.6 165.3 189.0 Group 3 kg... 102.4 133.9 153.9 161.4 187.1 Separable fat Group 1 kg 19.0"* 72.6* 69.1" 106.3" 119.4 *~ 139.8 Group 2 kg 7.4 25.5 27.0 39.3 45.8 99.0 Group3 kg... 31.6 60.0 65.7 82.1 92.4 a See table 1 for identification of groups. P<.05 within slaughter age. ~ P<.01 within slaughter age. interval were generally heavier than for groups 2 and 3, but the differences in average weight between groups tended to decrease at each age interval. The weight of separable lean after 36 months was nearly the same for the three groups. The weight of separable fat from the steers in group 1 was significantly (P~.01) higher than in group 2 due to feeding regimen. There were no steers in group 3 at 6 months. Among animals in the three groups of steers at 12, 18 and 24 months of age, the weight of separable fat was significantly different (P~.05). At 30 months, the differences were highly significant (P~.01). However, at 36 months, group 1 steers had 139.8 kg of separable fat; group 2, 99.0 and group 3, 92.4. The differences were not significant. The rapid increase
228 HINER AND BOND in weight of separable fat of the group 2 animals from 30 to 36 months more than doubled in weight (45.8 to 99.0 kilograms). Guenther et al. (1965) demonstrated that with groups of half-sib Hereford steers on a high plane of nutrition the increase in carcass weight was due to increased weight of muscle and fat with little change in bone weight. They reported that fat had increased more rapidly than muscle. Judge et al. (1969) reported that during the early feeding period (216 to 340 kg live weight) of half-sib Angus steers fed a hay diet as compared to a higher energy diet of silage the fat-free body weight was suppressed and the dry matter reduced but the ash content was increased in several of the muscles studied. The results showed increased fat-free body weight and dry matter from the half-sib animals finished on a high energy diet. Growth of muscles and lean the first 6 months as measured by ratio of weight increase from 6 to 12 months showed the muscles from group 3 increased most rapidly. TABLE 3. RATIO OF WEIGHT OF MUSCLE, SEPARABLE LEAN AND SEPARABLE FAT FROM 6 TO 12, 6 TO 18, 6 TO 24, 6 TO 30 AND 6 TO 36 MONTHS OF SLAUGHTER AGE Growth period--months 6 to 12 6 to 18 6 to 24 6to 30 6 to 36 Longissimus Group 1" 1.95 1.98 2.37 Group 2 2.51 2.71 3.62 Group 3 2.81 3.66 4.09 Semimembranosus Group 1 1.71 1.84 2.05 Group 2 2.12 2.54 3.31 Group 3 2.38 3.41 3.53 F ectus femoris Group 1 1.55 1.75 1.50 Group 2 2.33 2.52 2.86 Group 3 2.44 2.83 3.06 Biceps #moris Group 1 1.71 2.22 2.60 Group 2 2.39 3.03 4.14 Group 3 2.94 3.42 4.25 Semitendinosus Group 1 1.82 2.05 2.15 Group 2 2.97 3.27 4.81 Group 3 3.31 4.48 3.79 Psoas major Group 1 1.69 2.70 2.90 Group 2 2.27 3.49 4.66 Group 3 2.41 4.26 4.92 Adductor Group 1 I. 79 2.28 2.49 Group 2 1.76 2.32 3.10 Group 3 2.07 2.85 3.06 Triceps brachii Group 1 2.11 2.29 2.58 Group 2 2.63 3.00 4.09 Group 3 2.87 3.83 4.15 Separable lean Group 1 2.01 2.28 2.58 Group 2 2.44 2.76 3.62 Group 3 2.75 3.60 4.14 Separable fat Group 1 3.82 3.63 5.69 Group 2 3.45 3.65 5.31 Group 3 4.27 8.11 8.88 a See tabie 1 for identification of groups. 2.42 2.71 4.17 4.79 4.19 4.56 2.33 2.61 3.77 3.83 3.72 4.01 2.49 2.53 4.02 4.16 3.96 4.28 2.87 3.33 4.36 5.20 4.79 5.01 2.58 3.14 5.03 5.95 5.18 5.33 3.23 3.75 5.22 5.48 4.92 5.53 2.31 2.28 3.02 2.82 2.66 3.04 2.98 3.55 4.58 5.69 4.62 5.35 2.98 3.16 4.44 5.08 4.35 5.03 6.28 7.36 6.18 13.38 11.09 12.49
GROWTH OF MUSCLE AND FAT 229 The ratio of increase varied from 2.07 (207%) for the adductor which doubled its weight to 3.31 (331%) for the semitendinosus which more than tripled its 6-month weight (table 3). The ratio of weight of the muscle, lean and fat at 12, 18, 24, 30 and 36 months and its weight at 6 months was used to determine weight increase. The weight of ttie muscle, lean and fat from the steers in group 2 was used as the devisor to determine ratio increase for the group 3 steers. Total separable lean for the group 3 animals increased in weight 2.75 times and total separable fat 4.27 times in 6 months on full-feed. The increase in muscle growth as measured by weight increase for those animals on a restricted level (group 2) was slightly less than group 3, with rate of separable fat weight increase lowest at 3.5 times in 6 months. The rate of muscle weight increase and total lean were less rapid in the group 1 animals, only the triceps brachii and total lean doubling in weight during the 6 months on full-feed. These results are in agreement with Guenther et al. (1962) who reported that muscle development in beef occurs early (91 to 114 kg). At succeeding age periods of 18, 24, 30 and 36 months, the rate at which the muscle weights increased was generally most rapid for those animals in group 3, followed closely by those in group 2. At each age period, the full-fed group 1 animals continued to show less muscle weight increase than the other two groups. The weights of muscles from group 1 at 6 months were heavier, and while continuing to grow, it is at a slower rate than for ffroups 2 and 3. The weight changes shown in table 2 for each muscle from 6 to 36 months indicate that there were different rates of growth for certain muscles. The muscle that was most consistent in its rate of growth for the three groups of animals was the psoas major, a little used muscle on the ventral surface of the transverse processes of the lumbar vertebrae. The second most consistent was the semitendinosus muscle of the round, an active work-type muscle. At the other extreme or slowest growing, especially in the group 1 animals, were the rectus ]emoris and adductor, two smaller muscles of the round. In the case of the group 2 and 3 animals, the adductor muscle's rate of growth decreased most consistently. Hammond (1932), MeMeekan (1940a, b, c; 1941), Pomeroy (1941) and Callow (1947, 1948) have described similar growth patterns of muscle development in meat animals as affected by age and plane of nutrition. The rate of weight increase for separable fat was fairly consistent for groups 1 and 2 at 12, 18, 24 and 30 months, having increased in weight about six times. However, between 30 and 36 months, the rate of fat deposition for the group 2 steers suddenly increased from 6.2 to 13.4 times the original 6-month weight. Part of these differences in rate of increase can be accounted for by the difference in weight of fat in animals in groups 1 and 2 at 6 months of age. The same rate of fat deposition is noted for the group 3 animals during the 12-, 18- and 24-month growth periods, but at 30 months it increased from nearly nine to 11 times and to 12.5 times at 36 months over the 6-month weight. The results indicate that the rate of muscle and separable lean growth was more rapid as shown by weight changes in animals after the first 6 months of restricted development. The full-feeding of animals the last 6 months after 30 months on a restricted diet increased the rate of: fat deposition without significantly increasing the rate and amount of lean deposition. The largest ratio increase of all muscles, lean and fat weights occurred during the growth :period of 6 to 12 months. The increase was generally highest for the group 3 animals, with those in group 2 following closely and group 1 having the least. However, the increase in weight of all muscles, separable lean and separable fat from 12 to 18 months was considerably less than the preceding period for all three groups. The percent increase for the muscles, separable lean and fat between 18 and 24 months of are showed a more rapid rate of growth for all three groups, with the greatest gain being for group 3. These data again show that restricted feeding followed by full-feeding for 6 months prior to slaughter results in some added increase in growth of muscles and total separable lean and a rapid increase in weight of separable fat. These results are in agreement with Berg and Butterfield (1967) who have reported that muscle, fat and bone in beef cattle have differential growth rates during development. They concluded that muscle development had an immediate growth impetus, fat a high impetus and bone a low impetus, especially during the fattening period. These differential growth patterns influenced the carcass composition.
230 HINER AND BOND TABLE 4, MUSCLE WEIGHTS EXPRESSED AS A PERCENTAGE OF TOTAL SEPARABLE LEAN BY SLAUGHTER AGE AND FEEDING REGIMEN Age in months 6 12 18 24 30 36 Longissimus Groupl ~'b % 5.91 5.73 5.15 5.42 4.82 Group 2 % 5.52 5.69 5.43 5.53 5.19 Group 3 %... 5.64 5.62 5.46 5.33 Semirnembranosus Group 1 % 5.01 4.25 4.04 3.97 3.93 Group 2 '% 4.86 4.23 4.49 4.43 4.13 Group 3 %... 4.21 4.61 4.15 4.17 Rectus ]emoris Group 1 % 1.82 1.40 1,39 1.05 1.52 Group 2 '% 1.74 1.67 1.59 1.38 1.58 Group 3 %... 1.54 1,37 1.29 1.59 Biceps femoris Group 1 % 5.13 4.35 5.01 5.15 4.83 Group 2 % 5.00 4.91 5.49 5.72 4.90 Group 3 %... 5.35 4.74 5.13 5.52 Semitendinosus Group 1 % 2.23 2.01 1.98 1.85 1.94 Group 2 % 1.73 2.11 2.05 2.30 1.97 Group 3 %... 2.08 2,15 2.00 2,06 Psoas major Group 1 % 1.52 1.38 1.78 1.71 1.65 Group 2 % 1.50 1.40 1.89 1.94 1.77 Group 3 %... 1.32 1.78 1.79 1.71 Adductor Group 1 % i. 64 1.46 1,64 1.58 1.27 Group 2 % 1.99 1.44 1.68 1.71 1.35 Group 3 '%... 1.50 1.58 1.47 1.22 Triceps brachii Group 1 % 2,59 2,71 2.61 2.59 2.61 Group 2 % 2.41 2.60 2.62 2.72 2.48 Group 3 %... 2.51 2.56 2.41 2.56 a See table 1 for identification of groups, b Individual muscle weights as shown in table 2 were doub]ecl to obtain the weight of the pair of muscles before percentages were calculated. 5.03 5.21 5.00 4.14 3,64 3.87 1.45 1.43 1.48 5.40 5.12 4:98 2.21 2.02 1.83 1.81 1,62 1.65 1.17 1.11 1.20 2,83 2.70 2.56 Table 4 shows the percent each muscle was of the total separable lean at each age. Only three muscles, biceps femoris, psoas major and triceps brachii, contributed a higher percent to total lean after 36 months' growth than they contributed at 6 months. Except for group 1, where it remained the same, the weight of the semitendinosus muscle also increased as a percent of total lean. These four muscles had shown the largest weight increase. The longissimus, semimembranosus, rectus ]emoris and adductor muscles decreased during the 36 months' growing period. The area of a muscle, especially the crosssectional area of the longissimus between the 12th and 13th rib, has been used as a possib e predictor of carcass leanness and is used in estimating yield grades. At 6 months of age, the cross-sectional area (table 5) showed that all muscles except the semitendinosus, psoas major and triceps brachii from full-fed animals were significantly (P<.05 or P~.01) larger than from those on a restricted diet. However, after 6 months, the muscles from the restricted and restricted plus full-fed animals increased more rapidly in cross-sectional area than those from the full-fed animals. Except for the cross-sectional area of the biceps ]emoris at 12 months and longissimus and semimembranosus at 18 months, there were no statistically significant differences between groups. The cross-sectional area of the muscles from the full-fed group 1 steers was larger than group 2 or group 3 animals for the first 18 months. After this stage of growth, the muscles of the restricted and re-
GROWTH OF MUSCLE AND FAT 231 TABLE 5. CROSS-SECTION AREA OF EIGHT MUSCLES BY SLAUGHTER AGE AND FEEDING REGIMEN Age in months 6 12 18 24 30 36 Longissimus Group 1 ~ cm ~ 43.6* 66.6 76.8* 84.4 75.8 82.5 Group 2 cm ~ 24.1 53.2 63.3 76.4 78.3 87.7 Group 3 cm 2... 55.3 69.4 79.5 78.3 80.7 Semimembranosus Group 1 cm 2 80.3** 104.7 129.1",* 141.1 178.3 186.6 Group 2 cm ~ 44.3 86.3 108.3 139.7 158.6 156.7 Group 3 cm -... 84.3 132.2 154.6 151.5 165.2 Rectus #moris Group 1 cm 2 51.1" 57.5 65.8 72.0 98.4 97.7 Group 2 cm - 29.5 56.1 55.2 76.6 89.2 86.6 Group 3 cm 2... 57.0 72.4 82.8 91.6 104.1 Biceps femoris Group 1 cm 2 59.2* 93.7* 109.3 120.3 138.6 139.1 Group 2 cm 2 42.6 81.3 88.0 117.5 127.9 130.4 Group 3 cm "~... 83.6 103.5 123.9 121.7 136.9 Semitend~o#us Group 1 cm ~ 27.5 43.6 50.9 57.8 76.6 68.1 Group 2 cm 2 12.7 34.9 40.7 63.3 59.5 70.9 Group 3 cm 2... 36.5 53.8 52.2 64.6 65.8 Psoas major Group 1 crn ~ 20.2 28.6 33.8 32.3 37.7 38.7 Group 2 cm 2 13.3 21.2 26.5 36.5 39.4 39.0 Group 3 cm ~... 23.8 31.5 35.5 34.6 41.5 Adductor Group 1 cln - 45.5* 53.5 77.6 85.4 95.1 84.3 Group 2 cm ~ 30.5 51.8 65.7 84.9 91.4 83.4 Group 3 cm 2... 50.8 67.7 88.3 75.6 87.3 Triceps brachii Group 1 cm 2 64.3 121.9 143.7 141.0 148.0 167.0 Group 2 cm 2 23.6 91.3 92.8 132.1 133.6 146.8 Group 3 cm "~... 85.6 112.8 130.4 131.8 147.7 " See table 1 for identification of groups. P<.05 within slaughter age. ~ P<.01 within slaughter age. stricted plus full-fed animals were nearly the same as those from the full-fed animals. Summary Fifty-one Angus steers slaughtered at 6, 12, 18, 24, 30 and 36 months were used to study the growth of muscles, separable lean and separable fat in beef steers. These steers were randomly grouped as follows: group 1, full-fed; group 2, restricted to 0.34 to 0.50 kg of gain per day; group 3, fed the same as group 2 until 180 days (6 months) before slaughter then full-fed. All steers were individually fed the same diet. Three group 1 and three group 2 animals were slaughtered at 6 months of age. At 12, 18, 24, 30 and 36 months of age, nine animals were slaughtered, three from each group. Significant dif- ferences were found in slaughter and chilled carcass weights between the three groups of animals at each age period except 36 months; whereas, dressing percents were significantly different only at 6, 30 and 36 months, but not at 12, 18 and 24 months. The group 2 animals were lowest until 36 months of age, and the group 3 were intermediate except at 18 and 36 months of age. Rate of muscle growth varied among age groups regardless of feeding regimen. The psoas major and semiten'dinosus muscles tended to increase in weight more rapidly than the rectus ]emoris and adductor muscles. The weight of muscles in groups 2 and 3 increased more rapidly than in group 1. The increase in weight of separable lean was more rapid in groups 2 and 3 than in group 1. However, the increase in separable fat was more rapid in group 3,
232 HINER AND BOND having increased 12.49 times in weight from 6 to 36 months. The most rapid increase in muscle weight occurred between 6 and 12 months for the three groups. The weights of the psoas major, biceps ]emoris and triceps brachii increased their proportion of total lean as the animals matured. The semitendinosus muscle had approximately the same proportion of the total lean throughout the growth period. The remaining muscles, longissimus, semimembranosus, rectus ]emoris and adductor, decreased in their proportion of total lean. The longitudinal and cross-sectional areas of the several muscles showed differences similar to those found for the individual muscle weights. Literature Cited Berg, R. T. and R. M. Butterfield. 1967. Growth patterns of bovine muscle, fat and bone. J. Anita. Sci. 27:611. Callow, E. H. 1947. Comparative studies of meat. I. The chemical composition of fatty and muscular tissue in relation to growth and fattening. J. Agr. Sci. 37:113. Callow, E. H. 1948. Comparative studies of meat. II. The changes in the carcass during growth and fattening, and their relation to the chemical composition of the fatty and muscular tissue. J. Agr. Sei. 38:174. Guenther, J. J., L. S. Pope, G. V. Odeli and R. D. Morrison. 1962. The growth and development of beef calves from weaning to slaughter weight with reference to the effect of plane of nutrition. Okla. Agr. Exp. Sta., Misc. Pub. 67. Guenther, J. J., D. H. Bushman, L. S. Pope and R. D. Morrison. 1965. Growth and development of major carcass tissues in beef calves from weaning to slaughter weight, with reference to the effect of plane of nutrition. J. Anita. Sci. 24:1184. Hammond, J. 1932. Growth and Development of Mutton Qualities in the Sheep. Oliver and Boyd, Ltd., London. Judge, M. D., N. L. Firth, H. R. Johnson, W. V. Kessler, R. B. Harrington and R. R. Garrigers. 1969. Dietary effects on beef composition. III. Weight, composition and potassium-40 content of fat-free muscles. J. Agr. Sei. 72:303. McMeekan, C. P. 1940a. Growth and development in the pig with special reference to carcass quality characters. I. Age changes in growth and development. J. Agr. Sci. 30:292. McMeekan, C. P. 1940b. Growth and development in the pig with special reference to carcass quality characters. II. The influence of the plane of nutrition on growth and development. J. Agr. Sci. 30: 387. McMeekan, C. P. 1940c. Growth and development in the pig with special reference to carcass quality characters. III. Effect of the plane of nutrition on the form and composition of the bacon pig. J. Agr. Sci. 30:fill. McMeekan, C. P. 1941. Growth and development in the pig with special reference to carcass quality characters. IV. The use of sample joints and of carcass measurements as indices of the composition of the bacon pig. J. Agr. Sci. 31:1. Pomeroy, R. W. 1941. The effect of sub-maintenance diet on the composition of the pig. J. Agr. Sei. 31:50. Techniques and Procedures in Animal Production Research. 1959. Monogr. A.S.A.S.p. 228.