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Correlated responses in body composition and fat partitioning

to divergent selection for yearling growth rate in Angus cattle

1

_*

Diana Perry , P.F. Arthur

NSW Agriculture, Agricultural Research Centre, Trangie, NSW 2823, Australia

Received 21 August 1998; received in revised form 7 April 1999; accepted 20 April 1999

Abstract

Body composition data were collected in a serial slaughter experiment on 91 Angus steers produced after 12 years of divergent selection for yearling growth rate. Steers born in 1986 and 1987 from lines selected for high (High Line) and low (Low Line) growth rate from birth to weaning, and from an unselected line (Control Line) were slaughtered at 0 (birth) and circa 7, 12, 27, 35 and 45 months of age. Weights were obtained for dissected carcass muscle, bone, subcutaneous and intermuscular fat, non-carcass fat depots, visceral organs, hide and head, tail and distal legs. At the same stage of maturity of empty body weight or total fat, body components and fat partitions were a similar proportion of their mean mature weights in all three selection lines. At the same mean empty body weight of 360 kg, High Line steers had smaller proportions of carcass fat and a higher proportion of bone than steers from the other two lines, and the Low Line steers had a smaller proportion of muscle relative to the other lines. Relative to the High Line, Control and Low Line steers required 85 and 175 additional days, respectively, to attain this carcass weight. Mean mature empty body weight (body weight minus urine and gut contents) was 666617 (standard error, S.E.) kg, 588618 kg and 512616 kg for High, Control and Low Line steers, respectively. There were no significant (P,_{0.05) differences among the selection lines for the pattern of growth of the different body}

components from birth to maturity. At maturity, no significant selection line differences were obtained in body composition. Relative growth rates indicated that, with respect to body weight, dissected fat was late developing and bone, muscle and viscera early maturing, over the period from weaning to maturity. When compared at the same stage of maturity or level of fatness, steers from the line selected for fast growth rate would be heavier than unselected steers, but have the same body composition.  2000 Elsevier Science B.V. All rights reserved.

Keywords: Beef cattle; Relative growth; Selection; Body composition, Fat partitioning

1. Introduction

Many beef cattle improvement schemes are based on some measure of growth. The perceived benefits

*Corresponding author. Tel.: 161-2-6888-7404; fax: 161-2- _{of selecting for faster growing cattle include the} 6888-7201.

ability of stock to grow to market weights more

E-mail address: arthurp@agric.nsw.gov.au (P.F. Arthur)

1 quickly and to use feed more efficiently to attain

Present address: Meat Quality CRC, University of New England,

these weights. However there may be associated

Armidale, NSW 2351, Australia.

changes in body composition that could affect the and sire, with calves born to cows two years of age value of the carcass. or greater than eight years of age being excluded Selection for growth results in a difference in where possible. Twin born and hand reared calves mature size, with consequent difficulties in interpre- were also excluded. Calves nursed their dams on tation when doing comparative studies, as it is pasture throughout the preweaning period. The possible that much of the difference reported be- calves were castrated at about three months of age. tween genotypes may be related to differences in the After weaning at about seven months of age, the stage of maturity at which the comparison takes steers were fed by an automatic feeding system place (Webster, 1980). Thus any comparative work described by Herd (1991). They were fed a high that is done between animals selected for measures quality pelleted diet which supplied 10.9 MJ ME / kg of growth should encompass differences in the dry matter. They had access to this feed 24 h a day, mature size of the animals when interpreting differ- although the feeding system was programmed to feed ences in body composition. an animal only if it had not been fed in the previous This experiment was part of the evaluation of a half hour. There was thus the potential for each research project investigating the effect of divergent animal to be fed 48 times each day. The weight of selection for growth rate to yearling age (yearling feed offered each time was approximately 1 kg. growth rate) on growth, feed efficiency and body Three 1987 born steers which did not adapt to the composition. Following 12 years of selection the feeding system were dropped from the experiment. A divergence in growth between the lines selected for further 12 (nine 1986 born, three 1987 born) steers increased and decreased growth rate to yearling age died from bloat or other gastro–intestinal conditions. exceeded 25% for both males and females (Parnell et Data on dissected composition were collected in a al., 1997). This paper reports the effect of divergent serial slaughter experiment. For each of the two selection for yearling growth rate on body com- years, two steers per selection line were slaughtered position in steers, and the growth of these body at 0 (birth) and at circa 7, 12, 27 and 35 months of components to maturity, in a serial slaughter experi- age (three 1986 Control Line steers were slaughtered ment involving steers from 0 to 47 months of age. at 12 months). The remainder (12 from the 1986 calving, 18 from the 1987 calving) were grown until they were considered to be mature and then

slaug-2. Materials and methods htered in batches as quickly as facilities would allow. Steers were considered to be mature when their 2.1. Animals and design growth curves, based on weekly liveweight measure-ments, showed that they had effectively stopped Steers were derived from the three selection lines growing. Age at mature slaughter ranged from 44 to of Angus cattle at the Agricultural Research Centre, 47 months. Animals were randomly selected for each Trangie, New South Wales, Australia. These selec- slaughter. A schematic diagram of the slaughter tion lines were established in 1974 using 220 cow schedule is presented in Fig. 1. There were slaughter Angus breeding herd at the Centre. Firstly, 50 cows data available from a total of 91 steers.

were randomly chosen to form a Control Line, then

the remainder of the cows was divided into a High 2.2. Measurements Line and a Low Line, based on their own growth rate

Fig. 1. Schematic diagram of the slaughter schedule within each of the 1986 and 1987 born groups. S indicates slaughter points and numbers in parentheses indicate mean number of steers slaughtered per selection line per year born group.

non-carcass components (head, tail and the non- effect of line, age, year of birth and all first order carcass portions of the legs), and hide. Non carcass interactions was examined on proportional body fat partitions were according to those defined by composition in mature animals using a multivariate Thompson et al. (1987). The carcasses were halved model which analysed all components simultaneous-and each half then weighed. All components were ly. Non-significant (P._{0.05) interactions and the}

frozen at 2₂₀8_{C until required for analysis. age effect were sequentially omitted from the model}

The right side of each carcass was subsequently until the final model was obtained. The final multi-thawed, divided into commercial cuts, and then variate model contained terms only for the fixed dissected into muscle, bone, subcutaneous fat (as effects of selection line (P._{0.05) and year of birth}

define by Thompson et al. 1987) and intermuscular (P,_0.01).

tissue (fat, plus connective, vascular, nervous and Fat in the carcass (subcutaneous and intermuscu-lymphatic tissue). These components were then lar) and non-carcass (omental, mesenteric, kidney weighed. Muscles were cleaned of intermuscular and channel and scrotal fat) depots was expressed as tissue and trimmed of any tendinous tissue at right a proportion of total dissectible fat in the body. angles to the muscle fibres at the last vestige of Using a multivariate model the effect of line, age, muscle tissue, before being weighed. Bones were year of birth and all first order interactions on fat cleaned of all tissues except the periosteum. partitioning was examined. To avoid singularity, scrotal fat was first excluded from the analysis. As 2.3. Statistical analysis before non-significant (P._{0.05) interactions and the}

age effect were sequentially omitted until the final For both mature and immature animals the dissec- model was obtained. The analysis was then repeated ted weights of muscle, bone, subcutaneous fat and with scrotal fat included and kidney / channel fat intermuscular tissue in the carcass were multiplied excluded. In both cases the final multivariate model by two. Total body fat was calculated as the sum of contained non-significant terms for selection line and subcutaneous, intermuscular, omental, mesenteric, year of birth (P._0.05).

kidney and channel fat and scrotal fat. Empty body

weight was calculated as the liveweight immediately 2.3.2. Relative growth pattern of body tissues prior to slaughter, minus the weight of urine and The weights of body components and empty body digesta. weight were transformed to log base 10 values for allometric analysis. Plots of log component weight 2.3.1. Proportional body composition and fat against log empty body weight showed that the birth

partitioning in mature animals data deviated from linearity. Accordingly data from

Table 1

Unadjusted means for body and component weights (6SD) at one day of age for male calves from lines selected for divergent growth rate

Selection line

High Line Control Line Low Line

No. of animals 4 4 4

Liveweight (kg) 33.26_{5.0 30.9}6_{0.4 24.2}6_2.7

Empty body weight (kg) 31.56_{4.9 29.6}6_{0.4 22.9}6_2.5

a Dissected components (kg)

Muscle 10.86_{2.71 9.7}6_{1.50 7.5}6_1.24

Bone 4.96_{1.02 4.1}6_{0.80 3.5}6_0.49

b

Total fat 2.56_{0.45 2.1}6_{0.11 1.7}6_0.22

Total viscera 3.06_{0.37 2.8}6_{0.26 2.4}6_0.24

Fat partitions (kg)

Subcutaneous 0.36_{0.1 0.3}6_{0.06 0.3}6_0.04

Intermuscular 1.66_{0.33 1.4}6_{0.10 1.1}6_0.18

Kidney / channel 0.26_{0.05 0.2}6_{0.03 0.1}6_0.01

Omental 0.076_{0.01 0.07}6_{0.005 0.05}6_0.007

Mesenteric 0.26_{0.02 0.2}6_{0.03 0.1}6_0.02

Scrotal 0.046_{0.01 0.04}6_{0.02 0.03}6_0.01

a

Other components of empty body weight such as hide, head and tail are not presented. b

Total fat5subcutaneous1intermuscular1all non-carcass fat partitions.

shown in Table 1. Using data from the remaining 79 and growth coefficients for log weights of muscle, animals (weaning to maturity) the growth of dissec- bone, viscera and total fat relative to log empty body ted body components relative to the growth of the weight was examined using a multivariate allometric body as a whole, and of fat partitions relative to total analysis. Non-significant (P._{0.05) interactions}

fat, was examined using the allometric equation, were sequentially deleted from the model. The final

b

y5_{ax which was computed in the linear form as model for log body component weights included}

log y5_{log a}1_{b log x}1_{error, where y was the significant terms for selection line (P},_{0.01), year}

weight of the component and x the weight of the (P,_{0.05), log empty body weight (P},_{0.001) and}

‘‘whole’’. The terms a and b are coefficients, a (the the interaction between year and log empty body proportionality coefficient) being the value of y weight (P,_{0.05). Predicted means for the}

trans-when x is equal to 1.0, and b the growth rate of y formed data were converted to the original scale by

2

relative to x. Thus the relative growth patterns for taking the antilogarithm of log 10 y1_1.1513s _,

2

tissues were estimated from the growth coefficient b, where s _{is the sample variance of log y (Johnson}

and multiplicative difference in distribution of the and Kotz, 1970).

(P,_{0.01), year (P},_{0.001), log total fat (P}, _{significant (P}._{0.05) terms for year and line and}

0.001) and the interaction between year and log total significant terms for log scaled total fat weight (P,

fat (P,_{0.001). 0.001) and the interactions between year and line}

To determine growth relative to stage of maturity, (P,_{0.05) and year by log total fat (P},_0.001).

weights of each component, and of empty body weight, were scaled by their mean mature weight

within year and selection line, and then transformed 3. Results

to log base 10. Using this data from the 79 steers a

multivariate allometric analysis, as described above, 3.1. Proportional body composition at maturity was used to determine the growth of dissected

muscle, bone, visceral mass and total fat relative to There was a symmetrical divergence in mature stage of maturity of empty body weight, and of all empty body weight between the lines, with the High the dissected fat partitions relative to stage of Line steers being 78 kg heavier than Control Line maturity of total fat. Once again, non-significant steers whilst the Low Line steers were a further 76 (P._{0.05) interactions were sequentially deleted kg lighter (Table 2). Despite the differences in}

from the models. For growth relative to stage of mature weight, the multivariate analysis indicated maturity of empty body weight the final model that these animals had similar (P._{0.05) mean}

contained non-significant (P._{0.05) terms for line proportions of muscle, bone, visceral mass and total}

and year, and significant terms for log scaled empty fat (Table 2). Year of birth had an effect (P,_0.01)

body weight (P,_{0.001) and the year by log scaled on proportional distribution, with steers from the first}

empty body weight interaction (P._{0.01). For calving having a higher proportion of bone than}

growth of the fat partitions relative to stage of those from the second year. Within the mature group maturity of total fat the final model contained non- of steers there was no change in proportional

com-Table 2

Means for liveweight (6_{SD), empty body weight (}6_{S.E.) and dissectible components expressed as percentages of empty body weight}

(6_{S.E.) in mature steers from lines selected for divergent growth rate}

Trait Selection line

Means with different superscripts are significantly different (P,0.05) within row. A

Other components of empty body weight such as hide, head and tail are not presented. B

Y1, Y2: consecutive birth years. C

position with age, indicating that the animals had had a higher (P,_{0.05) growth impetus (b}

coeffi-achieved their mature composition. cient, Table 4) and was a lesser (P,_0.05)

propor-tion of empty body weight (a coefficient, Table 4) at 3.2. Fat partitioning at maturity the same body weight than in steers from the second year. Line (P,_{0.01) affected the proportions of}

Table 3 shows the mean total dissected fat weight, components at the same empty body weight (a and the proportion of this total fat made up by each coefficients, Table 4). At the same weight the Low of the fat partitions, for mature steers from the three Line had relatively more fat and less bone than the selection lines, adjusted for birth year. The mean Control Line, which in turn had more fat and less proportion of total fat weight made up by the various bone than the High Line. The Low Line also had less partitions was not affected (P._{0.05) by line or birth muscle than the High Line. Table 5 shows predicted}

year when examined by multivariate analysis. How- values for component weights at 360 kg empty body ever examination of the univariate coefficients indi- weight, which is equivalent to comparison at 240 kg cated that the Low Line had less omental fat as a carcass weight (suitable for the heavy domestic proportion of total fat than did the High Line (Table Australian trade). Multivariate allometric analysis of 3). There was no effect of age on partitioning of fat scaled data indicated no difference (P._{0.05) among}

within the mature group of steers. selection line or birth year in growth rates (b coefficients) relative to stage of maturity of empty 3.3. Growth to maturity body weight. The difference in composition among the selection lines evident when compared at the The multivariate allometric analysis indicated that same empty body weight was not apparent (P.

there was no difference among the selection lines in 0.05) at the same stage of maturity of empty body the rate of growth (b coefficients, Table 4) for any of weight. All components for each of the lines were a the body components relative to empty body weight, similar proportion of their mature weight at the same nor was there any significant line by year interaction stage of maturity of empty body weight. Table 5 (P._{0.05). In steers from the first year of birth, bone compares these proportions at 0.67 maturity of}

empty body weight (the mean for this data group). b coefficients for all scaled log components were similar to those obtained using log component

Table 3 _weights.

Means for total dissectible fat (6S.E.) and dissectible fat

com-Relative growth coefficients derived from the

ponents expressed as percentages of total fat weight (6S.E.) in

allometric analysis (Table 4) indicated that total fat

mature steers from lines selected for divergent growth rate,

adjusted for year of birth was late developing relative to body weight. That is, as the animal matured, fat made up an increasing

Selection line

proportion of empty body weight. Dissected muscle,

High Line Control Line Low Line

bone and total visceral mass grew at a slower rate

Number of animals 10 9 11 _{than body weight (i.e., developed early relative to}

a a,b b

Total fat (kg) 2356_{10 198}6_{11 175}6₁₀

body weight) for the period from weaning to maturi-ty.

Carcass depots

Multivariate allometric analysis indicated that

Subcutaneous fat (%) 39.96_{1.55 39.1}6_{1.62 40.9}6_1.49

Intermuscular tissue (%) 35.16_{0.96 36.3}6_{1.01 36.5}6_{0.92 there was no difference (P}._{0.05) among the}

selec-tion lines in the growth rate of any of the fat

Non-carcass depots

partitions relative to weight of total dissected fat, but

Kidney / channel (%) 7.46_{0.56 8.1}6_{0.59 7.1}6_0.54

there was a difference among years (P,_{0.001), with} Omental (%) 7.46_{0.53 7.5}6_{0.56 6.7}6_0.51

scrotal and kidney / channel fat being later developing

Mesenteric (%) 5.36_{0.33 4.6}6_{0.35 4.2}6_0.32

Scrotal (%) 4.86_{0.22 4.4}6_{0.23 4.6}6_{0.21 relative to total fat, in the first year of the experiment}

a compared to the second (Table 6). Steers from the

Means within a row with different superscripts are different

Table 4

Proportional (a 6S.E.) and growth (b6S.E.) coefficients relative to empty body weight for dissected components in steers selected for divergent growth rate (based on allometric log–log regression)

Trait a coefficient b coefficient

a,b a,c a

Coefficient Line effect Year effect Coefficient Year effect

Muscle 0.2201 H 0.0029 Y12_{0.0317 0.8783 Y1 0.0053}

6_{0.0790 C 0.0076 Y2 0.0317} 6_{0.0141 Y2}2_0.0053

L 20.0104 60.0770 60.0138

6_0.0043

Bone 0.4098 H 0.0195 Y120.2537 0.7398 Y1 0.0476

6_{0.1090 C 0.0036 Y2 0.2537} 6_{0.0195 Y220.0476}

L 20.0232 60.1066 60.0190

6_0.0060

Total fat 2_{4.2580 H 20.0219 Y1}2_{0.0573 1.6838 Y1 0.0117}

6_{0.1423 C 20.0064 Y2 0.0573} 6_{0.0254 Y2}2_0.0117

L 0.0283 6_0.1390 6_0.0248

6_0.0078

Viscera 0.9371 H 0.0074 Y1 0.1749 0.6451 Y12_0.0321

6_{0.1007 C} 2_{0.0003 Y2}2_0.1749 6_{0.0180 Y2 0.0321}

L 2_0.0072 6_0.0984 6_0.0176

6_0.0055

a

Coefficients in bold are significantly different within class (P,0.05). b

H, High Line; C, Control Line; L, Low Line. c

Y1, Y2; consecutive years of experiment.

Table 5

,_{0.001) and kidney / channel fat (P},_{0.05) when Least-squares mean (}6_{S.E.) for age, and predicted values for body}

components in steers from lines selected for divergent growth rate,

compared at the same weight of total fat than did

when compared at an empty body weight of 360 kg (weight

steers from the second year (a coefficients, Table 6).

constant) and at 0.67 stage of maturity of empty body weight

Line significantly (P,_{0.01) affected the proportion}

(maturity constant)

of total fat made up by the different partitions at the

Selection line

same weight of total fat (a coefficients, Table 6). The

High Line Control Line Low Line

Low Line had more subcutaneous fat than the

Control and High Lines, whilst the High Line had Weight constant

a b c

Age (days) 544628 629628 719629

more mesenteric fat than the Control Line, which in

a a b

Muscle 126.8 128.1 122.9

turn had more than the Low Line. When compared at _{a b c}

Bone 34.6 33.4 31.4

the same stage of maturity of total fat, using scaled a ab b

Viscera 33.8 33.2 32.7

a a b

log data, multivariate analysis showed no effect of _{Total fat 86.5 89.6 97.1} line on the maturing rate of the different fat

parti-d Maturity constant

tions (P._{0.05), although there was a difference in}

Muscle 0.70 0.70 0.71

maturing rate between the two years (P,_0.001).

Bone 0.75 0.73 0.74

There was also a significant interaction between line

Viscera 0.76 0.75 0.77

and year on the a coefficients (P,_0.05).

Subcuta-Total fat 0.53 0.53 0.52

neous and omental fat in the Low Line steers were a Empty body weight 446 394 343

lesser proportion of their mature weight, and mesen- a

Means within a row with different superscripts are different

teric and kidney / channel fat a greater proportion of _(P,_0.05). d

Table 6

Proportional (a6S.E.) and growth (b6S.E.) coefficients relative to total dissected fat for dissected fat partitions in steers selected for divergent growth rate (based on allometric log–log regression)

Trait a coefficient b coefficient

a,b a,c a

Coefficient Line effect Year effect Coefficient Year effect

Subcutaneous fat 2_{2.0045 H 20.0185 Y1 0.1128 1.3007 Y1}2_0.0210

6_{0.0849 C 20.0062 Y2}2_0.1128 6_{0.0169 Y2 0.0210}

L 0.0247 6_0.0838 6_0.0166

6_0.0086

Intermuscular fat 0.5087 H 2_{0.0014 Y1 0.0669 0.8215 Y1}2_0.0131

6_{0.0485 C 0.0037 Y2}2_0.0669 6_{0.0096 Y2 0.0131}

L 2_0.0051 6_0.0479 6_0.0095

6_0.0049

Mesenteric fat 0.0745 H 0.0484 Y1 0.1271 0.7351 Y12_0.0230

6_{0.1435 C 0.0068 Y2}2_0.1271 6_{0.0285 Y2 0.0230}

L 20.0552 60.1417 60.0281

6_0.0145

Omental fat 2_{0.7544 H 0.0160 Y1}2_{0.2316 0.9284 Y1 0.0470}

6_{0.1403 C 0.0107 Y2 0.2316} 6_{0.0279 Y2}2_0.0470

L 2_0.0266 6_0.1386 6_0.0275

6_0.0142

Kidney / channel fat 2_{1.6233 H} 2_{0.0170 Y120.3415 1.0928 Y1 0.0652}

6_{0.1417 C 0.0266 Y2 0.3415} 6_{0.0281 Y220.0652}

L 2_0.0096 6_0.1399 6_0.0279

6_0.0143

Scrotal fat 2_{2.0100 H 0.0096 Y121.0921 1.1277 Y1 0.2056}

6_{0.1455 C} 2_{0.0306 Y2 1.0921} 6_{0.0289 Y220.2056}

L 0.0209 6_0.1438 6_0.0285

6_0.0147

a

Coefficients in bold are significantly different within class (P,0.05). b

H, High Line; C, Control Line; L, Low Line. c

Y1, Y2; consecutive years of experiment.

when compared at the same stage of maturity of total 4. Discussion

fat. b coefficients for all scaled log components were

but rate of maturation in the two traits did not differ carcass fat relative to carcass weight (Robelin et al., significantly among the selection lines. 1978) were greater than 1. That is, fat is a late In this study there was no correlated response in developing component of the body. Relative growth mature body composition and in the growth of the coefficients for muscle reported in the literature are major body components, to divergent selection for not as consistent, although most suggest approxi-yearling growth rate. Most reports on cattle slaug- mately isometric growth relative to the body measure htered prior to maturity indicate that correlated to which they are being compared. Robelin et al. responses in proportional carcass composition to (1978) and Morris et al. (1993) reported coefficients selection for growth rate or size are either non not significantly different from 1 for muscle relative significant or very small (Andersen et al., 1974; to carcass weight and for saleable meat relative to Koch, 1978). Comparable information on body carcass weight. In this study the relative growth composition at maturity is limited. Morris et al. coefficient from weaning to maturity for muscle was (1993) found no difference in the growth of saleable less than 1. The coefficients reported by Fortin et al. meat, bone or fat trim relative to carcass weight (1980) for muscle growth relative to body weight between bulls from a weight-selected and an un- ranged from 0.85 to 1.159 depending on breed and selected line, and no difference between the herds in level of feed intake, and Shahin et al. (1993) the proportions of saleable meat and fat trim. This reported values of less than 1 for muscle relative to suggests that there would also have been no differ- side weight. This inconsistency could be due to ence at maturity, although they reported that, when varying nutritional treatments, or to the range of adjusted for differences in body weight, the selected weights over which the study was done. The non-line had slightly more bone than unselected animals. linearity of birth data for log muscle in our study Averaged over the two years of the experiment when plotted against log empty body weight illus-there was no difference in mature body composition trates the possibility of different patterns of growth between the lines. As maturing rate of body com- being reported if studies are conducted at different ponents also did not differ between lines, animals stages of maturity. Bone is considered to be an early from the three growth rate selection lines would have developing component in the body, as indicated by the same proportional composition if compared at the the growth coefficients of less than 1 obtained in this same stage of maturity of body weight. In fact, when study, and in the studies by Morris et al. (1993) and compared at 0.67 maturity of empty body weight Shahin et al. (1993). Fig. 2, which plots the main each component had attained a similar proportion of body components, as a percentage of empty body its mean mature weight in each line (Table 5). At the weight, against age, illustrates the patterns of growth same body weight, however, animals from the High described here.

Line would be less mature than animals from the The pattern of growth for subcutaneous and Control Line, which in turn would be less mature intermuscular fat, relative to total fat, was similar to than Low Line animals. This was evident when the that reported by numerous studies, as reviewed by lines were compared at the same empty body weight Kempster (1981). All studies showed that as total fat (Table 5), the High Line steers displaying the less (or total carcass fat) increased the proportion of mature pattern of higher bone and lower fat per- intermuscular fat decreased and that of subcutaneous centages. Market specifications for carcasses are fat increased. Truscott et al. (1983) reported growth usually based on ranges of weight and fatness. The coefficients (relative to total fat) of 1.14 for subcuta-results imply that relative to the Control Line, High neous and 0.83 to 0.90 for intermuscular fat, which Line cattle will need to be raised to slightly heavier are similar to those obtained in this study. The weights to achieve the required level of fatness while growth of kidney and channel fat is more variable. the opposite applies to Low Line cattle. Truscott et al. (1983) found it to be late developing The pattern of growth reported here is similar to (b5_{1.13) whereas Johnson et al. (1972) found it be}

maturity. However, meat animals are usually slaug-htered to particular market specifications or at a particular age. When slaughtered to attain a particular carcass weight, steers selected for high growth rate will be younger, less mature, and will thus have higher bone weights, lower levels of fat, and similar levels of muscle than will unselected animals. Where degree of fatness is an important market considera-tion, steers from a line selected for fast growth rate will be heavier at the same level of fatness than will unselected steers. This also applies where animals are slaughtered at a set age.

Acknowledgements Fig. 2. Dissected body components as percentages of empty body

weight at different ages.

Meat and Livestock Australia (formerly Meat Research Corporation) funded this study. We thank R. Barlow, who was instrumental in the design of the by Kempster et al. (1976) the growth of kidney /

selection experiment, our colleagues P.F. Parnell and channel fat relative to total carcass fat differed

R.M. Herd and J.M. Thompson of the University of markedly between groups, being early developing in

New England for advice throughout, and the staff of some and late in others. They found scrotal fat to

the cattle section at the Agricultural Research Centre, vary in this manner also. The pattern of development

Trangie, in particular C. Brennan, D. Mula and R. of fat partitions between this study and that of

Snelgar, who cared for the animals and did the major Truscott et al. (1983) was similar except for omental

part of slaughters and dissections. fat, which we found to be early developing relative

to total fat and they found to be late developing (b5_{1.08 to 1.22). In both this study and for the}

Hereford steers studied by Truscott et al. (1983) _References subcutaneous fat was a greater proportion of total fat

than was intermuscular fat in older animals. This Andersen, B.B., Fredeen, H.T., Weiss, G.M., 1974. Correlated

differs from other studies (Johnson et al., 1972; response in birth weight, growth rate and carcass merit under single-trait selection for yearling weight in beef Shorthorn

Kempster et al., 1976) and from the Friesian steers

cattle. Can. J. Anim. Sci. 54, 117–125.

of Truscott et al. (1983), where intermuscular fat

Archer, J.A., Herd, R.M., Arthur, P.F., Parnell, P.F., 1998.

was a higher proportion of total carcass fat at all _{Correlated responses in rate of maturation and mature size of} carcass weights. Fat is a tissue that can vary in cows and steers to divergent selection for yearling growth rate

development and site of deposition more readily than in Angus cattle. Livest. Prod. Sci. 54, 183–192.

Arthur, P.F., Parnell, P.F., Richardson, E.C., 1997. Correlated

other tissues (Berg and Butterfield, 1974), and can

responses in calf body weight and size to divergent selection

thus vary with nutrition. However the increasing

for yearling growth rate in Angus cattle. Livest. Prod. Sci. 49,

proportion of total fat in the carcass fat partitions as _305–312.

an animal increases in body weight seems standard. Berg, R.T., Butterfield, R.M., 1974. Growth of meat animals. In: Cole, D.J.A., Lawrie, R.A. (Eds.), Proceedings of the 21st Easter School in Agricultural Science, University of

Notting-ham, Meat, Butterworths, London. 5. Conclusion

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distribution in the bovine carcass. 1. The partitioning of fatty Robelin, J., Geay, Y., Beranger, C., 1978. Genetic variations in tissues between depots. Aust. J. Agric. Res. 23, 381–388. growth and body composition of male cattle. In: De Boer, H., Johnson, N.L., Kotz, S., 1970. In: Distributions in Statistics. 1. Martin, J. (Eds.), Patterns of Growth and Development in

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ton, MA. Shahin, K.A., Berg, R.T., Price, M.A., 1993. The effect of breed-Kempster, A.J., 1981. Fat partition and distribution in the carcas- type and castration on tissue growth patterns and carcass

ses of cattle, sheep and pigs: a review. Meat Sci. 5, 83–98. composition in cattle. Livest. Prod. Sci. 35, 251–264. Kempster, A.J., Cuthbertson, A., Harrington, G., 1976. Fat Thompson, J.M., Butterfield, R.M., Perry, D., 1987. Food intake,

distribution in steer carcasses of different breeds and crosses. 1. growth and body composition in Australian Merino sheep Distribution between depots. Anim. Prod. 23, 25–34. selected for high and low weaning weight. 4. Partitioning of Koch, R.M., 1978. Selection in beef cattle. III. Correlated dissected and chemical fat in the body. Anim. Prod. 45, 49–60. responses of carcass traits to selection for weaning weight, Truscott, T.G., Wood, J.D., MacFie, H.J.H., 1983. Fat deposition yearling weight and muscling score in cattle. J. Anim. Sci. 47, in Hereford and Friesian steers. 1. Body composition and 142–150. partitioning of fat between depots. J. Agric. Sci. (Camb.) 100, Morris, C.A., Baker, R.L., Bass, J.J., Jones, K.R., Wilson, J.A., 257–270.

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