GENETIC AND ENVIRONMENTAL EFFECTS ON ...

Genetic and environmental effects on carcass characteristics of Southdown x Romney lambs: I. Growth rate, sex, and rearing effects G. L. Bennett, A. H. Kirton, D. L. Johnson and A. H. Carter J ANIM SCI 1991, 69:1856-1863.

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GENETIC AND ENVIRONMENTAL EFFECTS ON CARCASS CHARACTERISTICS OF SOUTHDOWN x ROMNEY LAMBS: 1. GROWTH RATE, SEX, AND REARING EFFECTS G. L. Bennett', A. H. Kirton, D. L. Johnson and A. H. Carte3 Ruakura Animal Research Station, Private Bag, Hamilton, New Zealand ABSTRACT

Carcass data from more than 4,400 Southdown x Romney ewe and wether lambs collected over a 16-yr period were analyzed for the effects of sex, rearing status, and growth rate. Ewe lambs grew more slowly than wethers and had .78 kg less carcass weight at the same age. The carcass weight advantage for wethers was nearly all caused by heavier fat-free weight. Based on fat depths, the fat on ewe lambs was distributed in more anterior and ventral parts of the carcass relative to wether lambs. Lambs reared as twins had 1.73 kg less carcass weight and correspondingly reduced carcass measurements compared with lambs reared as singles. Sex and rearing status interacted for some traits. However, in no case was a significant sex difference reversed in single- and twin-reared lambs. Growth rate effects were determined by regressing average change in carcass measurements on average carcass weight gain over a 5-wk period. When carcass weight remained constant over a 5-wk period, fat weight increased by .12 kg, fat-free weight and muscle measurements decreased, and bone lengths increased. For each kilogram of increase in 5-wk carcass weight gain, the marginal increase in fat weight was .41 kg and that of fat-free weight was .59 kg. At the average 5-wk carcass weight gain of 1.4 kg, fat and fat-free gains were equal. As carcass weight gain increased above 1.4 kg, fat-free gain exceeded fat gain. Key Words: Carcass Composition, Sheep, Growth Rate, Twins, Carcass Weight, Sex Differences J. Anim. Sci. 1991. 69:1856-1863

Introduction

A large breeding project was begun at the Ruakura Animal Research Station in 1963 to study genetic variation in sire-transmitted effects on lamb survival, growth, and carcass composition when mated to Romney dams (Carter and Kirton, 1975). Approximately 350 sires were used to produce progeny to compare breeds available in New Zealand. Nearly

'Present address: Roman L. Hruska U.S. Meat Anim. Res. Center, ARS, USDA, P. 0.Box 166, Clay Center, NE 68933-0166. bamed. Received January 2, 1990. Accepted November 6, 1990.

10,OOO progeny were slaughtered and measured for carcass composition in this phase of

the study. Approximately half the sires used were Southdowns. A second phase of the project was carried out from 1973 to 1978. In this phase, Southdown rams selected for high growth rate as lambs were progeny-tested each year, followed by selection within family and line. Approximately 2,000 carcasses were evaluated in this phase. Carcass and growth data from progeny of Southdown sires were analyzed for genetic and nongenetic effects. Effects of rearing status, sex, and growth rate are reported in this paper. These results are useful for adjusting progeny and sib carcass data used in evaluation and selection schemes. Knowledge of rearing status and sex differences can be useful in the development of marketing strategies designed

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GROWTH RATE, SEX, AND REARING EFFECTS

to produce more uniform carcass composition. The effects of growth rate on developmental changes in carcass measurements were determined from the regression of changes on interyear and interperiod variation in average growth rate. Information on growth rate effects can also be useful in developing management plans to alter the composition of carcasses. Materlals and Methods

Experimental Design Data were collected on lambs resulting from matings of Southdown rams and mature Romney ewes. Beginning with the 1963 mating, rams were sampled from Southdown breeders in New Zealan& this sampling program was continued throughout the first 10 yr. In addition to the rams sampled from purebred breeders, an experimental flock of Southdowns was begun by sampling the industry in 1963 and 1964. Progeny of young rams from the experimental flock provided all data for the 1973 through 1978 matings. Throughout the 16 yr of the experiment, some rams that had been used in earlier years were used in subsequent years. In these analyses, progeny from second and later years of use were deleted. Rams were single-sire mated to 20 or more mature ewes. Lambs were identified at birth or shortly after to determine parentage. Lambs were docked and males were castrated at 3 to 4 wk of age. Lambs were weaned at about 14 wk of age and allowed to graze on predominantly rye grassclover pasture. In some years, various pre- and postweaning treatments crossclassified with sex, rearing status, and slaughter age were imposed on the progeny. Lambs were allocated to slaughter ages, balancing as much as practicable within sire group for sex, rearing status, birthday, and weaning weight. Lambs born from 1963 to 1972 were assigned to three slaughter ages. The first slaughter age ranged from 17 to 20 wk of age, depending on the year. The second and third slaughter ages were 5 and 10 wk, respectively, after the fust slaughter age. Within each slaughter age, lambs were assigned to weekly slaughter groups by ascending birth date to reduce the difference between intended and actual slaughter age. Lambs born from 1973 through 1978 were allocated in a similar way to slaughter at either 22 or 27 wk of age.

1857

Live weights of lambs were recorded at birth, docking, and weaning and at 2-wk intervals from weaning until slaughter. Lambs were weighed before being trucked to an abattoir and again after being held overnight without feed and water. Lambs were then slaughtered by standard methods. Table 1 describes weights and measurements that were recorded Dressed carcass weight included kidney and kidney fat. One side of each carcass was minced for chemical analysis w o n et al., 1962). Four traits were not measured in the first year (1963), as indicated in Table 1. Statistical Analysis Each year was analyzed separately because treatments were different from year to year and age adjustments were thought to differ depending on yearly grazing conditions. The minimal statistical model consisted of effects for intended slaughter age, sex, rearing status (twin or single), and the interaction of sex and rearing status. Deviations of birth date from the average birth date and actual slaughter age from the intended slaughter age were fitted as covariates. Each covariate was allowed to interact with intended slaughter age so that regressions specific to each slaughter age were obtained. In addition, pre- and postweaning treatment effects were included in the model when appropriate. Sex, rearing status, and their interaction effects were averaged across the 16 y-r to obtain pooled estimates. The SE of each pooled estimate was calculated from the variance among the 16 independent estimates. These estimates of effects are unbiased but do not have minimum variance. However, the large number of lambs still resulted in small

SE. It is well known that carcass traits change

as cmass weight changes. Changes in carcass traits may also differ due to the rate of carcass weight change. Rate of carcass weight gain can be altered by management and environment. The simplest model that describes the joint effects is as follows: Change = bo x days + bl x carcass weight change. When change is measured over a k e d number of days, then the model can be reduced to the following: Change = intercept + bl x carcass weight change. In this model, an intercept that is different from zero can be interpreted as the effect of rate of growth independent of weight change.


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B-TT

The effects of growth rate were estimated from average gain or change in carcass weights and measurements between adjacent slaughter ages. There were a total of 26 periods of 5 wk available for analysis across years (i.e., two periods, approximately 18 to 23 wk and 23 to 28 wk) each year from 1963 through 1972 and one period (22 to 27 wk) each year from 1973 through 1978. Unplanned variation in average growth rates was partly due to weather and management. Differences in least squares means between the start and end of each of the 26 periods were analyzed by regression.

ET AL..

Average changes in carcass traits during each of the 26 periods were regressed on the average changes in carcass weight for the corresponding periods. Results

The number of lambs having a complete set of data for live weights, carcass weight, fat and fat-free weights, eye-muscle area, and fat depth C and tissue depth J is shown in Table 2. Analyses of other traits resulted in minor differences in the number of lambs included in

TABLE 1. DESCRIPTION OF LIVE AND CARCASS WEIGHTS AND CARCASS MEASUREMENTS Trait

Description

Biahwt,kg Reweaning ADG,g/d Pullbodywkkg F'reslaughter wt, kg Hot carcass wt, kg Dressing % Fat % Protein % Water % Fat wt, kg Fat-free wt. kg Kidney fat %" S l b fat depth, mm

Weight at birth (Weaning -birth wt)/weaning age Weight off pastnre Live wt after ovexnight fast Dressed carcass wt including kidney and kidney fat immediatelyafter slaughter Hot carcass wt/preslaughter wt Ethm extract in sample from one minced carcass side corrected for drip loss Protein in mincedsample corrected for drip loss Water in mincedsample plus calculated drip loss Fat % xhot c~zcasswt Hot C~TCBSSwt-fat wt Kidney and pelvic fat divided by hot carcass wt Subcutaneous fat depth over M. trapeziusabove maximum depth of M. longissimus on section cut through carcass between 5th and 6th ribs Subcutaneousfat depth over M. latissimus dorsi on the carcass section between the 5th and 6th ribs at a right angle to the section that halved the carcass taken from the vertebral colamn Maximum M. longissimusdepth on carcass section cut between the 5th and 6th ribs Maximum width of M.longissimus on section of carcass between last thoracic and first lumbar vertebra at right angles to the vertebral column Maximum depth of M.longissimuson sectionof carcass takenbetween last thoracic and first lumba~vertebra Fat depth over maximum B muscle depth Depth of thickest fat layer over rib when sectioned for A, B,and C Area of M.longissimus where sectioned for A, B,and C Subcutaneousfat thickness where the plane of section when the carcass is halved down the backbone meets the plane where the leg is sectionedfrom the loin by a cut between the last lumbar and first sacral vertebra over the vertebral column Subcutaneousfat thickaess over M. obliquus on carcass section between last lumbar and first sacralvertebra Maximum subcutaneousfat thickness over M.gluteus medius on carcass on the section between last lumbar and first sacral vertebm Direct distance from tip of leg to crutch Length of tibia plus tarsus Mcasarement from ventral surface of cut between last lumbar and fmt sacral vertebra and the tip of the tibia --of left fore metacarpal bone Air-dried weight of left fore metacarpal bone cleaned of all tissue including periosteum

S2bfat depth, mm S3b muscle depth, mm A' muscle width, mm B' muscle depth, mm

E fat depth, mm

fi' fat depth, mm EP muscle area,cm2 LI'Jfat depth,mm

ubfat depth, mm L3" fat depth, m m crutch depth, mm F length, mm Leg length", Cannon bone length. mm Cannon bone wt, g

Wot recorded in 1963. hlustrated in Kirton et al. (1967). 'See diagram and definitions in Palsson (1939). 'Ihe present data differ from those of Palsson in that the cut between the last thoracic and first lumbar vertebra was made at right angles to the vertebral column and cut through one or two ribs instead of following the curve of the last rib as in Palsson.


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GROWTH RATE, SEX. AND REARING EPFECTS

the analyses. Of come, those traits not measured until 1964 (seeTable 1) had no data in 1963. Approximately 40% of lambs were born and reared as twins. Average differences between ewe and wether lambs and between lambs reared as singles or as t w i n s are shown in Table 3. Rearing status significantly affected all traits. Lambs reared as singles had heavier birth weights and grew more rapidly to weaning. The difference in carcass weight between lambs reared as singles or twins was nearly equally due to fat and fat-free weight. Other differences between lambs reared as singles or twins primarily reflect carcass weight differences. Ewe lambs grew more slowly than wethers and had less carcass weight at slaughter. The increased carcass weight of wether lambs was nearly all caused by differences in fat-free weight. Differences in anatomical shape between the sexes were apparent. Ewe carcasses had shorter tibia plus tarsus length 0,cannon bone length, and leg length but greater crutch Ewes had smaller and rounder eye depth 0. muscles than wethers, as indicated by longissimus width A, depth B, and area. The distribution of fat on ewe carcasses as measured by fat depths was diffexent fkom the distribution of fat on wether carcasses. The more dorsal fat depths (Sl, C, and L1) of ewe lambs were similar to or less than those of wether lambs. The more ventral fat depth measurements (S2, J, L2, and L3) of ewe lambs were greater than or similar to those of wether lambs. Comparison of the most dorsal fat depth with the more ventral fat depths on the same section (S1 vs S2, C vs J, and L1 vs L2 and L3) shows that the difference between ewe and wether lambs increases in the more ventral parts of the carcass. Comparison of the more anterior fat depths (Sl, S2, C, and J) with the most posterior measurements (Ll, L2, and L3) indicates that ewes are fatter than wethers in anterior portions of the carcass. Interactions between sex and rearing status (Table 3) were primarily due to changes in magnitude of differences between sexes or rearing status rather than to a reversal of differences. In no case was a significant sex effect reversed in singles vs twins or a significant rearing status effect reversed in ewes vs wethers. Intercepts and regression coefficients relating trait change to carcass weight change over

TABLE 2. NUMBER OF SLAUGHTER AGES, DF POR FMED EPPECTS, NUMBER OF SIRES, AND NUMBER OP LAMBS BY YEAR OF BDRTH

Year

1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 Totals

Fixed Slaughter effects, ams df

~-

No. of

No.of

sires

lambs

10 10 9 9 12 7 16 13 14 16 8 8 9 12 12 12 177

338 180 230 249 339 154 395 205 361 378 156 204 241 367 318 332 4.447

~~

3 3 3 3 3 3 3 3 3 3 2 2 2 2 2 2 42

17 17 17 16 14 13 13 13 13 13 10 10 10 11 10 10 207

5-wk periods (Table 4) indicate the effect of growth rate per se on the growth of specific tissues. Carcass weight change over a fixed time period is equivalent to carcass growth rate. The intercept measures specific tissue changes associated with no carcass weight change (i.e., a period of carcass weight stasis). A significant intercept is interpreted to mean that the trait changes over the 5-wk period even though there is no carcass weight change and, hence, that other factors such as time (age) are important in development. A significant positive regression coefficient indicates that change in the trait increased positively as carcass growth rate increased and that growth rate is important for determining change in that trait. Regression coefficients were sigruficant for all traits except dressing percentage, protein percentage, crutch depth F, and leg length. Significant and nonsignificant intercepts showed an interpretable pattern of change at carcass weight stasis. Although total fat weight increased, fat depths and kidney fat percentage indicated that most fat was deposited in the shoulder area and internally. Fat-free weight decreased and measurements of muscle area and dimensions all decreased significantly at the intercept. At the same time, leg bone lengths increased. Change in each trait per kilogram of gain at the average carcass weight gain of 1.405 kg is also shown in Table 4. At


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BENNE'IT ET AL.

the average carcass weight gain, one-half of the gain was fat. Figure 1 illustrates the regressions of fat and fat-free gains on carcass weight gains. Dlscussion

Differences between ewe and wether lambs in carcass weight are almost entirely due to growth of the fat-free carcass. Similar results were shown for rams, wethers, and ewes by Everitt and Jury (1966) and for rams and ewes by Fourie et al. (1970). Although fat weight did not differ between ewes and wethers, ewes had greater internal fat, and fat seemed to be distributed in more anterior and ventral parts of the carcass as measured by fat depths. Many of the differences between lambs reared as singles or twins can be accounted for by the difference in carcass weight. Using the average change per kilogram of carcass weight

gain from Table 4 to adjust for rearing status differences removed fat and fat-free weight differences. However, singles would have slightly greater dressing percentages and decreased fat depths when evaluated at the same carcass weights. Several workers have studied the effects of different intakes and patterns of intake on body composition using small numbers of sheep and experhentally controlled diets. The results reported here are of a different nature. The present results are based on large numbers of animals and natural variation in average dietary intake and quality, and possibly on other factors Occurring between years and periods within years. Nondiemy factors that could influence average weight gains include ambient temperature, disease, and parasites. Furthermore, some genetic change across years for growth rate, particularly in the sires,

TABLE 3. SEX AND REARING D-NCES

AND THEIR INTERACTIONS Single - twin

Ewe -wether

Interactionb

Traita

Mean

SE

Mean

SE

Mean

Birth wt, kg Reweaning growth rate, g/d FUllbodywkkg &slaughter wt, kg Hot cmcass wt, kg

-.25 -13 -1.74 -1.72 -.78 .1 1.2 -.2 -.9

.M* 1* .12* .lo* .07* .ll .13*

.83 33 2.71 2.68 1.73 1.6 3.3 -.7 -2.4 .89

.04* 1* .lo* .lo*

-.lo -2 -.34 -27 -.lo .1 .6 -.l -.5

.84

.03* .03* .06* .12* .lo* .13*

Dressing % Fat % Rotein % .w* Water % .09* -.M .03 Fat wt, kg Fat-free wt, kg .w* -.74 Kidney fat % .M* .33 S1 fat depth, mm -.09 .os S2 fat depth, m m .23 .lo* S3 muscle depth, mm -.34 .13* A muscle width, mm -1.48 .15* B muscle depth, mm -24 .13 C fat depth, mm .09 .w J fat depth, m m .48 .os* Eye muscle area, cm2 -.30 .06* L1 fat depth, mm .09* -.60 L2 fat depth, mm .os .01 L3 fat depth, mm -.w .os F crutch depth, mm .30, .71 T length, mm -3.42 .3o* Leg length, cm -.45 .os* Cannonbone length, mm -2.70 .14* Cannon bone wt, mm -2.55 .os* 'See Table 1 for definitions of the traits. bsingle ewe - twin ewe - single wether + twin wether. cUnweighted mean of all observations. *P < .05.

24 .59 1.41 1.25 1.22 1.47 .54 1.50 .71 1.23 1.60 .98 2.77 3.89

.a*

.lo* .2*

.os*

.17*

.w*

.os*

.04* .13* .05* .lo* .16* .lo* .W

26*

.99

.09*

1.92 2.02

.18* .09*

.M -.16

SE

.06 1

.17 .16 .09

.13 .26 .07 .20*

.06 .05*

.M

.w

.13 .30 -.01

.07 .15 .20 .17 .21

.09

23 20

.os*

.#

.16*

.05

.09

.30 -.06 26 -1.14 -.87 -.17 -.72 -.59


20 -26 .15 .64 .35* .09 .24*

.lo*

Overall mean' 4.44

200 29.67 27.61 13.40 49.3 27.1 14.8 542 3.72 9.68 2.1 2.14 6.17 22.89 49.11 27.03 2.52 7.30 9.07 6.47 11.42 5.39 222.50 159.99 34.13 9521 22.26

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GROWTH RATE, SEX, AND REARING EFFECTS

probably occurred. The trait used to summarize this natural variation in growth was average carcass weight gain during a 5-wk period. Average 5-wk carcass weight gain was 1.405 kg and the SD among 5-wk periods was .699 kg. The use of large numbers of lambs to accurately estimate average changes and a continuum of 26 average gains in carcass weight provides an opportunity to "field-test" more exacting experimental results based on much smaller numbers of lambs. The present results should be descriptive of the relationship between traits and carcass growth rate found in similar production situations. The applicability of these relationships to prediction of the effects of management changes on traits depends on the biological mechanism of the relationship. Changes in traits are expected to differ with stage of maturity. Growth near maturity should be fatter and have less bone and muscle than immature growth. Separate regressions of

average changes in fat and carcass weight for the earlier and later 5-wk periods for the first 10 yr (1963 to 1972) resulted in intercepts and regression coefl"icientsthat differed by .008 kg fat gain/5 wk and ,003 kg fat gain/ 5-wk carcass weight gain, respectively. Differences in maturity over this limited age range did not seem to be important. Many experimental results from immature lambs fed at maintenance or lower intakes have shown reductions in fat weight, but little more than might be expected from differences in weight gain (Burton and Reid, 1969; Kirton, 1970; Burton et al., 1974; Searle and Graham, 1975; Thomton et aL, 1979; Kirton et al., 1981). However, Elsley and McDonald (1964) and Hodge and Star (1984) found a greater reduction in body fat than would be expected from body weight loss with feed restriction. Rattray et al. (1973) and Notter et aI. (1983) found increased fat in animals held at maintenance. Hovell et al. (1983) have suggested that

TABLE 4. REGRESSION OF CHANGE IN MEASUREMENT ON CHANGE IN CARCASS WEIGHT DURING FIVE-WEEK TIME PERIODS Regression coefficient forcarcasswtchange

htefiept

Pull body wt, kg

Preslaughterwt, kg Dressing% Fat 96 Protein % Water % Fatwt,kg

Fat-fre wt, kg Kidney fat 96 s1 fat depth.mm s2 fat depth.mm S3 muscle depth, mm A muscle width, mm B muscle depth, mm C fat depth, mm J fat depth, mm EY muscle area, cm2 L1 fat depth, mm L2 fat depth, mm L3 fat depth, mm F crutch depth, mm T length,mm h g length, cm Cannon bone length. mm

Average change per kg carcass wt changeb

Est.

Est.

SE

Est.

SE

R2

-.07 -.14 .3 1 .o -.18 -.8 1 .12 -.12 .ll .15 .61 -.62 -1.12

23 .17 28

1.99 1.89 .3 1.1 -.15 -.E2 .41 .59 .13 .29 .48 1.15 1.07 1.54

.15* .11* .18 .19* .08 .18* .03* .03* .03* .07* .15* .16* .16* .23* .06* .15*

.88 .92

1.94 1.79

.07*

.09

.5

.56 .14 .47 .88 .94 .39 .41 .31 .68 .65 .66 .70 .77 .71 .69 .45 .52 .13 .53 .01 .36

1.73 -.28 -1.39 50

.09* .09*

-.84

.30*

.12 .28*

.os* .os* .os .ll .23* .25*

.w*

.35* .06 .09 -.03 23 -.34 .13* -. 10 22 -2s 50 .06 28 2.49 .77* 1.25 .24* .48 .31* .65 a* cannonbone Wtg 29 .17 %e Table 1 for definitions of the traits. bmtercept + 1.405 x regression coefficient)/1.405. *P < .os.

.44

126 .66 1.03 1.42 .88 -.93 1.24

.IO .66 .60

.#*

.14* .32* .18* .49

a* .19 .18* .11*


.55

50

20 .40 91 .71 28 .94 .48 1.24 .41 .%

1.25 .92 .E4

2.13 .45 1.13 .81

SE

.os*

.04* .09* .02* .02*

.02* .04* .07*

.a* .a* .11* .03* .08* .04* .lo* .22* .13* .24* .12* .lo* .09*

.os*

BENNETT ET AL.

1862

1.o

t

FAT GAIN

.5

0

- .5 1. 5

'

/ *r

I

0

FAT-FREE GAIN

I

.5

I

I

I

I

1.0

1.5

2.0

2.5

CARCASS WEIGHT GAIN, K G

Figure 1. Fat weight and carcass weight gains during 26,5-wk periods. Squares represent gains during the first period (approximately 18 to 23 wk) from 1963 to 1972, circles represent gains during the second period (approximately23 to 28 wk) from 1963 to 1972, and diamonds represent gains (approximately 22 to 27 wk) from 1973 to 1978.

35-kg lambs fed enough energy to maintain weight may not produce enough microbial amino acids to maintain body protein. The present data suggest that a period of carcass weight stasis slows but does not stop skeletal growth, decreases muscle and fat-free weight, and increases fat weight slightly. At the average carcass growth rate, onehalf of the gain was fat and onehalf was fat-free. As carcass growth rate increased, the proportion of gain that was fat decreased and the proportion that was fat-free increased. Conse quently, lambs grown at faster growth rates were leaner at the same carcass weights. The fastest growth rates in this experiment were less than might be expected with concentrate feeding. At very high growth rates, the relationship between growth rate and the partition of carcass weight gain to fat and fatfree gain may be reversed. On the other hand, the results may be extendable to periods of weight loss. Kirton et al. (1967) found that fat was 31% of carcass weight lost during a 3 d fast. The equation in Table 4 scaled to 3 d periods predicts that fat would make up 40% of carcass weight loss.

lmpllcatlons

When slaughtered at the same age, carcasses from ewe lambs were lighter and fatter than those from wethers. Ewe lambs must be slaughtered at younger ages and lighter weights to produce carcasses with a fat percentage similar to that of wethers. Lambs reared as twins must be slaughtered at older ages than lambs reared as singles but at similar weights to produce carcasses with the same fat percentage. Adjustments for sex and weight must be considered in selection programs. Slower growth, resulting from environmental and management effects, produced slightly fatter carcass weight gain than moderate growth rates, suggesting that the opportunity to improve carcass composition by changing growth rate by management is limited unless slower growth results in lighter carcass weights at slaughter. Literature Clted

Burton, J. H., M. Anderson and J. T. Reid. 1974. Some biological aspects of partial starvation. effect of weight loss and regrowth on body composition in sheep. Br. I. Nutr. 32:SlS.


GROWTH RATE, SEX, AND REARING EFpecTs Burton, J. H. and J. T. Reid. 1969. Interrelationshipsamong emrgy input, body size, age and body composition of sheep. J. Nu&. 97517. Carter, A. H. and A. H. Kinon. 1975. Lamb production performance of 14 sire breeds mated to New Zealand Romney ewes. Livest. Rod. Sci. 2157. Elsl9, F.W.H. and I. McDonald. 1964. Ihe effect of plane of nutrition on the carcases of pigs and lambs when variations in fat content are excluded. Anim. Prod. 6 141. Everitt, G. C. and K. E. Jnry. 1966. Effects of sex and gonadectomy on the growth and development of Southdown x Romney cross lambs. II. Effects on carcass grades, meaSllrementsand chemical composition. J. Agric. Sci. 66:15. Fourie, P.D., A. H. Kirton and K. E. Jury. 1970. Growth and development of sheep. 1I. Effect of breed and sex on the growth and carcass compositionof the Southdown and Romney a d their cross. NZ. J. Agric. Res. 13: 753. Hodge, R. W.and M.Stm. 1984. Comparison of the fat status of lambs during continnous growth and following nutritional restriction and subsequent re-alimentation. Aust. J. Exp. Agric. Anim. Husb. 24:150. Hove& F. D. DeB.. E. R. 0rskov. D.A. Grubb and N.A. MaCLcad. 1983. Basal urinary nitrogen excretion and growth response to supplemental protein by lambs close to energy equilibrium. Br. J. Nu&. 50173. Kirton, A. H. 1970. Effect of preweaning plane of nutrition

1863

on subsequent growth and carcass quality of lambs. Roc. N.Z. SOC. Anim. Rod. 30:106. Kirton, A. H., R. A. Barton and A. L. Rae. 1962. The efficiency of determining the chemical composition of lamb carcasses. I. Agric. Sci. 58:381. Kirton, A. H.,J. N. Clarke and A. H. Carter. 1%7. Effect of pre-slaughter fasting on liveweight, carcass weight, and w a s s composition of Southdown ram Iambs. NZ. J. Agric. Res. 10:43. Kirton,A.H.,D.P. ShcIair,B. B.Chrystal,C.E. Devhe andE. G. Woods. 1981. Effect of plane of nutrition on carc~sscomposition and the palatability of pasture-fed lamb. J. Anim. Sci. 52285. Nom, D. R., C. L. Femll and R.A. Field. 1983. Effects of breed and intake level on allometric growth patterns in ram lambs. J. Anim. Sci. 56:380. Palsson, H. 1939. Meat qualities in the sheep with special reference to Scottish breeds and crosses. J. Agric. Sci. 29:544. Rattray, P. V., W. N. Garrett, H. H. M9er, G. E.Bradford, N. E. East and N. Himnan. 1973. Body and carcass composition of Targhee and Fi-Terghee lambs. J. Anim. Sci. 37:892 Searle, T. W. and N. McC. Graham. 1975. Studies of weaner sheep during and after a period of weight stasis. II. Body composition. Aust. J. Agric. Res. 30135. Thornton, R F.,R L. Hood, P. N. Jones and V.M Re. 1979. Compensatory growth in sheep. Aust J. Agric. Res. 3 0 135.
