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Genetic studies on carcass traits in iceland twin ram lambs I. Estimates of genetic parameters on carcass traits, live weight at weaning and carcass weight
Livestock Production Science, 8 (1982) 489--505
489
Elsevier Scientific Publishing Company, Amsterdam - - P r i n t e d in The Netherlands
GENETIC STUDIES ON CARCASS TRAITS IN ICELAND TWIN RAM LAMBS I. ESTIMATES OF GENETIC PARAMETERS ON CARCASS TRAITS, LIVE WEIGHT AT WEANING AND CARCASS WEIGHT
S.S. THORSTEINSSON and H. BJORNSSON
Agricultural Research Institute, Keldnaholt, 110 Reykjam'k (Iceland) (Accepted 31 August 1981)
ABSTRACT
Thorsteinsson, S.S. and BjSrnsson, H., 1982. Genetic studies on carcass traits in Iceland twin ram lambs. I. Estimates of genetic parameters on carcass traits, live weight at weaning and carcass weight. Livest. Prod. Sci., 8: 489--505. For this analysis, data from 1826 entire twin ram lambs, progeny of 216 sires tested during the period 1958--1977, were used. In addition to live weight and carcass weight, 27 traits involving direct carcass measurements, scores and various derived expressions describing different features of carcass conformation were analysed. The data were adjusted by two methods: by the regression of measurements on carcass weight and by the regression of measurements on age of lambs. It is concluded that in studying carcass quality traits, correction for carcass weight is a better adjustment than correction for age. High heritability estimates were obtained for external carcass traits, 0.60--0.80 for linear measurements of length and depth, b u t generally lower, 0.40--0.60 for measurements of width, scores for fullness of leg and the derived expressions of carcass measurements. Cross-sectional measurements at the 12th rib, caulfat weight and carcass grade yielded heritability estimates of 0.12--0.44. Heritability estimates of live and carcass weights corrected for age of lambs were found to be 0.18 and 0.11, respectively. The possible effects of the design of the progeny testing program on the validity of the heritability estimates are discussed. Genetic and phenotypic relationships amongst 27 carcass traits are discussed as well as the relationships of these traits with live weight and carcass weight corrected for age of lambs. A genetic antagonism was found to exist between length of skeletal measurements and traits indicating fleshiness. It is suggested that the ratio of m i n i m u m circumference: length of the metacarpal is the single most useful criterion in selection for carcass quality.
INTRODUCTION
In 1957, a ram progeny testing program for improvement of conformation and carcass quality of lambs at weaning was initiated at the Experimental Sheep Farm, Hestur, of the Agricultural Research Institute in Iceland. This paper is concerned with estimates of phenotypic and genetic parameters for various carcass traits, weight at weaning, carcass and caulfat weight 0301-6226/82/0000-0000/$02.75 © 1982 Elsevier Scientific Publishing Company
490 of Iceland twin ram lambs. Another paper reports the use of these results in formulating selection indices for meat quality improvement (Arnason and Thorsteinsson, 1982). MATERIALS AND METHODS Source o f data
For this analysis, data from 1826 entire twin ram lambs of the Iceland sheep, progeny of 216 sires tested during the period 1958--1977, were used. Of these, 1753 lambs were the progeny of 171 short-legged blocky-type sires distributed over 20 years, 73 lambs were the progeny of 25 long-legged rangyt y p e sires distributed over 9 years (1961--1970), and finally, 28 lambs were the progeny of 12 randomly selected sires distributed over 3 years (1964-1966). Procedure and management
The sires to be tested were mostly selected from the breeding flock at Hestur. However, some were sired by A.I. rams, and a few were from pedigree flocks in the same area. The criteria on which the test sires were selected were: weight for age and t y p e of birth, twins preferred to singles, heartgirth, width of loin, length of the left fore cannon bone (metacarpal), scorns for fullness of loin, rump and leg. Pedigree of parents was taken into account, greatest attention being paid to their fecundity and milking ability. T w e n t y ewes from the breeding flock at ages three to seven were allocated to each ram. The ewes were divided into even lots according to age, live weight at previous weaning, length of the left fore cannon, fecundity, and points for d r e s ~ d carcass weight production in previous years. These lots were randomly allocated to the rams tested. The ewes were treated as a one lot during the whole year. All lambs were weighed at birth, ear-tagged and t y p e of birth and sex recorded. In the last week of September, the lambs were weaned at an average age o f 132 days, weighed, and all the males killed. At slaughter, weight of dressed carcass, caulfat and pelt were recorded and the left f o r e f o o t of each lamb was collected for dissection of the metacarpal. After rigor mortis the following measurements, as defined by P~lsson (1939), and illustrated in Fig. 1, were taken on the carcass: External measurements T = length of tibia + tarsus; F = leg length; G = width of gigot; Th = depth o f thorax; W = width of chest; U = circumference of chest;
491
K = l e n g t h o f carcass; Leg s c o r e = s c o r e f o r fullness o f leg o n a scale o f 1 - - 5 in a s c e n d i n g order o f merit; Carcass grade = c o m m e r c i a l e v a l u a t i o n o f t h e carcass o n a scale o f 1 - - 4 in d e s c e n d i n g order o f merit. Internal carcass m e a s u r e m e n t s ( o n cross s e c t i o n at t h e 1 2 t h rib) A = " w i d t h " o f l. dorsi; B = " d e p t h " o f I. dorsi;
-
/
K
CIRCUMf'ERtZNCE L [ NG T~
OF" CANNOI
Fig. 1. Carcass measurements.
492
C = thickness of back-fat over the deepest part of the l. dorsi; J = thickest layer of fat over rib; X = thickness of muscle layer (mixed with fat) on the lower half of the rib; Y = thickness of subcutaneous fat layer over X. Cannon (metacarpal) bone measurements (after dissection) MW = fresh weight of the left metacarpal; ML = length of the left metacarpal; MC = minimum circumference of the left metacarpal. Measurements T and F were taken with a ruler, b u t measurements K and U were taken with a linen tape. Measurements G, W, Th were taken with sliding callipers and measurements on cross section at the 12th rib with steel dividers. Statistical analysis The following nested linear model was used for the analysis Yhijk = I~h + Shi + dhij + ~ (Xhijk - - X h . . .) + ehijk where Yhijk is the observation on one trait, #h denotes the annual mean in year h, shi and dhij denote the random effects o f the jth dam of the ith sire in the hth year with dams nested within sires and sires within years and the variances of the sire and dam effects are a~ and a~, respectively, ~ is the coefficient for regression on a covariate, X, with_ the value Xhijk for the kth lamb of the hijth dam and the annual mean X h . . . . and ehijk is the residual random deviation with variance o~. All covariances among effects are assumed to be zero. The covariates used were age at weaning for adjustment to equal age, and carcass weight for adjustment to equal carcass weight. Only linear regression was used. A limited study had indicated that n o t much would be gained by using powers of carcass weight other than one. Harvey's (1972) least squares c o m p u t e r program was used for the analysis and estimation of genetic parameters, except for the standard errors of the heritability estimates which were calculated according to Falconer (1963). The degrees of freedom and expected values of mean squares in the analyses adjusted for carcass weight were Between sires 196 oe2 + 1.402 a2d + 8.298 o~ Dams/sires 1190 oe2 + 1.278 a~ Progeny/dams 419 o~ Heritabilities were estimated by 4 o~/(o~ + a~ + o~) with the population variances replaced b y their estimates. The p h e n o t y p i c and genotypic correlations between any two variables, X and Y, were calculated from the variance and covariance c o m p o n e n t s as (Os,yy 4r tldX Y ~" OeXY I ) / [ ( OsX 2 "1" O'~LY "t" OeX 2 ) (Osy 2 + 02dy "F O~y)l~
resp. OsX Y/(Osx Osy ) where % x r , ° ~ x r and Oexr are the sire, dam and environmental covariance components, respectively, between the variables X and Y.
493
R E S U L T S A N D DISCUSSION
The effects of adjustment T a b l e I p r e s e n t s t h e r e g r e s s i o n c o e f f i c i e n t s , w i t h i n sires, o f m e a s u r e m e n t s o n c a r c a s s w e i g h t a n d age o f l a m b , r e s p e c t i v e l y , a l o n g w i t h t h e i r s t a n d a r d e r r o r a n d c o e f f i c i e n t o f d e t e r m i n a t i o n (r2). TABLE I Regression coefficients (b) of traits on carcass weight and age, respectively Traits
Regression coefficient on Carcass wt. b
sb
Age of lambs r2
b
sb
r2
0.029 0.043 0.030 0.11 0.042 0.003 0.043 0.014 0.022 0.003 0.036 0.12
0.07 0.02 0.01 0.10 0.12 0.07 0.11 0.03 0.01 0.01 0.09 0.14
External measurements 1 Length of tibia, T (ram) 2 Length of leg, F (ram) 3 F--T (ram) 4 Length of carcass, K (mm) 5 Depth of thorax, Th (ram) 6 Carcass grade 7 Width of chest, W (ram) 8 W/Th × 100 9 G/F × 100 10 Legscore 11 Width of gigots, G (ram) 12 Circumference of chest, U (ram)
2.05 0.883 --1.17 9.54 4.12 -0.187 4.34 0.712 1.00 0.158 3.62 13.35
0.068 0.121 0.081 0.23 0.08 0.0061 0.07 0.037 0.06 0.008 0.06 0.16
0.36 0.03 0.12 0.51 0.64 0.37 0.68 0.19 0.15 0.20 0.70 0.81
0.297 0.202 --0.095 1.42 0.595 --0.026 0.587 0.088 0.090 0.010 0.432 1.83
0.733 0.748 0.328 0.727 1.055 0.201 0.673 0.860 0.543 0.086
0.039 0.027 0.015 0.024 0.033 0.011 0.026 0.059 0.017 0.0044
0.18 0.32 0.22 0.37 0.39 0.17 0.30 0.12 0.38 0.19
0.090 0.015 0.067 0.011 0.047 0.0058 0.079 0.010 0.126 0.014 0.026 0.0041 0.080 0.010 0.059 0.021 0.053 0.075 0.01050.0017
0.02 0.02 0.04 0.04 0.05 0.03 0.04 0.01 0.02 0.03
0.51 0.25 0.27 0.24 0.04
0.128 0.014 0.135 0.018 0.072 0.0091 --0.280 0.042 0.01960.0073 0.121 0.0081 0.286 0.017
0.05 0.04 0.04 0.02 0.00 0.13 0.16
Internal measurements 13 14 15 16 17 18 19 20 21 22
Width of i. dorsi, A (mm) Depth of I. dorsi, B (mm) Fat thickness over I. d., C (ram) Thickest fat on the rib, J ( m m ) C+J Subcutaneous fat over X, Y (mm) Thickness of the side, X (mm) B/A × 100 A × B/100 Caulfat (kg)
Cannon bone measurements 23 24 25 26 27 28 29
Cannon bone weight,M W (g) 1.18 0.029 Cannon bone length,M L (mm) 1.05 0.045 Cannon bone circumf.,MC (ram) 0.581 0.024 M C / M W × 100 --2.67 0.118 M C / M L x 100 0.171 0.022 Carcassweight (kg) Weaning weight (kg)
494 The adjustment for age of lambs accounted for only a small fraction of the variation in the carcass traits, while adjustment for carcass weight reduced this variation to a much greater extent, depending on the degree of the relationship of the trait with the weight of the carcass. Heritability estimates for carcass traits which are strongly related to the weight of the carcass, and more so phenotypically than genetically, increased most by the adjustment for carcass weight as compared to that for age. The opposite effects are observed when the genetic correlations with carcass weight were higher than their p h e n o t y p i c counterparts. In such cases the heritability estimates slightly decreased or were unaffected in spite of the reduction in phenotypic variation. Purser (1960) obtained an increase from 0.25 to 0.57 in the heritability of the cannon bone length when correction was made for b o d y weight. By adjustment for carcass weight the phenotypic correlations changed towards negativity, i.e., positive values were lowered or became negative, while the negative ones became more negative. The same trend was observed by Bradford and Spurlock (1972). These changes infer that variation in carcass weight will exaggerate any positive correlations among measurements related to the weight o f the carcass, while true negative correlations may be disguised. The elimination of this variation is therefore necessary in order to avoid the effect of weight on shape. On the other hand, phenotypic correlations among traits that are only slightly related to carcass weight are b u t little affected b y the m e t h o d of adjustment. The genetic correlations were, with few exceptions, similarly affected by the m e t h o d of adjustment as their phenotypic counterparts. However, the changes were considerably smaller due to the low heritability of carcass weight. The bulk of the variation eliminated by this adjustment is, therefore, of environmental or nonadditive genetic origin. In view of the rather consistent reduction of the phenotypic variation by adjustment for carcass weight, it is considered justified to limit the discussion of heritabilities and relationships among traits studied to the weight-adjusted measurements.
Factors affecting the validity of the results The data used in this study were derived from a progeny testing program and n o t from a study specifically aimed at estimating genetic parameters. Some features of the design could affect the validity of the results. The sires to be tested were selected for certain traits. The genetic variation among sires for these selected and related traits will, therefore, be less than in the breeding flock as a whole. The ewes allocated to the rams to be tested were balanced for certain traits as described in the Materials and Methods. This procedure aims at minimizing the variation between rams, b u t at the same time it increases the variation within rams. Both these deviations from randomness in the testing program will lead to the underestimation of the
495 heritabilities being studied. However, the effects of selection and balancing on the variation are to some degree counteracted by the inclusion of the leggy-type sires, that were selected from the flock in opposite direction to the block-type sires. The effects on the genetic parameters of excluding the leggy-type sires from the material were studied for most traits. The sire variance components were in many cases considerably reduced in the restricted data set. As a consequence, the heritabilities of the external measurements of length and depth and carcass grade were reduced by 13 to 29%, while the measurements of width and the internal measurements were little affected if at all. As an example, the heritability of cannon bone length was reduced from 0.82--0.71. Of the five traits on which the ewes were balanced, fecundity is presumably unrelated to the traits studied here, in particular since only twin ram lambs are included. Furthermore, three traits, live weight, points for dressed carcass weight and age, are likely to be expressed in the progeny primarily as an effect of size. Their effect would, therefore, be largely eliminated by the correction for carcass weight. This was studied for t w o of the traits, age of ewe and points for dressed carcass weight. It was concluded, that little or nothing would be gained by including them as covariates in the final analysis. The fifth trait balanced for, left fore cannon bone length of the ewe, is obviously closely related to several of the traits studied. The limitation of the data to twin ram lambs eliminates nearly one half of the dams of each sire. A random elimination of precisely one half of the dams would restore 59% of the variation among sires eliminated by balancing. Considering this, it can be concluded that the reduction of the heritability estimates due to balancing would generally be less than i 0 % of the true values.
Estimates of heritability The least squares means, their phenotypic standard deviations, the heritability estimates and their standard errors are given in Table II. All the external carcass traits and the cannon bone traits had low coefficients of variation, ranging from 2--7%, measures of I. dorsi, carcass weight and weaning weight had coefficients of variation around 6--12%, while carcass grade, leg score and the various fat measurements had coefficients of variation from 20--50%. The heritability estimates of external carcass traits and the cannon bone traits, with the exception of carcass grade, were high. For measurements of length and depth (skeletal size) the values are in range of 0.60--0.80, b u t are generally lower, ranging from 0.40--0.60, for measurements of width and the derived expressions descriptive of shape and compactness (fleshiness). Worth noting are the low heritability estimates of the ratio MC:MW, considering the high estimates of each of these traits per se. Results from 715 single-born lambs from the same progeny testing program revealed a substantially higher estimate (0.67) for this ratio. Intra-twin pair competition is a likely explanation of this discrepancy.
496 TABLE II Least squares means, phenotypic standard deviations, heritability estimates corrected for carcass weight and their standard error Traits
Least squares mean
ap
h2
SE
194.5 260.6 66.11 612.8 258.2 1.43 156.8 60.76 84.48 2.84 220.0 286.7
5.59 9.95 6.38 18.67 6.22 0.47 5.78 2.91 4.70 0.62 4.77 12.83
0.75 0.75 0.50 0.59 0.67 0.31 0.38 0.56 0.42 0.54 0.61 0.51
0.11 0.11 0.09 0.10 0.10 0.08 0.08 0.09 0.08 0.10 0.09 0.09
51.76 22.09 2.42 6.00 8.42 1.56 10.40 42.76
3.06 2.07 1.17 1.80 2.53 0.84 1.91 4.58
0.44 0.32 0.32 0.28 0.33 0.32 0.14 0.36
0.09 0.08 0.08 0.08 0.08 0.08 0.06 0.08
11.46 0.76
1.33 0.33
0.36 0.12
0.08 0.06
2.29 3.77 1.83 9.11 1.75 1.74 3.72
0.53 0.82 0.41 0.16 0.72 0.11 0.18
0.09 0.12 ** ** ** 0.07 0.07
External measurements 1 2 3 4 5 6 7 8 9 10 11 12
Length of tibia, T (ram) Length of leg, F (ram) F--T (ram) Length of carcass, K (mm) Depth of thorax, Th (ram) Carcass grade Width of chest, W(mm) W/Th x 100 G / F X 100 Leg score Width of gigots, G (ram) Circumference of chest, U(mm)
Internal measurements 13 14 15 16 17 18 19 20
Width of I. dorsi, A (mm) Depth of I. dorsi, B (mm) Fat thickness over I. d., C (mm) Thickest fat on the rib, J (mm) C+J Subcutaneous fat over X, Y (mm) Thickness of the side, X (ram) B / A X 100
21 A x B/100 22 Caulfat(kg)
Cannon bone measurements 23 24 25 26 27 28 29
Cannon bone weight, MW (g) Cannon bone length, ML (ram) Cannon bone circumf., MC (ram) MC/MW × 100 MC/ML x 100 Carcass weight* (kg) Weaning weight* (kg)
34.71 116.5 42.47 123.20 36.56 14.34 36.47
*Corrected for age of lambs. **Not calculated because negative dam variance component is replaced by zero. T h e r e a r e r a t h e r f e w d i r e c t l y c o m p a r a b l e r e s u l t s a v a i l a b l e in t h e l i t e r a t u r e f r o m o t h e r s t u d i e s o f e x t e r n a l c a r c a s s t r a i t s in s h e e p . H o w e v e r , B i c h a r d a n d Yalcin (1964), working with twin ram lambs, estimated the heritabilities of l e n g t h o f t i b i a , l e g l e n g t h a n d w i d t h o f g i g o t s as 0 . 8 3 , 0 . 7 8 a n d 0 . 0 2 , r e s p e c t i v e l y . T h e y also r e p o r t e d t h e h e r i t a b i l i t y e s t i m a t e s o f t h e l e n g t h a n d w e i g h t
497 of the cannon bone 0.40 and 0.07, respectively. Their estimates of length of tibia and leg length are in good agreement with the present results, whereas the low estimates of width of gigots and of cannon bone weight and length differ greatly from the present results. A possible explanation of their low estimates of these traits is the fact that no correction for carcass weight was made in their study. The area of I. dorsi, as measured by the product A × B, yielded a heritability estimate of 0.36, a value intermediate between those of the width and depth of the I. dorsi. The heritability estimate of this trait obtained here is in good agreement with those presented by Hilman et al. (1962), Carpenter (1963) and Field et ai. (1967). The subcutaneous fat measurements were moderately heritable. Caulfat yielded the lowest heritability estimate of the fat measurements. Generally, the heritability estimates of the subcutaneous fat measurements obtained here agree with other estimates reported (Carpenter, 1963; Timon, 1965; Munson, 1967; Smith et al., 1968; Botkin et al., 1969). Published heritability values for weaning weight of m u t t o n breeds, at ages 120--170 days, are in range of 0.00--0.20 (Bichard and Yalcin, 1964; Bowman and Broadbent, 1966; Vogt et al., 1967; Eikje, 1974). Bowman et al. (1968} and Bowman and Hendy (1972) obtained the low estimates of 0.02 and 0.11 for carcass weight of lambs, 35--45 kg live weight, and Smith et al. (1968) obtained 0.16 for carcass weight per day of age. Previously published heritability estimates of weaning weight in Iceland sheep are in range of 0 . 1 5 - 0 . 2 0 and those of carcass weight around 0.08 (Hallgrfmsson, 1971; J6nmundsson, 1975; Adalsteinsson and J6nmundsson, 1978). The heritability estimates of 0.18 for weaning weight and 0.11 for carcass weight presented here are in good agreement with these findings. Genetic and p h e n o t y p i c correlations The genetic and phenotypic correlations among the traitsstudied, corrected for carcass weight, are presented in rows and columns 1--27 of Table III. Rows and columns 28 and 29 in the table present the corresponding relationship of the same traitswith carcass weight and live weight corrected for age of lambs. Table IV presents the standard errors of the genetic correlation coefficients. In general the phenotypic and genetic correlations behave similarly. In most cases the phenotypic correlations are of the same sign and lower absolute value than their genetic counterparts. Consistently higher phenotypic values than their corresponding genetic ones occur in the relationships of width of chest with the fat measurements, both late maturing characteristics much affected by environment, e.g.,the plane of nutrition (P~Isson and Verg6s, 1952). It is difficult to account for such an intricate set of relationships amongst the 27 carcass traits in anything but general terms. External carcass traits,
498
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500 T A B L E IV S t a n d a r d errors f o r g e n e t i c c o e f f i c i e n t s o f c o r r e l a t i o n
1T 2F 3 F--T 4K 5 Th 6 Carcass grade 7W 8 W/Th 9 G/F 1 0 Leg s c o r e 11 G 12 U
F
F--T
K
Th
Carcass grade
W
W/Th G/F
Leg
G
U
A
B
0.02
0.08 0.02
0.08 0.06 0.07
0,09 0.08 0.09 0.10
0.12 0.11 0.12 0.14 0.13
0.13 0.13 0.15 0.14 0.13 0.17
0.12 0.12 0.13 0.13 0.14 0.15 0.03
0.12 0.13 0.15 0.12 0.12 0.16 0.11 0.09 0.05
0.11 0.11 0.12 0.12 0.12 0.14 0.14 0.12 0.10 0.12
0.11 0.10 0.12 0.12 0.08 0.15 0.14 0.13 0.14 0.13 0.12
0.12 0.12 0.13 0.13 0.12 0.16 0.15 0.14 0.14 0.13 0.13 0.14
0.13 D,14 0.15 0.15 0.14 0.17 0.16 0.14 0.14 0.13 0.14 0.15
0.14 0.15 0.16 0.14 0.13 0.17 0.14 0.11
13 A 14B 15C 16 J 17 C+J 18 Y 19 X 20 B/A 21AXB 22 Caulfat 23 24 25 26 27 28 29
MW ML MC MC/MW MC/ML Carcass wt.* Live w t . *
* C o r r e c t e d for age o f l a m b s .
descriptive of skeletal size, are generally highly positively intercorrelated. Width of chest and traits descriptive of shape, compactness and fullness of flesh are also generally highly intercorrelated. However, a rather strong negative relationship exists between the traits of these two groups of external measures. Carcass grade exhibits the same correlation pattern as traits descriptive of skeletal size in its relationship with traits indicative of compactness and fullness of flesh. This implies that large framed carcasses grade lower than small framed ones, as positive correlation between carcass grade and any trait implies lower grading because of the descending order of merit of the grading system. The external measures of skeletal size have low positive correlations with the width of 1. dorsi, but generally high negative correlations with internal measures of thickness, whether of muscle, fat or combination of both fat and muscle (X). External measures of width, shape and compactness show the reversed relationship with the internal traits. The consistency of the magnitude of the genetic correlations suggests that one trait in each group of the external measures may be sufficient to represent the whole group.
0.15
501
C
J
C+J
Y
X
B/A
A X B
Caul-
MW
ML
MC
fat
MC/ MW
MC/ ML
Carcass wt.
Live wt.
0.14 0.14 0.15 0.15 0.14 0.18 0.16 0.14 0.16 0.14 0.15 0.15
0.15 0.16 0.18 0.16 0.16 0.20 0.16 0.13 0.14 0.12 0.15 0.16
0.14 0.14 0.15 0.14 0.16 0.15 0.15 0.15 0.14 0.14 0.19 0.18 0.16 0.16 0.12 0.13 0.14 0.15 0.12 0.13 0.14 0.14 0.15 0.15
0.21 0.24 0.28 0.25 0.22 0.29 0.20 0.18 0.20 0.18 0.21 0.21
0.14 0.13 0.15 0.14 0.14 0.17 0.15 0.13 0.13 0.12 0.14 0.15
0.13 0.13 0.14 0.14 0.14 0.17 0.16 0.14 0.14 0.09 0.14 0.14
0.25 0.22 0.22 0.23 0.21 0.27 0.23 0.21 0.22 0.21 0.21 0.22
0.06 0.06 0.10 0.11 0.09 0.13 0.15 0.13 0.15 0.14 0.12 0.12
0.03 0.03 0.07 0.08 0.09 0.12 0.13 0.12 0.13 0.12 0.11 0.10
0.13 0.13 0.14 0.13 0.13 0.16 0.15 0.14 0.15 0.14 0.13 0.14
0.27 0.26 0.25 0.22 0.23 0.25 0.19 0.15 0.17 0.15 0.18 0.22
0.10 0.10 0.12 0.11 0.11 0.14 0.12 0.II 0.10 0.08 0.11 0.12
0.16 0.19 0.23 0.14 0.19 0.39 0.21 0.22 0.24 0.24 0.12 0.17
0.15 0.16 0.19 0.16 0.15 0.26 0.19 0.18 0.20 0.18 0.12 0.18
0.16 0.17
0.18 0.18 0.13
0.16 0.17 0.05 0.03
0.22 0.23 0.21 0.17 0.17 0.21
0.18 0.06 0.16 0.16 0.16 0.16 0.21
0.09 0.05 0.17 0.17 0.17 0.17 0.23 0.14
0.25 0.25 0.24 0.24 0.23 0.24 0.33 0.25 0.24
0.13 0.15 0.16 0.18 0.17 0.15 0.25 0.15 0.14 0.25
0.12 0.13 0.14 0.15 0.14 0.14 0.20 0.13 0.12 0.22
0.14 0.16 0.16 0.17 0.16 0.16 0.22 0.15 0.15 0.24
0.20 0.21 0.20 0.19 0.18 0.20 0.29 0.20 0.21 0.31
0.12 0.12 0.14 0.14 0.14 0.14 0.19 0.12 0.12 0.20
0.19 0.28 0.24 0.34 0.30 0.26 0.33 0.25 0.23 0.43
0.19 0.22 0.20 0.24 0.25 0.22 0.29 0.20 0.23 0.36
0.07
0.10 0.13
0.35 0.26 0.20
0.12 0.11 0.06 0.12
0.14 0.17 0.23 0.43 0.20
0.15 0.16 0.18 0.31 0.17 0.16
0.16 0.17 0.12 0.11 0.09
The fat measurements C, J, Y are highly positively intercorrelated, measurements J and Y being more highly correlated than either of these with C. An interesting feature is h o w the fat measurements differ in their relationship with depth of I. dorsi. Measurement C, and to a lesser extent Y, both purely subcutaneous fat measurements, are negatively associated with the depth of I. dorsi, B, while the reverse is true for measurement J, the thickest layer of fat on the side. The width and the area of I. dorsi, A and A × B, are independent of, or negatively correlated with all the fat measurements, including caulfat. The relationships of the cannon bone traits with other traits studied are of special interest, as the cannon bone length, i.e., short cannon, has been used as a selection criterion for meat quality improvement in Iceland sheep. Cannon bone length and weight behave much in the same manner as the external carcass measurements of length with respect to their association with other carcass measurements, external and internal. However, there are marked differences between these two traits regarding their degree of association with internal measurements of muscle and fat thickness. Cannon bone
502 weight is far more strongly negatively related to fat thickness than is cannon bone length. On the other hand, cannon bone weight is only slightly negatively related to the depth of I. dorsi, the ratio B : A and the area of l. dorsi, while cannon bone length is rather strongly negatively related to these traits. The circumference of the cannon bone is virtually uncorrelated with any external carcass traits, while it is weakly negatively related to fat thickness and slightly positively with muscle thickness. This agrees with the findings of Good et al. (1961) in cattle. They reported that, at equal live weight, cattle having a larger cannon bone circumference have larger I. dorsi area and less fat at the 12th rib. The ratio MC :MW shows a remarkably high genetic relationship with most of the external and internal measurements. This ratio alone appears to be an excellent indicator of carcass dimensions and fatness and, to a lesser extent, muscle thickness. This is in agreement with the theory put forward by P~lsson (1939, 1940) that a relatively light cannon bone with refined ends and a thick shaft is associated with early maturity, i.e., high degree of fatness and a high ratio B :A of the I. dorsi measurements. An interesting feature of the ratio MC:ML is its rather high positive genetic correlation with the measures of muscle thickness without any appreciable association with fat measures. Considering the high heritability (0.73) of this trait and its favourable relationships with fat and muscle, it appears to be an excellent criterion in selection for carcass quality. Carcass weight corrected for age of lambs is highly related to all measurements of carcass dimensions and bone weight, moderately with width of I. dorsi, and generally only slightly related to depth of I. dorsi and all fat measurements, and still less with all derived traits (ratios and differences) of the external measurements and with leg score. It is negatively, only slightly, associated with the shape index B : A and the ratio MC:ML, whereas it exhibits strong negative association with the ratio MC:MW. Live weight generally shows the same pattern as does carcass weight in its relationship with the carcass traits. ACKNOWLEDGEMENTS The authors thank Dr. H. P~lsson and Dr. S. Adalsteinsson for their advice, criticism and stimulating discussions and for their permission to use data collected by them from 1958--1963, and M.R. Thorgeirsson for his willing assistance at all times.
REFERENCES Adalsteinsson, S. and JSnmundsson, J.V., 1978. Pelt classification and live weight in Icelandic lambs. III. Direct and pedigree selection for weaning weight. J. Agric. Res. (Iceland), 10(1): 90--99.
503 Arnason, Th. and Thorsteinsson, S.S., 1982. Genetic studies on carcass traits in Iceland twin ram lambs. II. Analysis of principal components and construction of selection indices. Livest. Prod. Sci., 8: 507--517. Bichard, M. and Yalcin, B.C., 1964. Crossbred sheep production. II. Selection for growth rate and carcass attributes in the second cross lamb. Anita. Prod., 6: 179--187. Botkin, M.P., Field, R.A., Riley, M.L., Nolan, J.C., Jr. and Roehrkasse, G.P., 1969. Heritability of carcass traits in lambs. J. Anita. Sci., 29: 251--255. Bowman, J.C. and Broadbent, J.S., 1966. Genetic parameters of growth between birth and 16 weeks in Down cross sheep. Anita. Prod., 8: 129--135. Bowman, J.C. and Hendy, C.R.C., 1972. A study of retail requirements and genetic parameters of carcass quality in Polled Dorset Horn sheep. Anim. Prod., 14: 189--198. Bowman, J.C., Marshall, J.E. and Broadbent, J.S., 1968. Genetic parameters of carcass quality in Down cross sheep. Anita. Prod., 10: 183--191. Bradford, G.E. and Spurlock, G.M., 1972. Selection for meat production in sheep. Results of a progeny test. J. Anita. Sci., 34: 737--745. Carpenter, Z.L., 1963. Lamb sire progeny testing progress, Proc. 16th Ann. Reciprocal Meat Conf., 16: 9. Eikje, E.D., 1974. Studies on sheep production records. IV. Genetic, phenotypic and environmental parameters for weight o f lambs. Acta Agric. Scand., 24: 291--298. Falconer, D.S., 1963. Quantitative inheritance. In: Methodology in Mammalian Genetics, Holden Day, San Francisco, pp. 132--216. Field, R.A., Riley, M.L. and Botkin, M.P., 1967. Effect of sex and ram weight on composition of lambs. J. Anita. Sci., 2 6 : 8 9 4 (Abstr.). Good, D.C., Dahl, G.M., Wearden, S. and Weseli, D.J., 1961. Relationship among live and carcass characteristics of selected slaughter steers. J. Anita. Sci., 20: 698--701. Hallgffmsson, S., 1971. Heritability estimates of weaning weight and carcass weight in Iceland lamb. Mimeograph, Agric. Soc. Iceland, 3 pp. Harvey, W.R., 1972. Mixed model least squares and maximum likelihood computer program. Ohio State University, Columbus, OH. Hilman, M., Wheat, J.D., Menzies, C., Mackintosh, D.L. and Merkel, R.A., 1962. Heritability of and correlations between carcass and live animal traits in sheep. J. Anim. Sci., 2 1 : 9 7 3 (Abstr.). JSnmundsson, J.V., 1975. Factors affecting weaning weight and number of lambs. Licenciat Thesis, Agric. University, Norway. Munson, A.W., 1967. Association of various measurements with lamb carcass composition and preliminary estimates of some genetic parameters. Diss. Abstr. B. 27 : 4194-B. P~sson, H., 1939. Meat quality in the sheep with special reference to Scottish breeds and crosses. I. J. Agric. Sci., 29: 554--626. P~Isson, H., 1940. Meat quality in the sheep with special reference to Scottish breeds and crosses. II. J. Agric. Sci., 30: 1--64. P~Isson, H. and Verg6s, J.B., 1952. Effects of the plane of nutrition on growth and the development of carca~ quality in lambs. J. Agric. Sci., 42: 1--149. Purser, A.F., 1960. The use of correction for regression on a second character to increase the efficiency of selection. In: Biometrical Genetics. Proc. o f an Int. Syrup., Ottawa, 1958. Pergamon, Oxford, 1960, pp. 210--214. Smith, R.H., Kemp, J.D., Moody, W.G. and Cundiff, L.V., 1968. Heritability estimates o f some lamb carcass traits. Prog. Rep. Ky. Agric. Exp. Stn., No. 176: 24--25. Timon, V.M., 1965. Estimates of genetic and environmental factors affecting growth and carcass traits in lambs. European Assoc. for Animal Production, Commission on Animal Genetics, Noordwijk, June 23, 1965. Vogt, D.W., Carter, R.C. and McClure, W.H., 1967. Genetic and phenotypic parameter estimates involving economically important traits in sheep. J. Anim. Sci., 26: 1232-1238.
504 RESUME 'Fnorsteinsson, S.S. et Bj6rnsson, H., 1982. Etudes g~ndtiquas des caract~ristiques des carcasses chez les agneaux m~les jumeaux d'Islande. I. Estimation des param~tres g~n~tiques agissant sur les caract~ristiques des carcasses, le poids au sevrage et le poids de carcasse. Livest. Prod. Sci., 8 : 4 8 9 - - 5 0 5 (en anglais). On a utilis~ les r~sultats de 1826 agneaux m~es entiers n~s jumeaux, issus de 216 bdliers test~s entre 1958 et 1977. En plus du poids vif et du poids de carcasse, on a analys~ 27 caract~ristiques comprenant des mesures sur la carcasse, des notes et diverses appreciations d6crivant diffdrents caractdres de la conformation de la carcasse. Les donn~es ont dt~ corrig~es par deux m~thodes, ~ savoir la rdgression des mesures par rapport au poids de la carcasse et par rapport ~ l'~ge des agneaux. La correction par rapport au poids de la carcasse permet un meilleur ajustement que celle par rapport d l'~ge quand on 6tudie les caract~res de quatit~ de la carcasse. L'h6ritabilit~ obtenue est ~lev~e pour les caract~res externes de la carcasse, de 0,60 ~ 0,80 pour les mensurations de longueur et de profondeur, mais g6n~ralement plus faible, de 0,40 0,60, pour les mesures de largeur, les notes pour le rebondi de la cuisse et l'expression ddriv~e de ces mesures de la carcasse. L'h~ritabilit~ a 6t~ de 0,12 ~ 0,44 pour les mesures de la section de la 12~me cSte, le poids du gras interne et la classe de qualitd de la carcasse et de 0,18 et 0,11, respectivement, pour le poids vif et le poids de la carcssse, corrig~s pour l'~ge des agneaux. On discute les effets possibles du protocole du programme de testage sur la validit~ des estimations de 1~6ritabilit~. On discute les relations g~n~tiques et ph6notypiques entre les 27 caract~ristiques de la carcasse, ainsi que les relations entre ces caract~res et le poids vif et le poids de carcasse corrig~s pour l'~ge des agneaux. On constate une opposition g~n4tique entre les mesures de la longueur du squelette et les mesures de la charnure. Le rapport circonf~rence m i n i m u m : longueur du m4tacarpe appara~ comme le crit~re unique le plus utile dans la s~lection pour la qualit~ des carcasses. KURZFASSUNG
Thorsteinsson, S.S. und Bj~irnsson,H., 1982. Genetische Untersuchungen fiber Schlachtk~irpermerkmale bei Isl~/ndischenZwillingsbockl~/mmern. I. Sch~'tzwerte genetischer Parameter fiirSchlachtk6rpermerkmale, Lebendgewicht beim Absetzen und SchlachtkGrpergewicht. Livest. Prod. Sci., 8:489--505 (auf englisch). F~ir diese Analyse wurden Daten ~iber 1826 unkastrierte Zwillingsbockl//mmer herangezogen, Nachkommen von 216 B6cken, die in der Zeit 1958--1977 gepriift wurden. Zus//tzlich zum Lebend- und Sehlachtk6rpergewieht wurden 27 Merkmale untersucht, die direkte Messungen des Schlachtk~rpers, Bonitierungen und verschiedene abgeleitete Begriffe zur Beschreibung verschiedener Merkmale der Schlachtk~rperausformung einbezogen. Das Datenmaterial wurde nach 2 Methoden berechnet, dutch die Regression der Messwerte auf Sehlachtk~irpergewicht und dutch die Regression der Messwerte auf das Alter der L~/mmer. Es wird gefolgert, dass bei der Untersuchung yon Schlaehtk~irperqualit~/tsmerkmalen eine Korrektur auf Schlachtk~rpergewicht besser ist als eine Korrektur auf das Alter. Fiir die ~'usseren Schlachtk~irpermerkmale ergaben sich hohe Heritabilit//tssch~tzwerte yon 0,60 bis 0,80 fiir lineare Messungen der L~'nge und Tiefe abet generell niedrigere Werte yon 0,40 bis 0,60 f/Jr Messungen der Breite, fiir Bonltierungen in Bezug auf die Auspriigung der Keule und fiir den abgeleiteten Ausdruek der Schlachtk/irpermasse. Fiir die Querschnittsmessungen an der 12. Rippe, das Gewicht des Innenfettes und die Schlachtk/~rperqualit//t ergaben sich Heritabilit~/tssch~/tzwerte von 0,12 bis 0,44; Werte yon
505 0,18 und 0,11 wurden fiir das auf das Alter der L~/mmer korrigierte Lebend-, bzw. SchlachtkSrpergewicht ermittelt. Es werden die mSglichen Auswirkungen des Programmplanes ffir die Nachkommenpr(ifung auf die Sicherheit der Heritabilit~/tssch~/tzwerte diskutiert. Die genetischen uncl ph//notypischen Beziehungen zwischen 27 SchlachtkSrpermerkmalen werden ebenso diskutiert wle die Beziehungen zwischen diesen Merkmalen und dem auf das Alter der L~fmmer korrigierten Lebend- und Schlachtk~rpergewicht. Es wurde festgestellt, dass ein genetischer Antagonismus besteht zwischen der L~'nge yon Skelettmassen und Merkmalen, die die Fleischf/ille beschreiben. Es wird darauf hingewiesen, dass das Verh/fltnis yon Mindestumfang zu L~/nge der Mittelhand das einzige sehr brauchbare Kriterium bei der Selektion auf Schlachtk~rperqualit~/t ist.
489
Elsevier Scientific Publishing Company, Amsterdam - - P r i n t e d in The Netherlands
GENETIC STUDIES ON CARCASS TRAITS IN ICELAND TWIN RAM LAMBS I. ESTIMATES OF GENETIC PARAMETERS ON CARCASS TRAITS, LIVE WEIGHT AT WEANING AND CARCASS WEIGHT
S.S. THORSTEINSSON and H. BJORNSSON
Agricultural Research Institute, Keldnaholt, 110 Reykjam'k (Iceland) (Accepted 31 August 1981)
ABSTRACT
Thorsteinsson, S.S. and BjSrnsson, H., 1982. Genetic studies on carcass traits in Iceland twin ram lambs. I. Estimates of genetic parameters on carcass traits, live weight at weaning and carcass weight. Livest. Prod. Sci., 8: 489--505. For this analysis, data from 1826 entire twin ram lambs, progeny of 216 sires tested during the period 1958--1977, were used. In addition to live weight and carcass weight, 27 traits involving direct carcass measurements, scores and various derived expressions describing different features of carcass conformation were analysed. The data were adjusted by two methods: by the regression of measurements on carcass weight and by the regression of measurements on age of lambs. It is concluded that in studying carcass quality traits, correction for carcass weight is a better adjustment than correction for age. High heritability estimates were obtained for external carcass traits, 0.60--0.80 for linear measurements of length and depth, b u t generally lower, 0.40--0.60 for measurements of width, scores for fullness of leg and the derived expressions of carcass measurements. Cross-sectional measurements at the 12th rib, caulfat weight and carcass grade yielded heritability estimates of 0.12--0.44. Heritability estimates of live and carcass weights corrected for age of lambs were found to be 0.18 and 0.11, respectively. The possible effects of the design of the progeny testing program on the validity of the heritability estimates are discussed. Genetic and phenotypic relationships amongst 27 carcass traits are discussed as well as the relationships of these traits with live weight and carcass weight corrected for age of lambs. A genetic antagonism was found to exist between length of skeletal measurements and traits indicating fleshiness. It is suggested that the ratio of m i n i m u m circumference: length of the metacarpal is the single most useful criterion in selection for carcass quality.
INTRODUCTION
In 1957, a ram progeny testing program for improvement of conformation and carcass quality of lambs at weaning was initiated at the Experimental Sheep Farm, Hestur, of the Agricultural Research Institute in Iceland. This paper is concerned with estimates of phenotypic and genetic parameters for various carcass traits, weight at weaning, carcass and caulfat weight 0301-6226/82/0000-0000/$02.75 © 1982 Elsevier Scientific Publishing Company
490 of Iceland twin ram lambs. Another paper reports the use of these results in formulating selection indices for meat quality improvement (Arnason and Thorsteinsson, 1982). MATERIALS AND METHODS Source o f data
For this analysis, data from 1826 entire twin ram lambs of the Iceland sheep, progeny of 216 sires tested during the period 1958--1977, were used. Of these, 1753 lambs were the progeny of 171 short-legged blocky-type sires distributed over 20 years, 73 lambs were the progeny of 25 long-legged rangyt y p e sires distributed over 9 years (1961--1970), and finally, 28 lambs were the progeny of 12 randomly selected sires distributed over 3 years (1964-1966). Procedure and management
The sires to be tested were mostly selected from the breeding flock at Hestur. However, some were sired by A.I. rams, and a few were from pedigree flocks in the same area. The criteria on which the test sires were selected were: weight for age and t y p e of birth, twins preferred to singles, heartgirth, width of loin, length of the left fore cannon bone (metacarpal), scorns for fullness of loin, rump and leg. Pedigree of parents was taken into account, greatest attention being paid to their fecundity and milking ability. T w e n t y ewes from the breeding flock at ages three to seven were allocated to each ram. The ewes were divided into even lots according to age, live weight at previous weaning, length of the left fore cannon, fecundity, and points for d r e s ~ d carcass weight production in previous years. These lots were randomly allocated to the rams tested. The ewes were treated as a one lot during the whole year. All lambs were weighed at birth, ear-tagged and t y p e of birth and sex recorded. In the last week of September, the lambs were weaned at an average age o f 132 days, weighed, and all the males killed. At slaughter, weight of dressed carcass, caulfat and pelt were recorded and the left f o r e f o o t of each lamb was collected for dissection of the metacarpal. After rigor mortis the following measurements, as defined by P~lsson (1939), and illustrated in Fig. 1, were taken on the carcass: External measurements T = length of tibia + tarsus; F = leg length; G = width of gigot; Th = depth o f thorax; W = width of chest; U = circumference of chest;
491
K = l e n g t h o f carcass; Leg s c o r e = s c o r e f o r fullness o f leg o n a scale o f 1 - - 5 in a s c e n d i n g order o f merit; Carcass grade = c o m m e r c i a l e v a l u a t i o n o f t h e carcass o n a scale o f 1 - - 4 in d e s c e n d i n g order o f merit. Internal carcass m e a s u r e m e n t s ( o n cross s e c t i o n at t h e 1 2 t h rib) A = " w i d t h " o f l. dorsi; B = " d e p t h " o f I. dorsi;
-
/
K
CIRCUMf'ERtZNCE L [ NG T~
OF" CANNOI
Fig. 1. Carcass measurements.
492
C = thickness of back-fat over the deepest part of the l. dorsi; J = thickest layer of fat over rib; X = thickness of muscle layer (mixed with fat) on the lower half of the rib; Y = thickness of subcutaneous fat layer over X. Cannon (metacarpal) bone measurements (after dissection) MW = fresh weight of the left metacarpal; ML = length of the left metacarpal; MC = minimum circumference of the left metacarpal. Measurements T and F were taken with a ruler, b u t measurements K and U were taken with a linen tape. Measurements G, W, Th were taken with sliding callipers and measurements on cross section at the 12th rib with steel dividers. Statistical analysis The following nested linear model was used for the analysis Yhijk = I~h + Shi + dhij + ~ (Xhijk - - X h . . .) + ehijk where Yhijk is the observation on one trait, #h denotes the annual mean in year h, shi and dhij denote the random effects o f the jth dam of the ith sire in the hth year with dams nested within sires and sires within years and the variances of the sire and dam effects are a~ and a~, respectively, ~ is the coefficient for regression on a covariate, X, with_ the value Xhijk for the kth lamb of the hijth dam and the annual mean X h . . . . and ehijk is the residual random deviation with variance o~. All covariances among effects are assumed to be zero. The covariates used were age at weaning for adjustment to equal age, and carcass weight for adjustment to equal carcass weight. Only linear regression was used. A limited study had indicated that n o t much would be gained by using powers of carcass weight other than one. Harvey's (1972) least squares c o m p u t e r program was used for the analysis and estimation of genetic parameters, except for the standard errors of the heritability estimates which were calculated according to Falconer (1963). The degrees of freedom and expected values of mean squares in the analyses adjusted for carcass weight were Between sires 196 oe2 + 1.402 a2d + 8.298 o~ Dams/sires 1190 oe2 + 1.278 a~ Progeny/dams 419 o~ Heritabilities were estimated by 4 o~/(o~ + a~ + o~) with the population variances replaced b y their estimates. The p h e n o t y p i c and genotypic correlations between any two variables, X and Y, were calculated from the variance and covariance c o m p o n e n t s as (Os,yy 4r tldX Y ~" OeXY I ) / [ ( OsX 2 "1" O'~LY "t" OeX 2 ) (Osy 2 + 02dy "F O~y)l~
resp. OsX Y/(Osx Osy ) where % x r , ° ~ x r and Oexr are the sire, dam and environmental covariance components, respectively, between the variables X and Y.
493
R E S U L T S A N D DISCUSSION
The effects of adjustment T a b l e I p r e s e n t s t h e r e g r e s s i o n c o e f f i c i e n t s , w i t h i n sires, o f m e a s u r e m e n t s o n c a r c a s s w e i g h t a n d age o f l a m b , r e s p e c t i v e l y , a l o n g w i t h t h e i r s t a n d a r d e r r o r a n d c o e f f i c i e n t o f d e t e r m i n a t i o n (r2). TABLE I Regression coefficients (b) of traits on carcass weight and age, respectively Traits
Regression coefficient on Carcass wt. b
sb
Age of lambs r2
b
sb
r2
0.029 0.043 0.030 0.11 0.042 0.003 0.043 0.014 0.022 0.003 0.036 0.12
0.07 0.02 0.01 0.10 0.12 0.07 0.11 0.03 0.01 0.01 0.09 0.14
External measurements 1 Length of tibia, T (ram) 2 Length of leg, F (ram) 3 F--T (ram) 4 Length of carcass, K (mm) 5 Depth of thorax, Th (ram) 6 Carcass grade 7 Width of chest, W (ram) 8 W/Th × 100 9 G/F × 100 10 Legscore 11 Width of gigots, G (ram) 12 Circumference of chest, U (ram)
2.05 0.883 --1.17 9.54 4.12 -0.187 4.34 0.712 1.00 0.158 3.62 13.35
0.068 0.121 0.081 0.23 0.08 0.0061 0.07 0.037 0.06 0.008 0.06 0.16
0.36 0.03 0.12 0.51 0.64 0.37 0.68 0.19 0.15 0.20 0.70 0.81
0.297 0.202 --0.095 1.42 0.595 --0.026 0.587 0.088 0.090 0.010 0.432 1.83
0.733 0.748 0.328 0.727 1.055 0.201 0.673 0.860 0.543 0.086
0.039 0.027 0.015 0.024 0.033 0.011 0.026 0.059 0.017 0.0044
0.18 0.32 0.22 0.37 0.39 0.17 0.30 0.12 0.38 0.19
0.090 0.015 0.067 0.011 0.047 0.0058 0.079 0.010 0.126 0.014 0.026 0.0041 0.080 0.010 0.059 0.021 0.053 0.075 0.01050.0017
0.02 0.02 0.04 0.04 0.05 0.03 0.04 0.01 0.02 0.03
0.51 0.25 0.27 0.24 0.04
0.128 0.014 0.135 0.018 0.072 0.0091 --0.280 0.042 0.01960.0073 0.121 0.0081 0.286 0.017
0.05 0.04 0.04 0.02 0.00 0.13 0.16
Internal measurements 13 14 15 16 17 18 19 20 21 22
Width of i. dorsi, A (mm) Depth of I. dorsi, B (mm) Fat thickness over I. d., C (ram) Thickest fat on the rib, J ( m m ) C+J Subcutaneous fat over X, Y (mm) Thickness of the side, X (mm) B/A × 100 A × B/100 Caulfat (kg)
Cannon bone measurements 23 24 25 26 27 28 29
Cannon bone weight,M W (g) 1.18 0.029 Cannon bone length,M L (mm) 1.05 0.045 Cannon bone circumf.,MC (ram) 0.581 0.024 M C / M W × 100 --2.67 0.118 M C / M L x 100 0.171 0.022 Carcassweight (kg) Weaning weight (kg)
494 The adjustment for age of lambs accounted for only a small fraction of the variation in the carcass traits, while adjustment for carcass weight reduced this variation to a much greater extent, depending on the degree of the relationship of the trait with the weight of the carcass. Heritability estimates for carcass traits which are strongly related to the weight of the carcass, and more so phenotypically than genetically, increased most by the adjustment for carcass weight as compared to that for age. The opposite effects are observed when the genetic correlations with carcass weight were higher than their p h e n o t y p i c counterparts. In such cases the heritability estimates slightly decreased or were unaffected in spite of the reduction in phenotypic variation. Purser (1960) obtained an increase from 0.25 to 0.57 in the heritability of the cannon bone length when correction was made for b o d y weight. By adjustment for carcass weight the phenotypic correlations changed towards negativity, i.e., positive values were lowered or became negative, while the negative ones became more negative. The same trend was observed by Bradford and Spurlock (1972). These changes infer that variation in carcass weight will exaggerate any positive correlations among measurements related to the weight o f the carcass, while true negative correlations may be disguised. The elimination of this variation is therefore necessary in order to avoid the effect of weight on shape. On the other hand, phenotypic correlations among traits that are only slightly related to carcass weight are b u t little affected b y the m e t h o d of adjustment. The genetic correlations were, with few exceptions, similarly affected by the m e t h o d of adjustment as their phenotypic counterparts. However, the changes were considerably smaller due to the low heritability of carcass weight. The bulk of the variation eliminated by this adjustment is, therefore, of environmental or nonadditive genetic origin. In view of the rather consistent reduction of the phenotypic variation by adjustment for carcass weight, it is considered justified to limit the discussion of heritabilities and relationships among traits studied to the weight-adjusted measurements.
Factors affecting the validity of the results The data used in this study were derived from a progeny testing program and n o t from a study specifically aimed at estimating genetic parameters. Some features of the design could affect the validity of the results. The sires to be tested were selected for certain traits. The genetic variation among sires for these selected and related traits will, therefore, be less than in the breeding flock as a whole. The ewes allocated to the rams to be tested were balanced for certain traits as described in the Materials and Methods. This procedure aims at minimizing the variation between rams, b u t at the same time it increases the variation within rams. Both these deviations from randomness in the testing program will lead to the underestimation of the
495 heritabilities being studied. However, the effects of selection and balancing on the variation are to some degree counteracted by the inclusion of the leggy-type sires, that were selected from the flock in opposite direction to the block-type sires. The effects on the genetic parameters of excluding the leggy-type sires from the material were studied for most traits. The sire variance components were in many cases considerably reduced in the restricted data set. As a consequence, the heritabilities of the external measurements of length and depth and carcass grade were reduced by 13 to 29%, while the measurements of width and the internal measurements were little affected if at all. As an example, the heritability of cannon bone length was reduced from 0.82--0.71. Of the five traits on which the ewes were balanced, fecundity is presumably unrelated to the traits studied here, in particular since only twin ram lambs are included. Furthermore, three traits, live weight, points for dressed carcass weight and age, are likely to be expressed in the progeny primarily as an effect of size. Their effect would, therefore, be largely eliminated by the correction for carcass weight. This was studied for t w o of the traits, age of ewe and points for dressed carcass weight. It was concluded, that little or nothing would be gained by including them as covariates in the final analysis. The fifth trait balanced for, left fore cannon bone length of the ewe, is obviously closely related to several of the traits studied. The limitation of the data to twin ram lambs eliminates nearly one half of the dams of each sire. A random elimination of precisely one half of the dams would restore 59% of the variation among sires eliminated by balancing. Considering this, it can be concluded that the reduction of the heritability estimates due to balancing would generally be less than i 0 % of the true values.
Estimates of heritability The least squares means, their phenotypic standard deviations, the heritability estimates and their standard errors are given in Table II. All the external carcass traits and the cannon bone traits had low coefficients of variation, ranging from 2--7%, measures of I. dorsi, carcass weight and weaning weight had coefficients of variation around 6--12%, while carcass grade, leg score and the various fat measurements had coefficients of variation from 20--50%. The heritability estimates of external carcass traits and the cannon bone traits, with the exception of carcass grade, were high. For measurements of length and depth (skeletal size) the values are in range of 0.60--0.80, b u t are generally lower, ranging from 0.40--0.60, for measurements of width and the derived expressions descriptive of shape and compactness (fleshiness). Worth noting are the low heritability estimates of the ratio MC:MW, considering the high estimates of each of these traits per se. Results from 715 single-born lambs from the same progeny testing program revealed a substantially higher estimate (0.67) for this ratio. Intra-twin pair competition is a likely explanation of this discrepancy.
496 TABLE II Least squares means, phenotypic standard deviations, heritability estimates corrected for carcass weight and their standard error Traits
Least squares mean
ap
h2
SE
194.5 260.6 66.11 612.8 258.2 1.43 156.8 60.76 84.48 2.84 220.0 286.7
5.59 9.95 6.38 18.67 6.22 0.47 5.78 2.91 4.70 0.62 4.77 12.83
0.75 0.75 0.50 0.59 0.67 0.31 0.38 0.56 0.42 0.54 0.61 0.51
0.11 0.11 0.09 0.10 0.10 0.08 0.08 0.09 0.08 0.10 0.09 0.09
51.76 22.09 2.42 6.00 8.42 1.56 10.40 42.76
3.06 2.07 1.17 1.80 2.53 0.84 1.91 4.58
0.44 0.32 0.32 0.28 0.33 0.32 0.14 0.36
0.09 0.08 0.08 0.08 0.08 0.08 0.06 0.08
11.46 0.76
1.33 0.33
0.36 0.12
0.08 0.06
2.29 3.77 1.83 9.11 1.75 1.74 3.72
0.53 0.82 0.41 0.16 0.72 0.11 0.18
0.09 0.12 ** ** ** 0.07 0.07
External measurements 1 2 3 4 5 6 7 8 9 10 11 12
Length of tibia, T (ram) Length of leg, F (ram) F--T (ram) Length of carcass, K (mm) Depth of thorax, Th (ram) Carcass grade Width of chest, W(mm) W/Th x 100 G / F X 100 Leg score Width of gigots, G (ram) Circumference of chest, U(mm)
Internal measurements 13 14 15 16 17 18 19 20
Width of I. dorsi, A (mm) Depth of I. dorsi, B (mm) Fat thickness over I. d., C (mm) Thickest fat on the rib, J (mm) C+J Subcutaneous fat over X, Y (mm) Thickness of the side, X (ram) B / A X 100
21 A x B/100 22 Caulfat(kg)
Cannon bone measurements 23 24 25 26 27 28 29
Cannon bone weight, MW (g) Cannon bone length, ML (ram) Cannon bone circumf., MC (ram) MC/MW × 100 MC/ML x 100 Carcass weight* (kg) Weaning weight* (kg)
34.71 116.5 42.47 123.20 36.56 14.34 36.47
*Corrected for age of lambs. **Not calculated because negative dam variance component is replaced by zero. T h e r e a r e r a t h e r f e w d i r e c t l y c o m p a r a b l e r e s u l t s a v a i l a b l e in t h e l i t e r a t u r e f r o m o t h e r s t u d i e s o f e x t e r n a l c a r c a s s t r a i t s in s h e e p . H o w e v e r , B i c h a r d a n d Yalcin (1964), working with twin ram lambs, estimated the heritabilities of l e n g t h o f t i b i a , l e g l e n g t h a n d w i d t h o f g i g o t s as 0 . 8 3 , 0 . 7 8 a n d 0 . 0 2 , r e s p e c t i v e l y . T h e y also r e p o r t e d t h e h e r i t a b i l i t y e s t i m a t e s o f t h e l e n g t h a n d w e i g h t
497 of the cannon bone 0.40 and 0.07, respectively. Their estimates of length of tibia and leg length are in good agreement with the present results, whereas the low estimates of width of gigots and of cannon bone weight and length differ greatly from the present results. A possible explanation of their low estimates of these traits is the fact that no correction for carcass weight was made in their study. The area of I. dorsi, as measured by the product A × B, yielded a heritability estimate of 0.36, a value intermediate between those of the width and depth of the I. dorsi. The heritability estimate of this trait obtained here is in good agreement with those presented by Hilman et al. (1962), Carpenter (1963) and Field et ai. (1967). The subcutaneous fat measurements were moderately heritable. Caulfat yielded the lowest heritability estimate of the fat measurements. Generally, the heritability estimates of the subcutaneous fat measurements obtained here agree with other estimates reported (Carpenter, 1963; Timon, 1965; Munson, 1967; Smith et al., 1968; Botkin et al., 1969). Published heritability values for weaning weight of m u t t o n breeds, at ages 120--170 days, are in range of 0.00--0.20 (Bichard and Yalcin, 1964; Bowman and Broadbent, 1966; Vogt et al., 1967; Eikje, 1974). Bowman et al. (1968} and Bowman and Hendy (1972) obtained the low estimates of 0.02 and 0.11 for carcass weight of lambs, 35--45 kg live weight, and Smith et al. (1968) obtained 0.16 for carcass weight per day of age. Previously published heritability estimates of weaning weight in Iceland sheep are in range of 0 . 1 5 - 0 . 2 0 and those of carcass weight around 0.08 (Hallgrfmsson, 1971; J6nmundsson, 1975; Adalsteinsson and J6nmundsson, 1978). The heritability estimates of 0.18 for weaning weight and 0.11 for carcass weight presented here are in good agreement with these findings. Genetic and p h e n o t y p i c correlations The genetic and phenotypic correlations among the traitsstudied, corrected for carcass weight, are presented in rows and columns 1--27 of Table III. Rows and columns 28 and 29 in the table present the corresponding relationship of the same traitswith carcass weight and live weight corrected for age of lambs. Table IV presents the standard errors of the genetic correlation coefficients. In general the phenotypic and genetic correlations behave similarly. In most cases the phenotypic correlations are of the same sign and lower absolute value than their genetic counterparts. Consistently higher phenotypic values than their corresponding genetic ones occur in the relationships of width of chest with the fat measurements, both late maturing characteristics much affected by environment, e.g.,the plane of nutrition (P~Isson and Verg6s, 1952). It is difficult to account for such an intricate set of relationships amongst the 27 carcass traits in anything but general terms. External carcass traits,
498
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500 T A B L E IV S t a n d a r d errors f o r g e n e t i c c o e f f i c i e n t s o f c o r r e l a t i o n
1T 2F 3 F--T 4K 5 Th 6 Carcass grade 7W 8 W/Th 9 G/F 1 0 Leg s c o r e 11 G 12 U
F
F--T
K
Th
Carcass grade
W
W/Th G/F
Leg
G
U
A
B
0.02
0.08 0.02
0.08 0.06 0.07
0,09 0.08 0.09 0.10
0.12 0.11 0.12 0.14 0.13
0.13 0.13 0.15 0.14 0.13 0.17
0.12 0.12 0.13 0.13 0.14 0.15 0.03
0.12 0.13 0.15 0.12 0.12 0.16 0.11 0.09 0.05
0.11 0.11 0.12 0.12 0.12 0.14 0.14 0.12 0.10 0.12
0.11 0.10 0.12 0.12 0.08 0.15 0.14 0.13 0.14 0.13 0.12
0.12 0.12 0.13 0.13 0.12 0.16 0.15 0.14 0.14 0.13 0.13 0.14
0.13 D,14 0.15 0.15 0.14 0.17 0.16 0.14 0.14 0.13 0.14 0.15
0.14 0.15 0.16 0.14 0.13 0.17 0.14 0.11
13 A 14B 15C 16 J 17 C+J 18 Y 19 X 20 B/A 21AXB 22 Caulfat 23 24 25 26 27 28 29
MW ML MC MC/MW MC/ML Carcass wt.* Live w t . *
* C o r r e c t e d for age o f l a m b s .
descriptive of skeletal size, are generally highly positively intercorrelated. Width of chest and traits descriptive of shape, compactness and fullness of flesh are also generally highly intercorrelated. However, a rather strong negative relationship exists between the traits of these two groups of external measures. Carcass grade exhibits the same correlation pattern as traits descriptive of skeletal size in its relationship with traits indicative of compactness and fullness of flesh. This implies that large framed carcasses grade lower than small framed ones, as positive correlation between carcass grade and any trait implies lower grading because of the descending order of merit of the grading system. The external measures of skeletal size have low positive correlations with the width of 1. dorsi, but generally high negative correlations with internal measures of thickness, whether of muscle, fat or combination of both fat and muscle (X). External measures of width, shape and compactness show the reversed relationship with the internal traits. The consistency of the magnitude of the genetic correlations suggests that one trait in each group of the external measures may be sufficient to represent the whole group.
0.15
501
C
J
C+J
Y
X
B/A
A X B
Caul-
MW
ML
MC
fat
MC/ MW
MC/ ML
Carcass wt.
Live wt.
0.14 0.14 0.15 0.15 0.14 0.18 0.16 0.14 0.16 0.14 0.15 0.15
0.15 0.16 0.18 0.16 0.16 0.20 0.16 0.13 0.14 0.12 0.15 0.16
0.14 0.14 0.15 0.14 0.16 0.15 0.15 0.15 0.14 0.14 0.19 0.18 0.16 0.16 0.12 0.13 0.14 0.15 0.12 0.13 0.14 0.14 0.15 0.15
0.21 0.24 0.28 0.25 0.22 0.29 0.20 0.18 0.20 0.18 0.21 0.21
0.14 0.13 0.15 0.14 0.14 0.17 0.15 0.13 0.13 0.12 0.14 0.15
0.13 0.13 0.14 0.14 0.14 0.17 0.16 0.14 0.14 0.09 0.14 0.14
0.25 0.22 0.22 0.23 0.21 0.27 0.23 0.21 0.22 0.21 0.21 0.22
0.06 0.06 0.10 0.11 0.09 0.13 0.15 0.13 0.15 0.14 0.12 0.12
0.03 0.03 0.07 0.08 0.09 0.12 0.13 0.12 0.13 0.12 0.11 0.10
0.13 0.13 0.14 0.13 0.13 0.16 0.15 0.14 0.15 0.14 0.13 0.14
0.27 0.26 0.25 0.22 0.23 0.25 0.19 0.15 0.17 0.15 0.18 0.22
0.10 0.10 0.12 0.11 0.11 0.14 0.12 0.II 0.10 0.08 0.11 0.12
0.16 0.19 0.23 0.14 0.19 0.39 0.21 0.22 0.24 0.24 0.12 0.17
0.15 0.16 0.19 0.16 0.15 0.26 0.19 0.18 0.20 0.18 0.12 0.18
0.16 0.17
0.18 0.18 0.13
0.16 0.17 0.05 0.03
0.22 0.23 0.21 0.17 0.17 0.21
0.18 0.06 0.16 0.16 0.16 0.16 0.21
0.09 0.05 0.17 0.17 0.17 0.17 0.23 0.14
0.25 0.25 0.24 0.24 0.23 0.24 0.33 0.25 0.24
0.13 0.15 0.16 0.18 0.17 0.15 0.25 0.15 0.14 0.25
0.12 0.13 0.14 0.15 0.14 0.14 0.20 0.13 0.12 0.22
0.14 0.16 0.16 0.17 0.16 0.16 0.22 0.15 0.15 0.24
0.20 0.21 0.20 0.19 0.18 0.20 0.29 0.20 0.21 0.31
0.12 0.12 0.14 0.14 0.14 0.14 0.19 0.12 0.12 0.20
0.19 0.28 0.24 0.34 0.30 0.26 0.33 0.25 0.23 0.43
0.19 0.22 0.20 0.24 0.25 0.22 0.29 0.20 0.23 0.36
0.07
0.10 0.13
0.35 0.26 0.20
0.12 0.11 0.06 0.12
0.14 0.17 0.23 0.43 0.20
0.15 0.16 0.18 0.31 0.17 0.16
0.16 0.17 0.12 0.11 0.09
The fat measurements C, J, Y are highly positively intercorrelated, measurements J and Y being more highly correlated than either of these with C. An interesting feature is h o w the fat measurements differ in their relationship with depth of I. dorsi. Measurement C, and to a lesser extent Y, both purely subcutaneous fat measurements, are negatively associated with the depth of I. dorsi, B, while the reverse is true for measurement J, the thickest layer of fat on the side. The width and the area of I. dorsi, A and A × B, are independent of, or negatively correlated with all the fat measurements, including caulfat. The relationships of the cannon bone traits with other traits studied are of special interest, as the cannon bone length, i.e., short cannon, has been used as a selection criterion for meat quality improvement in Iceland sheep. Cannon bone length and weight behave much in the same manner as the external carcass measurements of length with respect to their association with other carcass measurements, external and internal. However, there are marked differences between these two traits regarding their degree of association with internal measurements of muscle and fat thickness. Cannon bone
502 weight is far more strongly negatively related to fat thickness than is cannon bone length. On the other hand, cannon bone weight is only slightly negatively related to the depth of I. dorsi, the ratio B : A and the area of l. dorsi, while cannon bone length is rather strongly negatively related to these traits. The circumference of the cannon bone is virtually uncorrelated with any external carcass traits, while it is weakly negatively related to fat thickness and slightly positively with muscle thickness. This agrees with the findings of Good et al. (1961) in cattle. They reported that, at equal live weight, cattle having a larger cannon bone circumference have larger I. dorsi area and less fat at the 12th rib. The ratio MC :MW shows a remarkably high genetic relationship with most of the external and internal measurements. This ratio alone appears to be an excellent indicator of carcass dimensions and fatness and, to a lesser extent, muscle thickness. This is in agreement with the theory put forward by P~lsson (1939, 1940) that a relatively light cannon bone with refined ends and a thick shaft is associated with early maturity, i.e., high degree of fatness and a high ratio B :A of the I. dorsi measurements. An interesting feature of the ratio MC:ML is its rather high positive genetic correlation with the measures of muscle thickness without any appreciable association with fat measures. Considering the high heritability (0.73) of this trait and its favourable relationships with fat and muscle, it appears to be an excellent criterion in selection for carcass quality. Carcass weight corrected for age of lambs is highly related to all measurements of carcass dimensions and bone weight, moderately with width of I. dorsi, and generally only slightly related to depth of I. dorsi and all fat measurements, and still less with all derived traits (ratios and differences) of the external measurements and with leg score. It is negatively, only slightly, associated with the shape index B : A and the ratio MC:ML, whereas it exhibits strong negative association with the ratio MC:MW. Live weight generally shows the same pattern as does carcass weight in its relationship with the carcass traits. ACKNOWLEDGEMENTS The authors thank Dr. H. P~lsson and Dr. S. Adalsteinsson for their advice, criticism and stimulating discussions and for their permission to use data collected by them from 1958--1963, and M.R. Thorgeirsson for his willing assistance at all times.
REFERENCES Adalsteinsson, S. and JSnmundsson, J.V., 1978. Pelt classification and live weight in Icelandic lambs. III. Direct and pedigree selection for weaning weight. J. Agric. Res. (Iceland), 10(1): 90--99.
503 Arnason, Th. and Thorsteinsson, S.S., 1982. Genetic studies on carcass traits in Iceland twin ram lambs. II. Analysis of principal components and construction of selection indices. Livest. Prod. Sci., 8: 507--517. Bichard, M. and Yalcin, B.C., 1964. Crossbred sheep production. II. Selection for growth rate and carcass attributes in the second cross lamb. Anita. Prod., 6: 179--187. Botkin, M.P., Field, R.A., Riley, M.L., Nolan, J.C., Jr. and Roehrkasse, G.P., 1969. Heritability of carcass traits in lambs. J. Anita. Sci., 29: 251--255. Bowman, J.C. and Broadbent, J.S., 1966. Genetic parameters of growth between birth and 16 weeks in Down cross sheep. Anita. Prod., 8: 129--135. Bowman, J.C. and Hendy, C.R.C., 1972. A study of retail requirements and genetic parameters of carcass quality in Polled Dorset Horn sheep. Anim. Prod., 14: 189--198. Bowman, J.C., Marshall, J.E. and Broadbent, J.S., 1968. Genetic parameters of carcass quality in Down cross sheep. Anita. Prod., 10: 183--191. Bradford, G.E. and Spurlock, G.M., 1972. Selection for meat production in sheep. Results of a progeny test. J. Anita. Sci., 34: 737--745. Carpenter, Z.L., 1963. Lamb sire progeny testing progress, Proc. 16th Ann. Reciprocal Meat Conf., 16: 9. Eikje, E.D., 1974. Studies on sheep production records. IV. Genetic, phenotypic and environmental parameters for weight o f lambs. Acta Agric. Scand., 24: 291--298. Falconer, D.S., 1963. Quantitative inheritance. In: Methodology in Mammalian Genetics, Holden Day, San Francisco, pp. 132--216. Field, R.A., Riley, M.L. and Botkin, M.P., 1967. Effect of sex and ram weight on composition of lambs. J. Anita. Sci., 2 6 : 8 9 4 (Abstr.). Good, D.C., Dahl, G.M., Wearden, S. and Weseli, D.J., 1961. Relationship among live and carcass characteristics of selected slaughter steers. J. Anita. Sci., 20: 698--701. Hallgffmsson, S., 1971. Heritability estimates of weaning weight and carcass weight in Iceland lamb. Mimeograph, Agric. Soc. Iceland, 3 pp. Harvey, W.R., 1972. Mixed model least squares and maximum likelihood computer program. Ohio State University, Columbus, OH. Hilman, M., Wheat, J.D., Menzies, C., Mackintosh, D.L. and Merkel, R.A., 1962. Heritability of and correlations between carcass and live animal traits in sheep. J. Anim. Sci., 2 1 : 9 7 3 (Abstr.). JSnmundsson, J.V., 1975. Factors affecting weaning weight and number of lambs. Licenciat Thesis, Agric. University, Norway. Munson, A.W., 1967. Association of various measurements with lamb carcass composition and preliminary estimates of some genetic parameters. Diss. Abstr. B. 27 : 4194-B. P~sson, H., 1939. Meat quality in the sheep with special reference to Scottish breeds and crosses. I. J. Agric. Sci., 29: 554--626. P~Isson, H., 1940. Meat quality in the sheep with special reference to Scottish breeds and crosses. II. J. Agric. Sci., 30: 1--64. P~Isson, H. and Verg6s, J.B., 1952. Effects of the plane of nutrition on growth and the development of carca~ quality in lambs. J. Agric. Sci., 42: 1--149. Purser, A.F., 1960. The use of correction for regression on a second character to increase the efficiency of selection. In: Biometrical Genetics. Proc. o f an Int. Syrup., Ottawa, 1958. Pergamon, Oxford, 1960, pp. 210--214. Smith, R.H., Kemp, J.D., Moody, W.G. and Cundiff, L.V., 1968. Heritability estimates o f some lamb carcass traits. Prog. Rep. Ky. Agric. Exp. Stn., No. 176: 24--25. Timon, V.M., 1965. Estimates of genetic and environmental factors affecting growth and carcass traits in lambs. European Assoc. for Animal Production, Commission on Animal Genetics, Noordwijk, June 23, 1965. Vogt, D.W., Carter, R.C. and McClure, W.H., 1967. Genetic and phenotypic parameter estimates involving economically important traits in sheep. J. Anim. Sci., 26: 1232-1238.
504 RESUME 'Fnorsteinsson, S.S. et Bj6rnsson, H., 1982. Etudes g~ndtiquas des caract~ristiques des carcasses chez les agneaux m~les jumeaux d'Islande. I. Estimation des param~tres g~n~tiques agissant sur les caract~ristiques des carcasses, le poids au sevrage et le poids de carcasse. Livest. Prod. Sci., 8 : 4 8 9 - - 5 0 5 (en anglais). On a utilis~ les r~sultats de 1826 agneaux m~es entiers n~s jumeaux, issus de 216 bdliers test~s entre 1958 et 1977. En plus du poids vif et du poids de carcasse, on a analys~ 27 caract~ristiques comprenant des mesures sur la carcasse, des notes et diverses appreciations d6crivant diffdrents caractdres de la conformation de la carcasse. Les donn~es ont dt~ corrig~es par deux m~thodes, ~ savoir la rdgression des mesures par rapport au poids de la carcasse et par rapport ~ l'~ge des agneaux. La correction par rapport au poids de la carcasse permet un meilleur ajustement que celle par rapport d l'~ge quand on 6tudie les caract~res de quatit~ de la carcasse. L'h6ritabilit~ obtenue est ~lev~e pour les caract~res externes de la carcasse, de 0,60 ~ 0,80 pour les mensurations de longueur et de profondeur, mais g6n~ralement plus faible, de 0,40 0,60, pour les mesures de largeur, les notes pour le rebondi de la cuisse et l'expression ddriv~e de ces mesures de la carcasse. L'h~ritabilit~ a 6t~ de 0,12 ~ 0,44 pour les mesures de la section de la 12~me cSte, le poids du gras interne et la classe de qualitd de la carcasse et de 0,18 et 0,11, respectivement, pour le poids vif et le poids de la carcssse, corrig~s pour l'~ge des agneaux. On discute les effets possibles du protocole du programme de testage sur la validit~ des estimations de 1~6ritabilit~. On discute les relations g~n~tiques et ph6notypiques entre les 27 caract~ristiques de la carcasse, ainsi que les relations entre ces caract~res et le poids vif et le poids de carcasse corrig~s pour l'~ge des agneaux. On constate une opposition g~n4tique entre les mesures de la longueur du squelette et les mesures de la charnure. Le rapport circonf~rence m i n i m u m : longueur du m4tacarpe appara~ comme le crit~re unique le plus utile dans la s~lection pour la qualit~ des carcasses. KURZFASSUNG
Thorsteinsson, S.S. und Bj~irnsson,H., 1982. Genetische Untersuchungen fiber Schlachtk~irpermerkmale bei Isl~/ndischenZwillingsbockl~/mmern. I. Sch~'tzwerte genetischer Parameter fiirSchlachtk6rpermerkmale, Lebendgewicht beim Absetzen und SchlachtkGrpergewicht. Livest. Prod. Sci., 8:489--505 (auf englisch). F~ir diese Analyse wurden Daten ~iber 1826 unkastrierte Zwillingsbockl//mmer herangezogen, Nachkommen von 216 B6cken, die in der Zeit 1958--1977 gepriift wurden. Zus//tzlich zum Lebend- und Sehlachtk6rpergewieht wurden 27 Merkmale untersucht, die direkte Messungen des Schlachtk~rpers, Bonitierungen und verschiedene abgeleitete Begriffe zur Beschreibung verschiedener Merkmale der Schlachtk~rperausformung einbezogen. Das Datenmaterial wurde nach 2 Methoden berechnet, dutch die Regression der Messwerte auf Sehlachtk~irpergewicht und dutch die Regression der Messwerte auf das Alter der L~/mmer. Es wird gefolgert, dass bei der Untersuchung yon Schlaehtk~irperqualit~/tsmerkmalen eine Korrektur auf Schlachtk~rpergewicht besser ist als eine Korrektur auf das Alter. Fiir die ~'usseren Schlachtk~irpermerkmale ergaben sich hohe Heritabilit//tssch~tzwerte yon 0,60 bis 0,80 fiir lineare Messungen der L~'nge und Tiefe abet generell niedrigere Werte yon 0,40 bis 0,60 f/Jr Messungen der Breite, fiir Bonltierungen in Bezug auf die Auspriigung der Keule und fiir den abgeleiteten Ausdruek der Schlachtk/irpermasse. Fiir die Querschnittsmessungen an der 12. Rippe, das Gewicht des Innenfettes und die Schlachtk/~rperqualit//t ergaben sich Heritabilit~/tssch~/tzwerte von 0,12 bis 0,44; Werte yon
505 0,18 und 0,11 wurden fiir das auf das Alter der L~/mmer korrigierte Lebend-, bzw. SchlachtkSrpergewicht ermittelt. Es werden die mSglichen Auswirkungen des Programmplanes ffir die Nachkommenpr(ifung auf die Sicherheit der Heritabilit~/tssch~/tzwerte diskutiert. Die genetischen uncl ph//notypischen Beziehungen zwischen 27 SchlachtkSrpermerkmalen werden ebenso diskutiert wle die Beziehungen zwischen diesen Merkmalen und dem auf das Alter der L~fmmer korrigierten Lebend- und Schlachtk~rpergewicht. Es wurde festgestellt, dass ein genetischer Antagonismus besteht zwischen der L~'nge yon Skelettmassen und Merkmalen, die die Fleischf/ille beschreiben. Es wird darauf hingewiesen, dass das Verh/fltnis yon Mindestumfang zu L~/nge der Mittelhand das einzige sehr brauchbare Kriterium bei der Selektion auf Schlachtk~rperqualit~/t ist.