Genetic correlations among sex-limited traits in beef cattle

M. D. MacNeil, L. V. Cundiff, C. A. Dinkel and R. M. KochGenetics Correlations among Sex-Limited Traits in Beef Cattle

1984, 58:1171-1180.J ANIM SCI

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G E N E T I C C O R R E L A T I O N S A M O N G S E X - L I M I T E D T R A I T S IN BEEF C A T T L E 1

M. D. MacNeil 2'3, L. V. Cundiff 4 , C. A. Dinkel 3 and R. M. Koch 2

US Department of Agriculture, Clay Center, NE 68933 ; South Dakota State University, Brookings 57007

and University of Nebraska-Lincoln, Clay Center 68933

Summary

Data from a comprehensive germ plasm evaluation program were used to estimate genetic correlations of reproductive and maternal traits of beef females with growth and carcass traits of their steer paternal half-sibs. The data set consisted of 187 sires with approximately four female and five male progeny each. Heritability estimates for age at puberty, weight at puberty, conceptions/service, gestation length, calving difficulty, progeny birth weight, progeny preweaning daily gain and maturq weight measured on females were .613 + .177, .700 -+ .114, .026 -+ .126, .298 -+ .175, .217 -+ .175, .374 -+ .174, .094 -+ .161 and .540 +.150, respectively. Postweaning daily gain, carcass weight, fat trim weight and retail product weight measured on male half-sibs had estimated heritabilities of .363 -+ .090, .441 -+ .093, .502 -+ .093 and .451 -+ .093, respectively. The estimated genetic correlations suggest that selection for postweaning daily gain would result in increased age and weight at puberty, increased mature weight, improved fertility, reduced maternal gestation length, reduced maternal

1 Published in cooperation with and a contribution from Regional Project NC-1, Improvement of Beef Cattle Through Breeding Methods. Published with the approval of the Director of the South Dakota Agr. Exp. Sta. as Publication No. 1936 of the Journal Series and as Paper No. 7238 Journal Series, Nebraska Agr. Exp. Sta., Lincoln. Appreciation is expressed to Jeannie Pittam and Kathy Leising for secretarial assistance.

2Univ. of Nebraska, Roman L. Hruska U.S. Meat Animal Research Center, Clay Center, NE 68933.

3Dept. of Anita. Sci., P.O. Box 2170, South Dakota State Univ., Brookings 57007.

4Roman L. Hruska U.S. Meat Animal Research Center, Agr. Res. Service, Clay Center, NE 68933.

Received July 22, 1983. Accepted September 27, 1983.

calving difficulty, increased maternal birth weight and reduced maternal preweaning gain. Predicted correlated responses to selection for reduced fat trim at a constant age were increased age and weight at puberty, increased mature weight, reduced maternal fertility, reduced maternal preweaning gain and increased maternal gestation length, birth weight and calving difficulty. Consequences of selection for increased age constant retail product weight or carcass weight appear to be increased age and weight at puberty, increased mature weight, improved fertility, increased maternal gestation length and maternal birth weight but reduced maternal difficulty and reduced maternal preweaning gain. (Key Words: Genetic Correlation, Heritability, Sex-Limited Traits, Beef Cattle.)

I ntroduction

Knowledge of genetic correlations is needed for multiple trait evaluation of individuals (Hazel, 1943; Henderson and Quass, 1976)and prediction of correlated responses to selection (Falconer, 1960). Correlations among components of merit modify the optimum weighting of components and the overall effectiveness of selection for merit (Dickerson, 1976). The importance of these correlations in under- standing the effects of selection has been well documented (Bell, 1974; Roberts, 1979).

For many pairs of economically important growth and carcass characteristics, estimates of the genetic correlations have been reported (reviews by Preston and Willis, 1974; Wolde- hawariat et al., 1977). However, there is a paucity of estimates for which one trait must be measured on females and the other on males. This lack of information hinders the effective choice of selection objectives (Dickerson, 1976; Niebel and Van Vleck, 1982). The objective of this study was to estimate from experimental

1171

JOURNAL OF ANIMAL SCIENCE, Vol. 58, No. 5, 1984



1172 MacNEIL ET AL.

data the genetic correlations between reproductive and maternal traits of beef females and growth and carcass traits of paternal half-sib steers.

Materials and Methods

This study includes data on calves born at the Roman L. Hruska U.S. Meat Animal Re- search Center during 1970, 1971 and 1972. Straightbred Hereford and Angus cows were mated to either Hereford, Angus, Jersey, South Devon, Limousin, Charolais or Simmental sires to calve in the spring of each year. The sires of each breed were assumed to be random samples of the respective breeds. Calves were born in the spring from late February to early May. Bull calves were castrated within 24 h of birth, and all calves had access to creep feed from mid-July until weaning in late October at approximately 215 d of age. Smith et al. (1976c) have characterized the preweaning performance of these breed types and reported in more detail their preweaning management.

After weaning, heifers were fed ad libitum a diet of approximately 50% corn silage (IFN 3-28-250) and 50% grass haylage (IFN 3-00-964) with supplemental protein and minerals to meet NRC (1963) requirements. Heifers were observed for estrous activity twice daily from approximately 250 to 510 d of age, except in 1971 then estrous detection ceased at about 480 d of age. A breeding season of approximately 65 d durat ion was commenced when the average age of all heifers was 430 d. During the first two-thirds of the breeding season, all heifers were artificially inseminated with semen from either Hereford, Angus, Devon, Holstein or Brahman bulls. In the latter one-third of the breeding season, natural service Hereford and Angus sires were used. Addit ional details regarding the management of these heifers were reported previously (Laster et al., 1976; Notter et al., 1978).

Age at puber ty was defined as the age of first observed standing estrus. Weight at puberty was calculated as the sum of weaning weight and postweaning average daily gain times the number of days from weaning to observed estrus. Some individuals were not observed to be in estrus and were excluded from the age and weight at puber ty data sets. Because the number of individuals excluded was small (< 7%) the impact on the estimates of heritabili ty and genetic correlation should be trivial.

For those heifers mated by artificial insemi- nation, the exact date of service was known, as was calving date. If the interval between service and calving was 285 + 10 d, it was assumed the heifer conceived to that service. Fert i l i ty was expressed as the ratio of numbers of conceptions (either 0 or 1) to number of services. Concep- tions/service has the advantage relative to services/conception because data from all individuals are used. Also relative to conception to first service, all available information on each individual is used. The interval from each recorded breeding date to calving was calculated, and the length of the interval nearest 285 d was taken as gestation length. At calving, the difficulty of birth was classified as either not difficult (unobserved, no difficulty or minor manual assistance) or difficult (assistance with calf puller or surgery). Calving difficulty scores for heifers with abnormally presented calves were not used in this study. All calves were weighed within 12 h of birth and again at weaning. Daily gain was calculated as weaning weight minus birth weight divided by weaning age. Only records from calves born when heifers were 2 yr old were used in this study.

Cows were maintained continuously on improved pastures and fed supplemental silage and(or) hay as conditions warranted. All females were weighed four times annually: at brand clipping (approximately 60 d before calving), at the beginning and end of breeding (60 to 90 d and 100 to 140 d after calving, respectively) and at weaning (approximately 200 d after calving). Mature weight was calculated as the average of these four weights taken when each female was 7 yr old.

The postweaning management of steer calves was different than that of their half-sib heifers. After a 25- to 30-d adaptat ion period, steers were implanted with 36 mg diethylsti lbestrol and assigned to feedlot pens replicated by sire breed groups. Steers were fed ad libitum in fenceline bunks for each pen. The diet was changed periodically as the steers matured such that the energy density of the diet increased from approximately 2.61 Meal of metabolizable energy after weaning to about 2.86 Meal of metabolizable energy before slaughter. Details of the feeding regimen have been presented by Smith et al. (1976b). Steers were stratified by age and breed group and within strata were assigned randomly to one of three slaughter groups at approximately monthly intervals. The initial slaughter group was killed after 190 d on



GENETIC CORRELATIONS AMONG SEX-LIMITED TRAITS 1173

feed in 1971, 169 d in 1972 and 194 d in 1973. Before s laughter , s teers were fas ted 12 h, t r a n s p o r t e d to a c o m m e r c i a l s laughter p lan t and s laughtered t he n e x t morn ing . T he r ight side of each carcass was t r a n s p o r t e d to Kansas S ta t e Univers i ty . Carcass sides were cut in to wholesa le cuts w i th n o t m o r e t h a n 8 m m ex te rna l fat . T h e lean t r im f rom the wholesa le cuts was t h e n t r i m m e d to con t a in a p p r o x i m a t e l y 25% fat and retai l cuts were f ab r i ca ted . Detai ls o f the cu t t ing p rocedures have been p resen ted previously (Koch et al., 1976) .

Pos twean ing dai ly gain was ca lcu la ted for each s teer as t he l inear regress ion o f we igh t o n age. On ly those weights (n = 8) t a k e n be fo re the s laughter of t he ini t ial kill g roup each yea r were used. Reta i l p r o d u c t we igh t was t he sum of lean t r im and retai l cu t weights . Fa t t r im was the sum of ex te rna l fa t f r o m t he wholesa le cuts, add i t iona l fa t t r i m m e d f r o m wholesa le cuts to achieve 25% fat in the lean t r im and t he k idney , hear t and pelvic fa t ( k i d n e y inc luded) .

Separa te analyses of var iance were c o n d u c t e d for each hei fer and s teer sire breed . F o r each sire b reed and t ra i t , an ini t ial mode l t h a t inc luded all ma in ef fec ts t h o u g h poss ibly to af fec t t h a t t ra i t and t he two- fac to r i n t e r ac t i ons of ma in ef fec ts were f i t t ed ( tab le 1) a f te r t he sire effects had b e e n abso rbed . Covar ia tes for wean ing age and days o n feed were inc luded in the mode l s for carcass weight , retai l p r o d u c t weight and fa t t r im we igh t fo r a d j u s t m e n t to a c o n s t a n t s laughter age. On ly t he covar ia te

wean ing age was inc luded in the m o d e l for pos twean ing dai ly gain. T w o - f a c t o r inter- ac t ion t e r m s were de le ted one at a t i m e (least s igni f icant f i rs t) if t h e y d id n o t a p p r o a c h s ignif icance (P> .20 ) . Main ef fec ts were l ikewise de le ted if t h e y were no t a t e r m in any two- f ac to r i n t e r a c t i o n t h a t r ema ined in t h e m o d e l and did n o t a p p r o a c h signif icance. Occas iona l ly a source of va r ia t ion was o m i t t e d f r o m the m o d e l for a pa r t i cu la r t ra i t even if i t a p p r o a c h e d s ignif icance in one sire b r eed b u t was un im- p o r t a n t in t he o thers . In the analysis o f Here fo rd and Angus he i fer and s teer sire b reeds any measu reab l e i n t e r ac t i on of b reed o f sire w i th b reed of d a m was c o m p l e t e l y c o n f o u n d e d w i th the b reed of d a m source of var ia t ion .

The f inal mode l s for each b reed of sire and t ra i t are ind ica ted in t ab le 2. Sire e f fec ts were f i t t ed in t he f inal mode l s r a t h e r t h a n a b s o r b e d and a leas t -squares m e a n was e s t ima ted fo r each sire. The b e t w e e n and wi th in sire c o m p o n e n t s of var iance (o~ and O~v, respec t ive ly) were also e s t ima ted in t he f inal mode l s w i th t he d i rec t p r o c e d u r e o f Harvey (1970) .

Her i t ab i l i t y was c o m p u t e d as:

4o~/(O2s + O2w).

The he r i t ab i l i t y of calving d i f f icu l ty was ad jus ted to the n o r m a l basis w i th t he c o r r e c t i o n der ived b y R o b e r t s o n and Lerne r (1949) . S u b s e q u e n t e x a m i n a t i o n s b y Van Vleck ( 1 9 7 2 ) and Olausson and RiSnningen ( 1 9 7 5 ) d e m o n -

TABLE 1. FIXED MAIN EFFECTS AND COVARIATES INCLUDED IN INITIAL ANALYSES OF VARIANCE MODELS a

Female trait b Male trait c

Source of variation AP WP CS GL CD BW MG MW DG CW FT RP

Year x x x x x x x x x x x x Breed of dam x x x x x x x x x x x x Age of dam x x x x x x x x x x x x Breed of service sire x x x x x Calf sex x x x x b 1 (initial age) x x x x b 2 (days fed) x x x

aAU possible ~wo-factor interactions among main effects included in the model for each trait were also included in the initial models. Sires were absorbed.

bAp = Age at puberty (d); WP = Weight at puberty (kg); CS = Conceptions/service; GL = Gestation length (d); CD = Calving difficulty (0 = unassisted, i = assisted); BW = Birth weight (kg); MG = Preweaning daily gain of progeny (kg); MW = Average of four weights taken at 7 yr of age (kg).

CDG = Postweaning daily gain (kg); CW = Carcass weight (kg); FT = Fat trim from one-half carcass (kg); RP = Retail product from one-half carcass (kg).



1174 M a c N E I L E T A L .

T A B L E 2 . R E D U C E D A N A L Y S I S O F V A R I A N C E M O D E L S

S o u r c e o f v a r i a t i o n A P W P

F e m a l e t r a i t a M a l e t r a i t b

CS G L C D BW M G M W D G CW F T R P

Sire x x Year (Y) x x Breed of dam (B) x x Age of dam (A) x x Breed of service

sire (S) Sex (X) Y * A x

Y * B

Y * S

B * A

B * S

S * X

b 1 (initial age) b~ (days fed)

X X X X X X IK X X X

X X X X X X X X X X

X X X X X X X X

X X X X X X

X X X X X

X X

X X X g X

X X X

X X X

aAp = Age at puberty (d); WP = Weight at puberty (kg); CS = Conceptions/service; GL = Gestation length (d); CD = Calving difficulty (0 = unassisted, 1 = assisted); BW = Birth weight (kg); MG = Preweaning daily gain of progeny (kg); MW = Average of four weights taken at 7 yr of age (kg).

bDG = Postweaning daily gain (kg); CW = Carcass weight (kg); FT = Fat trim from one-half carcass (kg); RP = Retail product from one-half carcass (kg).

strated the utility of this procedure, particularly when the incidence level is intermediate.

The correlation of sire progeny means (r m) was estimated separately for each breed of sire, and these correlations were pooled by the Z-transformation procedure originally described by Fisher (1921). The correlations of sire progeny means of male traits with calving difficulty were subjected to adjustment to a multivariate normal basis using the multi- plicative correction developed by Olausson and R6nningen (1975). An adaptation of the method of path coefficients (Wright, 1934) was used in this study to estimate genetic correlations. Essentially, this strategy was employed by Calo et al. (1973) to estimate the genetic correlations between growth traits of Holstein- Friesian bulls and milk production (kg) of half-sib cows. Similar applications of path coefficients have been made in the study of genotype x environment interactions (Dickerson 1962; Yamada, 1962). Genetic correlations (rg) among male and female traits were estimated a s :

/ .25h~k x 25h~ky rg = r m / /

~/ 1 + (kx-1) (h~/4) 1+ (ky-1)(h~/4) ,

where h2x and h~ are estimated heritabilities for traits x and y, respectively, and k x and ky are effective numbers of progeny/sire for traits x and y.

The expression CRy = hyrgoy/h x, derived from the formulas for direct and correlated response to mass selection (Falconer, 1960), was used to assess the magnitude and direction of predicted correlated response in trait y (CRy) per standard deviation of direct response in trait x. Symbols used in the previous equa- tion are defined as follows:

h x and hy = square roots of heritabilities for traits x and y;

rg = genetic correlation of traits x and y and

Oy = phenotypic standard deviation of trait y.

Results and Discussion

Heritabilities. The average values of heritabilities and effective numbers of progeny for the traits studied are presented in table 3. The data for this study were generated from an experiment whose primary objective was breed evaluation. Therefore, more sires with fewer progeny each were sampled than would be



GENETIC CORRELATIO1NS/~MONG SEX-LIMITED TRA,ITS 1175

TABLE 3. AVERAGE HERITABILITIES (h=), EFFECTIVE NUMBERS OF PROGENY PER SIRE (k), TOTAL SIRE AND RESIDUAL DEGREE;S OF ~FtREEDOM (df s AND df, RE[gPECTIVELY)

AND WITHIN SIRE COM~PONE~ff~I'S OF VARIANCE (a ~ )

Traita df s k df o 2 h 2

AP 187 4.05 613 1710. .613 • WP 187 4.19 6,47 686.0 .700 • CS 187 3.78 57'1 .1345 .026 • .126 GL 184 2.86 37t5 35.28 .298 • CD 184 2.88 377 .1761 .217 • BW 184 2.87 377 14.38 .374 • .174 MG 181 3.11 425 .0398 .094 • .161 MW 180 3.30 452 1901. .540 • .150

DG 187 $,38 903 .0144 .363 • .090 CW 187 5.26 875 542.3 .441 • .093 FT 187 5,26 875 26.68 .502 • .093 RP 187 5.21 868 40.70 .451 •

i _ , i , r " ' �9 . . . . - - . . . .

aFemale traits: AP = Age at puberty (d); WP = Weight at puberty (l~.g); CS = Conceptions/service; GL = Gestation length (d); CD -- Calving difficulty (0 = unassisted, 1 = assisted)~ ~BW ~- Birth weight (kg); MG = Pre- weaning daily gain of progeny (kg); MW = Average of four waig'hts taken g 7 yr of age (ks). Male traits: DG = Postweaning daily gain (ks); CW = Careus weight (kg); FT -- Fat trim from one-half carcass (kg); RP = Retail product from one-half carcass (kg).

opt imum for estimation of heritabili t iy and genetic correlation. As a result, sampling variances of est imated genetic statistics are large relative to estimates from a populat ion with the same tota l number of individuals in fewer families with more members per family.

Heritabil i ty estimates for age and weight ~ first observed estrus (puberty) estimated from these data (.61 and .70, respectively) arc somewhat higher than estimates previously reported (average, ,31 and .64, respectively; Arije and Wiltbank, 1971; Smith et al., 1976a; Laster et al., 1979), A port ion of this difference might be explained by the reduction of pre- l iminary models with a resultant decrease in the residual component of variance. Other authors have tended to utilize unreduced models in the estimation of heritabili ty.

Fert i l i ty, whether measured as calving rate, conception/service or services/conception has generally been found to be lowly heritable. Lindley et al. (1958) and Milagres et al. (1979) reported the heritabil i ty of services/conception to be .00 znd .64, respectively. Dearborn et al. (1973) examined reproduction and fitness traits, and reported heritabili ty estimates for conception on first service (.22), concep- t ions/estrous cycle exposed (.27) and pregnancy rate at the end of the breeding season (.09). In the present study, heritabili ty for conceptions/ service was estimated to be .03.

Gestation length and calf birth weight have

been imp'licated in the incidence of dystocia and result~ant calf morta l i ty (Dickerson et al., 1974; Cure, d i l l et al., 1975; Gravert, 1975; Cundiff et al . , 1982a). Heritabili ty estimates for gestation len~gth, birth weight and incidence of dystocia, con'sidered as traits of the calf, have been repor ted as .55, .33 and .30, respectively (Cundiff et al., 1982b). Lindley et al. (1958) estimated an a, verage heritabil i ty of .08 for gestation length. Cundiff et al. (1975) reported estimates of heri tabi l i ty for incidence of dystocia to be very low (0 and - . 0 7 ) in Hereford- and Angus-sired c~.lves out of 2- and 3-yr-old Hereford and Angut~ cows. Other estimates of hcritabili ty for calving difficulty in first calf h~jfers, also measured as a trait of the calf, average .25 (Menissier, 1975). Estimates of heritabili ty for calf bir th weight as a trait of the call, summarized by Woldehawariat et al. (1977), averaged .4.

In the present study, gestation length, calf birth weight, calving diff icul ty and preweaning daily gain were treated as traits of the dam. Maternal effects (gM), and to a lesser degree, direc~ effects (gI) contr ibute to the expression of these traits as (�89 + gM). Therefore, inter- prel:ation of parameter estimates for these traits should consider not only gI and gM, but also any possible covariance of gI and gM.

In this s tudy, heri tabil i ty estimates were .30, .37 and .22 for gestation length, calf birth weight and calving difficulty, respectively.



1176 MacNEIL ET AL.

Some previous studies (Everett and Magee, 1965; Brown and Galvez, 1969) have found direct and maternal components of birth weight to be at least moderately heritable. In contrast, Burfening et al. (1981) and Bourdon and Brinks (1982) fou, nd calving difficulty, birth weight and gestation length to be lowly heritable when measured as traits of the dam. Philipsson (1976) reported heritability estimates lot: gestation length, birth weight and calving difficulty treated both as traits of the individual and the dam. In that study, heritability esT:i- mates for calving difficulty were essentially equal wether treated as a trait of the calf or of its dam and averaged approximately .10. Heritability estimates for birth weight and gestation length treated as traits of the caTtf or of its dam were .19 vs .06 and .50 vs .13, respectively.

Estimated with these data, the herit'ability for preweaning gain of the calf as a trait, of the dam was .09. Heritability of preweani.ng gain measured as a trait of the dam could be lower than heritability of either direct effects or maternal effects on preweaning gairi, possibly due to a negative covariance between the direct and maternal effects (Koch and Clark, 1955, Deese and Koger, 1967; Hoherlboken and Brinks, 1971; Koch, 1972). Even so, previous studies (Deese and Koger, 1967.; Hohenboken and Brinks, 1971) suggest the total heritability (Willham, 1972) lies in the range .17 to .34,

so,mewhat higher than the estimate reported here. However, in these data, the heritability of p'reweaning gain as a trait of the calf was also found to be low (Koch et al., 1982a).

The amount of feed eaten by a cow is highly correlated with her weight. Therefore, an indicator of cow size may be important in the evaluation of alternative selection objectives. Mature weight has been one commonly used and highly heritable measure of size (h2=.74, Brinks et al., 1962; h2=.55, Brinks et al., 1964; h2=.28, Brown et al., 1972; and h2=.42, Smith et al., 1976a). The estimated heritability of the average of four weights taken at 7 yr of age was .54 in this study.

Heritability estimates for daily gain, carcass weight, retail product weight and trimmed fat weight found in this study (.36, .44, .45 and .50, respectively) were comparable with other literature estimates. In a larger study from which these steers are a subsample, Koch et al. (1982a) also estimated heritability for daily gain, half carcass weight, retail product weight and trimmed fat weight (.57, .43, .58 and .48, respectively). Earlier studies (Cundiff et al., 1971; Dinkel and Busch, 1973; Koch, 1978) also confirm the intermediate to high heritability of these traits. Therefore, selection to increase (or decrease) any of these traits measured on steers should be effective.

Genetic Correlations. Presented in table 4 are estimated genetic correlations of male and

TABLE 4. ES'rIMATED GENETIC CORRELATIONS BETWEEN GROWTH AND COMPOSITION TRAITS MEASURED ON MALES AND

REPRODUCTION AND PRODUCTIVITY TRAITS MEASURED ON FEMALE HALF-SIBS

Male trait a

Female traits b DG CW FT RP

AP .16 .17 --.29 .30 WP .07 .07 --.31 .08 CS 1.33 .61 .21 .28 GL -.10 .03 --.07 .13 CD -.60 --.31 --.36 --.02 BW . 3 4 . . 3 7 - - . 0 7 .30 MG -1.02 --1.00 --1.25 --.26 MW .07 .21 --.09 .25

aMale traits: DG = Postweaning daily gain (kg); CW = Carcass weight (kg); FT = Fat trim from one-half carcass (kg); RP = Retail product from one-haif carcass (kg).

bFemaie traits: AP = Age at puberty (d); WP = Weight at puberty (kg); CS = Conceptions/service; GL = Gestation length (d); CD = Calving difficulty (0 = unassisted, 1 = assisted); BW = Birth weight (kg); MG = Pre- weaning daily gain of progeny (kg); MW = Average of four weights taken at 7 yr of age (kg).




female traits studied. The sampling variances of these estimates are unknown but presumed high.

Postweaning daily gain, carcass weight and retail product weight at a constant age appear to have similar genetic associations with the complex of female reproductive and productivity traits studied. Previous studies have found the genetic correlations among daily gain, carcass weight and retail product weight at the same age to be positive and large (Cundiff et al., 1971; Dinkel and Busch, 1973; Koch et al., 1982b). Therefore, this result was not unexpected.

The weight of t r immed fat from steer carcasses at a constant age appeared t o have genetic associations with puberty and weight traits of females opposite those of daily gain and retail product weight. Dinkel and Busch (1973) reported genetic correlations at constant age and weight of - . 8 1 and - . 8 8 for fat trim weight with postweaning daily gain and retail product weight, respectively. Koch et al. (1982b) estimated the genetic correlation of postweaning daily gain and fat trim weight to be .40 and the correlation of retail product weight and fat tr im weight to be - . 1 1 . Cundiff et al. (1969) estimated the genetic correlation for retail product and fat tr im at constant age to be +.55, hut at constant weight to be - . 9 0 .

The correlations of progeny birth weight and calving difficulty with postweaning daily gain

pose an anomaly. At the phenotypic level, increases in birth weight generally have been associated with increases in calving difficulty (Bellows et al., 1971; Notter et al., 1978; Nelson and Beavers, 1982). Likewise, the genetic correlation between direct effects for birth weight and calving difficluty is generally accepted as being positive (Cundiff et al., 1982b). These data suggest that selection based on postweaning daily gain would tend to increase maternal birth weight and decrease maternal calving difficulty simulatneously (table 5).

There is ample evidence to show that genetic correlation of direct effects on pre- and postweaning gain is positive (Preston and Willis, 1974; Woldehawriat et al., 1977), supposedly due to pleiotropic effects of genes for growth. Progeny preweaning daily gain expressed as a trait of the dam is influenced by both direct and maternal effects, while postweaning daily gain of paternal half-sibs of the dam is indicative only of direct effects. Therefore, the negative genetic correlation of maternal preweaning daily gain with postweaning daily gain observed in this s tudy suggests an antagonism of direct and maternal effects for growth. Willham (1972) suggested a possible negative correlation of direct and maternal effects on preweaning growth. However, these findings appear to be in disagreement with the report of Calo et al. (1973). Among Holstein-Fresian bulls pedigree

TABLE 5. PREDICTED CORRELATED RESPONSES IN FEMALE REPRODUCTION AND PRODUCTIVITY TRAITS PER STANDARD DEVIATION OF DIRECT RESPONSE

IN GROWTH AND COMPOSITION TRAITS

Female trait b Selection criteria a and (standard deviation) (mean) DG (.126) CW (16.6) FT (5.52) RP (6.77)

AP (374) 9.34 9.43 14.4 15.7 WP (284) 2.81 2.63 10.5 2.86 CS (.512) c .061 -.018 .024 GL (238) --.559 .166 .333 .652 CD (.377) --.201 -.102 .101 --.007 BW (31.6) 1.37 1.46 .241 1.08 MG (.641) c c c -.024 MW (537) 3.99 11.5 .438 12.8

aDG = Postweaning daily gain (kg); CW = Carcass weight (kg); FT -- Fat trim from one-half carcass (kg); RP = Retail product from one-half carcass (kg). Selection applied to increase daily gain, carcass weight and retail product weight and decrease fat trim weight.

bAp = Age at puberty (d); WP = Weight at puberty (kg); CS -- Conceptions/service; GL = Gestation length (d); CD = Calving difficulty (0 = unassisted, 1 -- assisted); BW = Birth weight (kg); MG = Pre- weaning daily gain of progeny (kg); MW = Average of four weights taken at 7 yr of age (kg).

CEstimated genetic correlation has absolute value greater than 1.0.



1178 MacNEIL ET AL.

selected for increased milk production, low but positive genetic correlations of estimated merit for milk production and average daily gain were found.

Retail product weight and carcass weight at a constant age, as would be expected from the high genetic correlation between them (Cundiff et al., 1971; Koch et al., 1982b), had similar genetic correlations with each of the female traits measured (table 4). As both retail product weight and carcass weight also had essentially equal heritabilities, the predicted correlated responses (table 5) to selection for either trait are also similar. The predicted correlated responses to selection for either retail product weight or carcass weight are greater in magnitude than those for daily gain selection, due pri- marily to the higher heritabilities of the former traits.

Selection for increased carcass weight or retail product weight of steers at a constant slaughter age should result in heifers older and heavier at puberty and that have improved fertility. The gestation length of heifers would increase slightly as would calf birth weight, although parturition would occur with less difficulty. The inconsistency of a larger calf and less calving difficulty is perhaps explained by the increased size of the heifer herself.

The notion of selection for decreased fat recently has received considerable attention (Van Demark, 1976). While this course of action may result in more desirable carcasses, the genetic correlation found in this study foretell of possible genetic antagonisms. Females from sires selected for reduced fat trim of steer progeny would be expected to reach puberty later and at a heavier weight, have reduced fertility and be larger at 7 yr of age (table 5). These data also suggest a longer first gestation with the resultant calf born heavier and with greater difficulty.

The preceding results document the existence of unfavorable genetic correlations between component traits of female productivity and progeny carcass value. Therefore, the concept of specialized sire and dam lines (Smith, 1964) appears to have some merit in beef production. Alternatively, selection indexes that incorporate both carcass value traits and maternal productivity traits provide logical selection objectives in general purpose populations. Genetic progress in index-selected general purpose populations would be reduced relative to progress that could be made from crossing specialized sire and dam lines (Smith, 1964).

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