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Sire and Maternal Grandsire Effects on Calving Difficulty and Calf Mortality in Israeli Holsteins1
Sire and Maternal Grandsire Effects on Calving Difficulty and Calf Mortality in Israeli Holsteins z M. RON, R. B A R - A N A N , and J. I. W E L L E R Institute of Animal Science Agricultural Research Organization The Volcani Center P.O. Box 6 Bet Dagan 50250, Israel ABSTRACT
Associations among sire and maternal grandsire evaluations for calving difficulty and calf mortality were investigated in the Israeli Holstein population. Primiparous and multiparous calvings were analysed separately. Genetic correlations between calving difficulty and calf mortality were close to unity for primiparous calvings but lower for multiparous calvings. Genetic correlations between primiparous and multiparous sire evaluations were .94 and .72 for calving difficulty and calf mortality. Genetic correlations between sire and maternal grandsire evaluations were 6 for primiparous calvings and not-significant for multiparous calvings for both traits. These results correspond to findings that the direct genetic effect is larger than the maternal effect but that there is a slight negative correlation between them. We suggest that sire evaluations for calving difficulty of multiparous cows be used as a preliminary prediction of heifer calving difficulty and calf mortality and that the separate calving trait evaluations be combined into an overall calving selection index. INTRODUCTION
Progeny testing of sires for calving difficulty (DIFF) and calf mortality (MORT) was started in Israel in 1964. Since 1976, eight independent analyses have been computed thrice yearly. Bulls are evaluated separately for primiparous
and multiparous calvings, both as sires of calves and as maternal grandsires (MGS), for DIFF and MORT. Whereas sire evaluations estimate one-half of the sires' breeding value for these traits, MGS evaluations include one-quarter of the sires' breeding value as grandsire of the calf and one-half of the sires' maternal breeding value as sire of the cow. Correlations between direct and maternal effects have been negative for both DIFF (6, 11, 16) and MORT (7). Thompson and coworkers (16, 18) suggested that the negative association between direct and maternal effects on DIFF in US Holsteins was caused by small calves that are delivered easily and then become small heifers that may have difficulty in giving birth. To minimize the risk of DIFF and MORT in primiparous cows, matings in Israel are planned so that nearly all heifers are bred to bulls with positive evaluations for both production and calving traits as mates of primiparous cows and as sires of heifers (1). Better calving performance benefits farmers beyond the direct economic impact on production and reproduction in the subsequent lactation (8, 10, 12, 14, 17) by reducing the need to assist in births and to dispose of dead calves or dams. Objectives of the present study were to investigate the association between sire and MGS effects on DIFF and MORT in primiparous and multiparous calvings and to suggest improvements of selection and breeding policies for increasing the reproductive efficiency of the national dairy population. MATERIALS AND METHODS
Received July 11, 1984. 1Research sponsored by the US-Israel Binational Agricultural Research and Development Fund, Project No. US-334-81. 1986 J Dairy Sci 69:243-247
Eighty-seven sires in 3 5 5 herds were evaluated by contemporary comparison (13) for DIFF and MORT from 1977 to 1982. Mean heifer calving age was 24 mo. Primiparous and multiparous calvings were analyzed separately for each
243
244
RON ET AL.
trait. Eight separate analyses were used to estimate sire and MGS effects. D I F F and MORT were scored as all-or-none traits. For D I F F : 0 = normal birth and 100 = assisted birth and for MORT: 0 = live birth and 100 = stillbirth or calves dead within 48 h of calving. The analysis model for sire evaluations was:
with sire evaluations for primiparous calvings excluded to evaluate young sires that have not been mated to heifers. Index values were computed only for sires with repeatabilities > .6 for all traits included in the indices. Product m o m e n t and rank correlations were computed among the sires' selection index values for the different indices.
Yijk = Si + HYSj + eij k where Yijk is the D I F F or MORT score of k th calving, progeny o f the ith sire, occurring in the jth herd-year-season; Si is the effect of the i th sire; HYSj is the effect of the jth herd-yearseason, and eij k is the residual effect not explained by the other factors. Three calving seasons were defined: February through May, June through September, and October through January. Sire and residual effects were random, and the HYS effect was fixed. The sire component of variance was assumed to be .0152 and .0125 of the residual variance for primiparous and multiparous calvings for both traits. The same analysis model was used for the MGS analyses except that the sire effect was replaced by the MGS effect and the MGS component of variance was .0125 and .01 for primiparous and multiparous calvings. Repeatabilities were computed (2). Productm o m e n t correlations (r)and genetic correlations (rg) (4) were estimated among evaluations of sires with repeatabilities of >.6. Correlations were also computed for a subset of 14 sires with > .9 repeatability (r14). Selection indices to predict sire effects for D I F F and MORT in heifer calvings as functions of multiparous sire evaluations and primiparous MGS evaluations were constructed by stepwise regression. Sires were evaluated for overall calving performance by four selection policies: 1) all evaluations were weighted equally; 2) MORT and D I F F evaluations were weighted 2:1; 3) sire and MGS evaluations were weighted 2:1; and 4) alternatives 2 and 3 combined. Although the actual economic weights are not k n o w n and are subject to change, MORT is likely to have a higher economic value than DIFF, and more economic weight is likely to be assigned to sire effects that occur earlier and more frequently than MGS effects. Evaluations were computed with all eight traits included in the indices and Journal of Dairy Science Vol. 69, No. 1, 1986
RESULTS A N D DISCUSSION
Frequencies of D I F F and MORT were 9.2 and 8.0% in primiparous calvings and 3.6 and 4.1% in multiparous calvings. The standard deviations (SD) of sire evaluations for D I F F and MORT are in Table 1. The SD of sire evaluations for D I F F in primiparous calvings was three times as large as for multiparous calvings, but coefficients of variation were similar. The SD of sire evaluations were larger than those of MGS evaluations. The SD of sire evaluations in primiparous calvings of the 14 extensively used sires were small, but those of their MGS evaluations were large. These results may be due to the breeding policy followed in Israel. As heifers are generally bred to sires evaluated as favorable for calving traits in multiparous calvings, bulls are selected by their evaluations as sires but not as maternal grandsires. The SD of the MGS evaluations of sires with >.9 repeatability were greater than those of sires with >.6 repeatability because in c o n temporary comparison, evaluations of effects with small variance components and few observations are regressed toward the mean. Correlations among evaluations are in Table 2. The r14 are in all cases nearly equal to rg. Correlations between D I F F and MORT of sire and MGS effects were higher for primiparous than for multiparous calvings. The rg between D I F F and MORT sire effects in heifer calvings was near unity. The rg between evaluations derived from primiparous and muhiparous calvings ranged from .37 for MGS effects on MORT to .94 for sire effects on DIFF. Correlations between sire and MGS effects for D I F F and MORT were in the range of .5 to .7 for primiparous calvings. This is similar to the rg of .52 for MORT in primiparous calvings of Finnish Ayrshires (5). These results agree with findings that there is a slight negative rg between direct and maternal effects for both D I F F (6,
PRODUCTION TECHNICAL NOTE
245
P o s i t i v e c o r r e l a t i o n s b e t w e e n sire a n d M G S effects are corroborated by positive correlations b e t w e e n D I F F a n d c o w size. L a r g e - f r a m e D u t c h Red and White heifers experienced greater
11, 16) and MORT (7) but that the direct calf e f f e c t is s e v e r a l t i m e s l a r g e r t h a n t h e m a t e r n a l effect and masks the negative relationship between calf and maternal effects (18).
TABLE 1. Basic statistics of sire transmitting values for calving difficulty (DIFF) and perinatal calf mortality (MORT) in percent. Parity
Effect
Character
nt
Range
SD
CV 2
Range 3
SD 3
1
Sire
DIFF MORT DIFF MORT DI F F MORT DIFF MORT
37 37 87 87 62 62 49 49
17.1 11.8 9,1 8.7 6.3 4.4 4.6 3.6
3.02 2.35 2.08 2.07 1.06 1.03 .86 .85
32.8 28.4 22.6 25.9 27.8 25.1 23.9 20.7
7.7 7.4 9.2 8.7
2.13 2.12 2.61 2.56
MGS a >1
Sire MGS 4
1 N u m b e r of sires of > .6 repeatability. 2 Coefficient o f variation, in percent. 3 Fourteen sires with > 6 0 0 calvings and > 9 0 0 calvings of heifer mates and daughters. 4 Maternal grandsire.
TABLE 2. Correlations between calving traits) Parity
Effect
Character
n
r
rg
r ~4
Correlations between calving difficulty (DIFF) and calf mortality (MORT) 37 59 87 49
Sire
1
>1 1 >1
MGS
.86*** .71"** .79"** .56"**
.96 .83 .98 .66
.93*** .96***
Correlations between heifer and cow calvings Sire MGS
DIFF MORT DIFF MORT
33 33 45 45
.84*** .65"** .47*** .32*
.94 .72 .54 .37
Correlations between sire and MGS effects DIFF MORT DIFF MORT
1
>1
32 32 30 30
.52** .56***
.59 .64
.52" .67**
.II
-.13
t n = N u m b e r of sires of > . 6 repeatability, MGS = maternal grandsire, rg = genetic correlations following Calo et al. (4) adjustment, r , , = correlation for 14 sires with > 6 0 0 and > 9 0 0 calvings of heifer m a t e s and daughters. *.Ol
Journal o f Dairy Science Vol. 69, No. 1, 1986
246
RON ET AL.
calving difficulty than smaller framed heifers (9), and MORT by MGS was correlated positively (.2) with live weight of both heifer and cow daughters in Finnish Ayrshire cattle (5). In Israel, the between-sire correlation for bull and heifer growth rate was .67 (15), and correlations between sire evaluations for weight of fattening bulls at 435 days and DIFF and MORT in calvings of heifer mates were .31 and .21 (15). The results also show that genetic control of both DIFF and MORT is different in primiparous and multiparous calvings. Higher correlations for primiparous calvings suggest that the direct effect in primiparous calvings is larger than the maternal effect, but not in multiparous calvings, which is in accordance with previous findings (3, 16). In the mating scheme in Israel, selection of bulls for mating to heifers is based on four evaluations: sire evaluations for DIFF and MORT in multiparous calvings, and MGS evaluations for DIFF and MORT in primiparous calvings. Stepwise regression technique was used to construct an index of these evaluations to predict sire performance on primiparous calvings. The DIFF for sires of multiparous calvings had the highest coefficient of determination with sire evaluations in primiparous calvings for both DIFF and MORT, .75 and .71. None of the other evaluations had significant effect when added to the model. Regressions of DIFF and MORT of primiparous sire evaluations on multiparous DIFF evaluations were 3.4 + .4 and 2.5 + .3 (P<.0001); i.e., each percentage increase in DIFF in multiparous sire evaluations is expected to increase DIFF and MORT by 3.4 and 2.5% in primiparous calvings. Van Vleck and Edlin (19) developed a mixed model procedure for analysis of DIFF in primiparous and multiparous cows as correlated traits. Because of significant genetic correlations, it seems advantageous to extend this technique to analysis of all eight calving traits. By selection index theory, weighting factors can be computed as functions of genetic and economic relationships between the various traits. However, because of close genetic linkage between calving traits, selection policies based on different weightings for these traits should result in nearly identical ranking of bulls. Rank and product-moment correlations among four different overall calvings performance indices,
Journal of Dairy Science Vol. 69, No. 1, 1986
with weightings for sire and MGS effects on DIFF and MORT in primiparous and multiparous calvings as described, were all above .95. Therefore, relative weights assigned to the traits had little effect in determining the sire's overall calving performance. We suggest that: 1) sire evaluations for DIFF in cow calvings be used as a preliminary prediction of heifer calving performance for both DIFF and MORT; and 2) all of the eight separate calving trait evaluations be combined into an overall calving selection index.
ACKNOWLEDGMENTS
The authors are indebted to Barbara Reichman for statistical analyses. This research constitutes Contribution No. 1287-E, 1984 series, from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. REFERENCES
1 Bar-Anan, R. 1977. A breeding strategy for reducing perinatal calf mortality in heifer calvings. Page 149 in Current topics in veterinary medicine and science. Eur. Econ. Commun. Seminar 1977. 2 Bar-Anan, R. 1982. Parameters for evaluating Israeli-Holstein milk recording data. Mimeo, Inst. Anita. Sci., Agric. Res. Org., Volcani Center, Bet Dagan, Israel. 3 Bar-Anan, R., M. Soller, and J. C. Bowman. 1976. Genetic and environmental factors affecting the incidence of difficult calving and perinatal calf mortality in Israeli Friesian dairy herds. Anim. Prod. 22:299. 4 Calo, L. L., R. E. McDowell, L. D. Van Vleck, and P. D. Miller. 1973. Genetic aspects of beef production among pedigree Holstein-Friesians selected for milk production. J. Anim. Sci. 37:676. 5 Lindstrom, U. B., and V. Vilva. 1977. Frequency of stillborn calves and its association with production traits in Finnish cattle breeds. Z. Tierz. Zuchtungshiol. 94: 27. 6 Martinez, M. L., A. E. Freeman, and P. J. Berger. 1982. Genetic relationship between early calf mortality and calving difficulty. J. Dairy Sci. 65 (Suppl. 1):87. (Abstr.) 7 Martinez, M. L., A. E. Freeman, and P. J. Berger. 1983. Age of dam and direct-maternal effects on calf livability. J. Dairy Sci. 66:1714. 8 McDaniel, B. T. 1981. Economic impact of calving difficulty in Holstein heifers. J. Dairy Sci. 64(Suppl. 1):81. (Abstr.) 9 Meijering, A. 1983. Morphological aspects of dystocia in dairy heifers. J. Dairy Sci. 66(Suppl. 1): 108. (Abstr.)
PRODUCTION TECHNICAL NOTE
10 Meijering, A. 1984. Dystocia and stillbirth in cattle A review of causes, relations and implications. Livest. Prod. Sci. 11:143. 11 Philipsson, J. 1976. Studies on calving difficulty, stillbirth and associated factors in Swedish cattle breeds. III. Genetic parameters. Acta Agric. Scand. 26:211. 12 Philipsson, J. 1976. Studies on calving difficulty, stillbirth and associated factors in Swedish cattle breeds. V. Effects of calving performance and stillbirth in Swedish Friesian heifers on productivity in the subsequent lactation. Acta Agric. Scand. 26:230. 13 Robertson, A., and J. M. Rendel. 1954. The performance of heifers got by artificial insemination. Appendix: The contemporary comparison. J. Agric. Sci. 44:184. 14 Ron, M., R. Bar-Anan, and G. R. Wiggans. 1984. Factors affecting conception rates of Israeli Holstein -
247
cattle. J. Dairy Sci. 67:854. 15 Soller, M, and R. Bar-Anan. 1974. Correlated effects of selection for rate of gain in dairy cattle. 1st World Congr. Genet. AppI. Livest. Prod., (Madrid) 3:689. 16 Thompson, J. R., A. E. Freeman, and P. J. Berger. 1981. Age of dam and maternal effects for dystocia in Holsteins. J. Dairy Sci. 64:1603. 17 Thompson, J. R., E. J. Pollak, and C. L. Pelissier. 1982. Effects of calving difficulty on production and reproduction in the subsequent lactation in large California dairy herds. J. Dairy Sci. 65(Suppl. 1):87. (Abstt.) 18 Thompson, J. R., and J.E.O. Rege. 1984. Influences of dam on calving difficulty and early calf mortality. J. Dairy Sci. 67:847. 19 Van Vleck, L. D., and K. M. Edlin. 1984. Multiple trait evaluation of bulls for calving ease. J. Dairy Sci. 67:3025.
Journal of Dairy Science Vol. 69, No. 1, 1986
Associations among sire and maternal grandsire evaluations for calving difficulty and calf mortality were investigated in the Israeli Holstein population. Primiparous and multiparous calvings were analysed separately. Genetic correlations between calving difficulty and calf mortality were close to unity for primiparous calvings but lower for multiparous calvings. Genetic correlations between primiparous and multiparous sire evaluations were .94 and .72 for calving difficulty and calf mortality. Genetic correlations between sire and maternal grandsire evaluations were 6 for primiparous calvings and not-significant for multiparous calvings for both traits. These results correspond to findings that the direct genetic effect is larger than the maternal effect but that there is a slight negative correlation between them. We suggest that sire evaluations for calving difficulty of multiparous cows be used as a preliminary prediction of heifer calving difficulty and calf mortality and that the separate calving trait evaluations be combined into an overall calving selection index. INTRODUCTION
Progeny testing of sires for calving difficulty (DIFF) and calf mortality (MORT) was started in Israel in 1964. Since 1976, eight independent analyses have been computed thrice yearly. Bulls are evaluated separately for primiparous
and multiparous calvings, both as sires of calves and as maternal grandsires (MGS), for DIFF and MORT. Whereas sire evaluations estimate one-half of the sires' breeding value for these traits, MGS evaluations include one-quarter of the sires' breeding value as grandsire of the calf and one-half of the sires' maternal breeding value as sire of the cow. Correlations between direct and maternal effects have been negative for both DIFF (6, 11, 16) and MORT (7). Thompson and coworkers (16, 18) suggested that the negative association between direct and maternal effects on DIFF in US Holsteins was caused by small calves that are delivered easily and then become small heifers that may have difficulty in giving birth. To minimize the risk of DIFF and MORT in primiparous cows, matings in Israel are planned so that nearly all heifers are bred to bulls with positive evaluations for both production and calving traits as mates of primiparous cows and as sires of heifers (1). Better calving performance benefits farmers beyond the direct economic impact on production and reproduction in the subsequent lactation (8, 10, 12, 14, 17) by reducing the need to assist in births and to dispose of dead calves or dams. Objectives of the present study were to investigate the association between sire and MGS effects on DIFF and MORT in primiparous and multiparous calvings and to suggest improvements of selection and breeding policies for increasing the reproductive efficiency of the national dairy population. MATERIALS AND METHODS
Received July 11, 1984. 1Research sponsored by the US-Israel Binational Agricultural Research and Development Fund, Project No. US-334-81. 1986 J Dairy Sci 69:243-247
Eighty-seven sires in 3 5 5 herds were evaluated by contemporary comparison (13) for DIFF and MORT from 1977 to 1982. Mean heifer calving age was 24 mo. Primiparous and multiparous calvings were analyzed separately for each
243
244
RON ET AL.
trait. Eight separate analyses were used to estimate sire and MGS effects. D I F F and MORT were scored as all-or-none traits. For D I F F : 0 = normal birth and 100 = assisted birth and for MORT: 0 = live birth and 100 = stillbirth or calves dead within 48 h of calving. The analysis model for sire evaluations was:
with sire evaluations for primiparous calvings excluded to evaluate young sires that have not been mated to heifers. Index values were computed only for sires with repeatabilities > .6 for all traits included in the indices. Product m o m e n t and rank correlations were computed among the sires' selection index values for the different indices.
Yijk = Si + HYSj + eij k where Yijk is the D I F F or MORT score of k th calving, progeny o f the ith sire, occurring in the jth herd-year-season; Si is the effect of the i th sire; HYSj is the effect of the jth herd-yearseason, and eij k is the residual effect not explained by the other factors. Three calving seasons were defined: February through May, June through September, and October through January. Sire and residual effects were random, and the HYS effect was fixed. The sire component of variance was assumed to be .0152 and .0125 of the residual variance for primiparous and multiparous calvings for both traits. The same analysis model was used for the MGS analyses except that the sire effect was replaced by the MGS effect and the MGS component of variance was .0125 and .01 for primiparous and multiparous calvings. Repeatabilities were computed (2). Productm o m e n t correlations (r)and genetic correlations (rg) (4) were estimated among evaluations of sires with repeatabilities of >.6. Correlations were also computed for a subset of 14 sires with > .9 repeatability (r14). Selection indices to predict sire effects for D I F F and MORT in heifer calvings as functions of multiparous sire evaluations and primiparous MGS evaluations were constructed by stepwise regression. Sires were evaluated for overall calving performance by four selection policies: 1) all evaluations were weighted equally; 2) MORT and D I F F evaluations were weighted 2:1; 3) sire and MGS evaluations were weighted 2:1; and 4) alternatives 2 and 3 combined. Although the actual economic weights are not k n o w n and are subject to change, MORT is likely to have a higher economic value than DIFF, and more economic weight is likely to be assigned to sire effects that occur earlier and more frequently than MGS effects. Evaluations were computed with all eight traits included in the indices and Journal of Dairy Science Vol. 69, No. 1, 1986
RESULTS A N D DISCUSSION
Frequencies of D I F F and MORT were 9.2 and 8.0% in primiparous calvings and 3.6 and 4.1% in multiparous calvings. The standard deviations (SD) of sire evaluations for D I F F and MORT are in Table 1. The SD of sire evaluations for D I F F in primiparous calvings was three times as large as for multiparous calvings, but coefficients of variation were similar. The SD of sire evaluations were larger than those of MGS evaluations. The SD of sire evaluations in primiparous calvings of the 14 extensively used sires were small, but those of their MGS evaluations were large. These results may be due to the breeding policy followed in Israel. As heifers are generally bred to sires evaluated as favorable for calving traits in multiparous calvings, bulls are selected by their evaluations as sires but not as maternal grandsires. The SD of the MGS evaluations of sires with >.9 repeatability were greater than those of sires with >.6 repeatability because in c o n temporary comparison, evaluations of effects with small variance components and few observations are regressed toward the mean. Correlations among evaluations are in Table 2. The r14 are in all cases nearly equal to rg. Correlations between D I F F and MORT of sire and MGS effects were higher for primiparous than for multiparous calvings. The rg between D I F F and MORT sire effects in heifer calvings was near unity. The rg between evaluations derived from primiparous and muhiparous calvings ranged from .37 for MGS effects on MORT to .94 for sire effects on DIFF. Correlations between sire and MGS effects for D I F F and MORT were in the range of .5 to .7 for primiparous calvings. This is similar to the rg of .52 for MORT in primiparous calvings of Finnish Ayrshires (5). These results agree with findings that there is a slight negative rg between direct and maternal effects for both D I F F (6,
PRODUCTION TECHNICAL NOTE
245
P o s i t i v e c o r r e l a t i o n s b e t w e e n sire a n d M G S effects are corroborated by positive correlations b e t w e e n D I F F a n d c o w size. L a r g e - f r a m e D u t c h Red and White heifers experienced greater
11, 16) and MORT (7) but that the direct calf e f f e c t is s e v e r a l t i m e s l a r g e r t h a n t h e m a t e r n a l effect and masks the negative relationship between calf and maternal effects (18).
TABLE 1. Basic statistics of sire transmitting values for calving difficulty (DIFF) and perinatal calf mortality (MORT) in percent. Parity
Effect
Character
nt
Range
SD
CV 2
Range 3
SD 3
1
Sire
DIFF MORT DIFF MORT DI F F MORT DIFF MORT
37 37 87 87 62 62 49 49
17.1 11.8 9,1 8.7 6.3 4.4 4.6 3.6
3.02 2.35 2.08 2.07 1.06 1.03 .86 .85
32.8 28.4 22.6 25.9 27.8 25.1 23.9 20.7
7.7 7.4 9.2 8.7
2.13 2.12 2.61 2.56
MGS a >1
Sire MGS 4
1 N u m b e r of sires of > .6 repeatability. 2 Coefficient o f variation, in percent. 3 Fourteen sires with > 6 0 0 calvings and > 9 0 0 calvings of heifer mates and daughters. 4 Maternal grandsire.
TABLE 2. Correlations between calving traits) Parity
Effect
Character
n
r
rg
r ~4
Correlations between calving difficulty (DIFF) and calf mortality (MORT) 37 59 87 49
Sire
1
>1 1 >1
MGS
.86*** .71"** .79"** .56"**
.96 .83 .98 .66
.93*** .96***
Correlations between heifer and cow calvings Sire MGS
DIFF MORT DIFF MORT
33 33 45 45
.84*** .65"** .47*** .32*
.94 .72 .54 .37
Correlations between sire and MGS effects DIFF MORT DIFF MORT
1
>1
32 32 30 30
.52** .56***
.59 .64
.52" .67**
.II
-.13
t n = N u m b e r of sires of > . 6 repeatability, MGS = maternal grandsire, rg = genetic correlations following Calo et al. (4) adjustment, r , , = correlation for 14 sires with > 6 0 0 and > 9 0 0 calvings of heifer m a t e s and daughters. *.Ol
Journal o f Dairy Science Vol. 69, No. 1, 1986
246
RON ET AL.
calving difficulty than smaller framed heifers (9), and MORT by MGS was correlated positively (.2) with live weight of both heifer and cow daughters in Finnish Ayrshire cattle (5). In Israel, the between-sire correlation for bull and heifer growth rate was .67 (15), and correlations between sire evaluations for weight of fattening bulls at 435 days and DIFF and MORT in calvings of heifer mates were .31 and .21 (15). The results also show that genetic control of both DIFF and MORT is different in primiparous and multiparous calvings. Higher correlations for primiparous calvings suggest that the direct effect in primiparous calvings is larger than the maternal effect, but not in multiparous calvings, which is in accordance with previous findings (3, 16). In the mating scheme in Israel, selection of bulls for mating to heifers is based on four evaluations: sire evaluations for DIFF and MORT in multiparous calvings, and MGS evaluations for DIFF and MORT in primiparous calvings. Stepwise regression technique was used to construct an index of these evaluations to predict sire performance on primiparous calvings. The DIFF for sires of multiparous calvings had the highest coefficient of determination with sire evaluations in primiparous calvings for both DIFF and MORT, .75 and .71. None of the other evaluations had significant effect when added to the model. Regressions of DIFF and MORT of primiparous sire evaluations on multiparous DIFF evaluations were 3.4 + .4 and 2.5 + .3 (P<.0001); i.e., each percentage increase in DIFF in multiparous sire evaluations is expected to increase DIFF and MORT by 3.4 and 2.5% in primiparous calvings. Van Vleck and Edlin (19) developed a mixed model procedure for analysis of DIFF in primiparous and multiparous cows as correlated traits. Because of significant genetic correlations, it seems advantageous to extend this technique to analysis of all eight calving traits. By selection index theory, weighting factors can be computed as functions of genetic and economic relationships between the various traits. However, because of close genetic linkage between calving traits, selection policies based on different weightings for these traits should result in nearly identical ranking of bulls. Rank and product-moment correlations among four different overall calvings performance indices,
Journal of Dairy Science Vol. 69, No. 1, 1986
with weightings for sire and MGS effects on DIFF and MORT in primiparous and multiparous calvings as described, were all above .95. Therefore, relative weights assigned to the traits had little effect in determining the sire's overall calving performance. We suggest that: 1) sire evaluations for DIFF in cow calvings be used as a preliminary prediction of heifer calving performance for both DIFF and MORT; and 2) all of the eight separate calving trait evaluations be combined into an overall calving selection index.
ACKNOWLEDGMENTS
The authors are indebted to Barbara Reichman for statistical analyses. This research constitutes Contribution No. 1287-E, 1984 series, from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. REFERENCES
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