Estimation of Genetic Trends and Correlations for Jersey Cattle

Estimation of Genetic Trends and Correlations f o r Jersey C a t t l e P. J. BLANCHARD, R. W. EVERETT, and S. R. SEARLE 1 Department of Animal Science Cornell University Ithaca, NY 14853

ABSTRACT

Genetic correlations were estimated between sire evaluations for 14 type traits, 4 production traits, and stayability to 36, 48, 60, and 72 mo of age from type information and 469,460 lactation records on daughters of 3789 Jersey sires. Most correlated genetically with stayability were production traits with estimates that ranged from .44 to 1.01. Of the stayability measures, stayability at 48 mo of age had the highest correlation with production and other stayability measures. Modified Contemporary Comparison and Northeast Artificial Insemination Sire Comparison sire evaluations on daughter records had correlations of .77 and .71 for milk and fat. The Northeast Artificial Insemination Sire Comparison was correlated with stayability whereas the Modified Contemporary Comparison was correlated with type evaluations. Estimates of yearly genetic means indicate substantial improvement in recent years for production and stayability. Animals freshening from 1969 to 1977 have improved 290 kg of milk, 10.5 kg of fat, and 1.3, 4.0, 4.5, and 3.1 units of stayability for 36-, 48-, 60-, and 72-mo stayability because of improved sires only. This is an annual improvement due to sires of 36 kg of milk, 1.3 kg of fat, and .2, .6, .8, and .6 units of stayability. INTRODUCTION

Stayability, defined as survival to a specific age having had the opportunity to reach that

Received March 12, 1982. 1Biometrics Unit. 1983 J Dairy Sci 66:1947-1954

age, is important economically because it reduces herd replacements needed, increases proportion of cows with mature production, and increases intensity of selection for herd replacements. Animals with stayability have the opportunity to produce more offspring, which results in automatic selection for stayability (1). Selection for longevity is difficult because of low heritability (1, 9, 12, 14, 17), which suggests longevity is influenced by traits with low heritability such as reproductive problems (12). Yet there are significant differences among sires in their daughters' lifetime yields that indicate possible relationships between stayability and production (8, 16). Type is defined as the comparison of the physical appearance of cows with that of the ideal cow as envisioned by the breed association. Type has economic importance at sales and in the show ring; however, specific economic weights for components of type have been impossible to calculate. Prediction of daughter production has been taken for granted as an important aspect of sire evaluation by all dairy producers. However, the relative emphasis placed on production, type, and stayability varies among dairy producers and, thus, reduces the rate of genetic progress for production traits. Because artificial insemination (AI) organizations retain only bulls with more than adequate type evaluations, there should be little possibility of a decline of type. Everett et al. (4) showed production and type were negatively associated in Holstein cattle. Grantham et al. (6) also found consistently small negative genetic correlations between type and production. The purpose of this study was to investigate relationships among production, stayability, and type of Jersey cattle in national data. The objectives were to estimate product moment and genetic correlations among production, stayability, and type as well as yearly pheno-

1947

1948

BLANCHARD ET AL.

typic, genetic, and environmental means for production and stayability. MATERIALS AND METHODS

Data were obtained from the American Jersey Cattle Club (AJCC), the United States Department of Agriculture (USDA), and the Dairy Records Processing Laboratory (DRPL) at Cornell University. The AJCC contributed 3789 sire evaluations for production calculated by the USDA Modified Contemporary Comparison (MCC) and 469,460 lactation records from Jersey cows initiated between 1959 and 1978. The USDA furnished sire evaluations for type on 2796 AI and non-AI Jersey sires. The DRPL provided pedigree information for Jersey sires that was used to calculate the inverse of the relationship matrix in the Northeast AI Sire Comparison (NEAISC) for production evaluations on 885 AI sires and stayability evaluations on 889 AI sires. Lactation records in the NEAISC were 305-day mature equivalent (ME) records with at least 907 kg of milk and 32 kg of fat and at most 15,876 kg of milk and 680 kg of fat. The MCC sire evaluation (3) ranks sires taking into consideration genetic trend and the genetic ability of herdmates and uses pedigree information of the bull to form genetic groups of sires. Official Dairy Herd Improvement (DHI) and Dairy Herd Improvement Registry (DHIR) in-progress and complete 305-day ME records were used. The NEAISC evaluations of production were calculated on DHI, DHIR, and alternate morning and evening sampling (AM/ PM) 305-day ME first lactation milk and fat records of daughters of AI sires. The model for the NEAISC (5, 13) is Yijklmn = hi + gj + Sjk + .Sgl + -51m + eijklm n

[1]

where Yijklmn is a record in the i th herd-yearseason by the n th daughter of the kth sire in the jth sire group having maternal grandsire m that is in the lth sire group. Herd-year-seasons (h) and sire groups (g) are considered to be fixed effects, and sires (s) and residual (e) are random effects. The mixed model equations contain two maternal grandsire (MGS) effects (13): MGS effects of the daughters, which were obtained Journal of Dairy Science Vol. 66, No. 9, 1983

from the lactation data, and MGS effects of the bulls used in calculating the inverse of the relationship matrix (A-I), which were obtained from DRPL pedigree tapes. The MGS of the daughter is included in model [1] to eliminate some of the possible bias from nonrandom mating, i.e., dairy producers mating the best bulls to the best cows and inferior bulls to inferior cows. Calculation of A-1 (7) using bulls, bulls' sires, and maternal grandsires results in the generation of base sires. Base sires are bulls which have no daughters and which are sires or maternal grandsires of two or more bulls with daughters or granddaughters. The A-1 is multiplied by the ratio of residual variance to sire variance (7). This ratio was 15 and 40 for production and stayability measures. The stayability observation for a cow was zero if the cow failed to survive past a certain age and was a one if the cow survived. Cows sold for dairy purposes after reaching a specific age would not be included in stayability evaluations beyond that age. The opportunity of an animal to reach a specific age was defined as 3636-, 48-, 60-, and 72-mo stayability and each stayability was a separate trait. Type evaluations of Jersey sires provided by the USDA were calculated by mixed model (11). Type scores were adjusted for age and stage of lactation. Genetic groups were defined for AI and non-AI sires by year of birth of the sire. Genetic correlations between sires for production, stayability, and type traits were estimated (2) with assumptions (15). The estimated breeding value of the i th sire (EBV i) is based on the average of n i daughter records (one record per daughter), which are effectively deviations from herd-year-season averages. Such a daughter average can be represented as yi. = si +~i. where n ~i. = 2.; eij/ni, l=l with si and eij random variables with zero means, variances Os2 and Oe 2, respectively, and

GENETIC TRENDS OF JERSEYS with all covariances among si's and eij's zero. The the variance o f Yi. is Var(Yi.) = as2

+

O2elni =

Os2(ni +

N o w suppose that we have N sires. F r o m exp e c t e d values over the p o p u l a t i o n of sires, for X= 1 - l / N ,

2 2 aelOs)ln i,

N

N (EBV i -- ~ EBVi/N) 2 i=1 i=1

and the p o p u l a t i o n regression coefficient of si on Yi. is 2 2 bi = Os2/Var(yi.) = ni/(ni + Oe/Os)

1949

estimates N Xaa2 £ bi i=I

Because 2s i = ai = additive genetic merit of sire i, we have 4Os2

2 =

unbiasedly, Also,

o a

with

a similar result for EBV~.

and we take N

N (EBV i -- ~ EBVi/N ) X i=l i=1

EBV i = 2biYi.

N (EBV i -- • EBV~/N) i=l

with Var(EBVi) = 4b2(0 2 + oe2/ni) = 4b2osl(ni +

a2ela2s)lnil

estimates N XOaa' ~ bibi i=l

= 4b~ os2/bi = 4bios2 = bioa2 Similarly, for some o t h e r trait measured on n[ daughters, say t

t

unbiasedly. Therefore, over a whole sample o f bulls, we pool covariances and variances, and on this basis we take the p r o d u c t - m o m e n t correlation r ( E B V , E B V ' ) as an estimate of

#

Yij = si + eij Oaa' ~bib~ 2

we have

2

r

X/--OaS b i a a' ~ b i '

'

~bib~

= ni/(n i +

(°aa'/aa °a') ~/(~bi)(l~bl ) EBV] = 2biYi. '-'

~bibi Pad X/(~bi)(~b,i)

and Var(EBV[) = bi' o a'2 F u r t h e r m o r e , the covariance between EBV i and EBV~ is

where s u m m a t i o n s for i = 1 . . . . , N, and Pad represent the genetic correlation of interest. Its e s t i m a t o r is

^

Cov(EBVi,EBV~) = 4bib~Cov(si,s~)

Pad =

X/(~bi)(~b[) ~bib ~

r(EBV,EBV')

[21

= bib~oaa' where ~Taa' is the covariance b e t w e e n the additive genetic merits of the t w o traits.

Heritabilities for the traits (3, 8, 10) studied and numbers o f sires with evaluations for each trait are in Table 1. Numbers o f evaluations for Journal of Dairy Science Vol. 66, No. 9, 1983

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BLANCHARD ET AL.

feet and legs were less than the other traits because of a change of scoring cows by AJCC. The number of AI sires evaluated for production and stayability by the NEAISC is considerably less than the AI and non-AI sires evaluated by MCC. Yearly phenotypic means were calculated by averaging all AI first lactation records and stayability observations by year of freshening. Yearly genetic means were estimated by replacing each AI cow's record with her sire's NEAISC production or stayability evaluation and then averaging by year of first freshening. Yearly environmental means were the difference between phenotypic and genetic means. RESULTS A N D DISCUSSION

Product m o m e n t and estimated genetic correlations between production, stayability, and

type sire evaluations are in Table 2. Final type score was related to the 13 type components (Table 2). It was most correlated with general appearance (.86), and components of type correlating least with final score were feet and legs (.43 and .50). These correlations agree with (10) with the exception of suspensory ligament and mammary system which have a lower correlation with final score. Relationships between sire evaluations for type and production and stayability in Table 2 indicate the best component of type for predicting production and stayability is dairy character. These data indicate that general appearance, stature, breed character, feet, legs, chest and barrel, rear udder, and suspensory ligament are insignificant indicators to a sire's NEAISC transmitting ability for both production and stayability.

TABLE 1. Number of sires evaluated and heritabilities of production, stayability, and type traits for estimating genetic correlations.

Trait

Heritability

No. ~of sires evaluated

Final type Score

.23

2,796

General appearance Stature Breed character Back, rump, and tail Feet Legs

.19 .43 .12 .23 .23 .09

2,796 2,796 2,796 2,796 1,782 1,782

Dairy character Body capacity

.20 .14

2,796 2,796

Mammary system Fore Rear Teats Suspensory ligament

.26 .22 .21 .19 .11

2,796 2,796 2,796 2,796 2,796

Modified Contemporary Comparison production Northeast Artificial Insemination Sire Comparison production

.19 .251

3,789 885

Stayability 36 mo 48 mo 60mo 72 mo

.101 .101 .101 .101

889 832 779 732

1Estimates used in the Northeast Artificial Insemination Sire Comparison production and stayability sire evaluations. Journal of Dairy Science Vol. 66, No. 9, 1983

T A B L E 2. P r o d u c t m o m e n t correlations (above diagonal) and estimates of genetic correlations (below diagonal) between sire evaluations. 1

T

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Final t y p e score General appearance Stature Breed c h a r a c t e r Back, r u m p , and tail Feet Legs Dairy character Chest and barrel Fore udder Rear u d d e r Teats S u s p e n s o r y ligaments M a m m a r y system MCC 1 milk MCC 1 fat NEAISC 2 milk NEAISC 2 fat 36-mo stayability 48-mo stayability 60-too stayability 72-mo stayability

1.69 1.05 1.58 1.29 .92 1.31 1.22 1.51 1.32 1.33 1.10 1.38 1.39 .41 .43 .12 .15 -.01 .14 .05 .05

2

3

4

5

6

7

8

9

10

11

12

13

14

.86

.59 .72

.74 .76 .53

.67 .76 .46 .56

.43 .50 .31 .36 .31

.50 .56 .38 .44 .36 .74

.62 .59 .42 .56 .34 .30 .36

.72 .76 .69 .68 .58 .39 .49 .50

.68 .50 .21 .44 .49 .20 .26 .24 .41

.68 .46 .18 .40 .44 .19 .23 .29 .36 .75

.56 .37 .12 .31 .37 .17 .20 .17 .29 .70 .63

.63 .42 .17 .34 .37 .21 .23 .25 .32 .68 .72 .65

.74 .50 .20 .42 .47 .21 .27 .28 .39 .88 .88 .78 .79

1.32 1.66 1.51 1.10 1.50 1.18 1.63 .99 .92 .75 .94 .97 .39 .39 .00 .02 -.14 .00 -.13 -.12

1.07 .82 .61 .94 .76 1.35 .37 .35 .23 .35 .54 .20 .23 -.07 .01 --.17 -.03 -.13 --.12

1.21 .84 1.23 1.23 1.55 .94 .86 .68 .81 .89 .53 .55 .13 .17 -.06 .10 -.06 --.06

.66 .94 .66 1.22 .95 .86 .73 .81 .89 .05 .05 -.24 -.23 --.28 -.17 -.36 -.31

1.88 .64 .88 .42 .40 .37 .49 .44 .27 .26 .08 .11 .14 .17 .12 .13

.95 1.34 .70 .60 .52 .66 .69 .38 .39 .09 .16 .28 .32 .18 .16

1.07 .47 .58 .35 .55 .55 .95 .92 .70 .67 .38 .51 .49 .46

.86 .75 .62 .74 .80 .35 .40 .02 .10 -.05 .08 --.06 -.04

1.46 1.40 1.50 1.68 --.04 .00 -;29 -.24 -.20 -.18 -.23 --.25

1.26 1.59 1.68 .06 .06 -.11 -.11 -.10 -.07 -.11 -.06

1.44 1.52 --.11 -.05 --.24 -.18 --.13 --.14 --.17 -.08

1.70 .11 .14 -.13 -.08 .02 -.04 .00 --.04

.00 .04 -.20 --.16 -.14 -.11 -.16 --.11

15

16

17

.22 .22 .08* .20 .20 .00" .12 .14 --.05* .24 .25 .08* .03* .03* --.16 .13 .12 .06* .14 .14 .05* .49 .47 .47 .17 .19 .01" -.02* .00" -.20 .03* .03* --.07* -.05* -.03* -.16 .05* .06* --.08* . 0 0 " .02* --.13 .92 .77 1.49 .65 1.10 .93 .90 1.02 1.26 .44 .44 .88 .73 .70 1.01 .70 .70 .94 .75 .73 .90

18

19

20

21

22

.10" -.01" .08* .03* .03* .01" -.09* .00" -.08* -.07* .00"-.11"-.02"-.08"--.07" .11" -.03* .05* - . 0 3 * --.03* - . 1 5 - . 1 7 - . 1 0 " - . 2 1 --.18 .07* .09* . 1 0 " .07* .07* .09* .15 .17 .09* .08* .45 .23 .31 .29 .26 .07* - . 0 3 * .04"-.04"--.02" --.16 --.12 - . 1 1 " - . 1 4 - . 1 4 - . 0 7 * --.06* - . 0 4 * - . 0 6 * --.03* - . 1 2 --.08* - . 0 8 * - . 1 0 " --.04* -.05* .01" -.02* . 0 0 " .02* - - . 1 1 " - . 0 8 * -.07* --.09* --.06* .62 .26 .44 .42 .43 .71 .27 .42 .42 .42 .89 .57 .64 .59 .55 .59 .64 .60 .55 .92 .75 .70 .58 1.00 1.24 .85 .73 .96 1.18 1.43 .80 .90 .99 1.26 1.39

Z ,.q

Z r~

©

oq

* Nonsignificant at .01 probability a n d no significance was established for genetic correlations. 1 Modified C o n t e m p o r a r y Comparison. <

2 N o r t h e a s t Artificial Insemination Sire Comparison.

O Ox

Z 9 ",0

1952

BLANCHARD ET AL.

All of the sire evaluations for components of type were consistently more correlated with MCC production evaluations than with NEAISC production evaluations by an average of .13. The product moment correlations of .77 and .71 between MCC and NEAISC for milk and fat indicate the two sire evaluation systems are evaluating sires differently. These correlations indicate only 50 to 59% of the variation in each method of AI sire evaluation is accounted for by the other method. It appears the MCC and NEAISC are different measurements, and caution should be used in comparing them. Correlations of MCC milk or fat with stayability ranged from .26 to .44 compared to NEAISC milk or fat correlations of .55 to .64, which are similar to correlations in Holstein data (14). One would expect much higher correlations between MCC evaluations for milk and fat and stayability because MCC uses multiple lactations that should better indicate stayability or longevity. Correlations among stayability evaluations were high and similar to those in Holstein data (4). Genetic correlations were estimated by equation [2], and many of the estimates are more than 1.0. Equation [2] is sensitive to the correct choice of Oe/Os 2 2 and small ni. In fact, if ni were large for all bulls, bi and b~ in equation [2] would be 1.0. In these data, heritabilities were assumed correct, and ni were smaller than those in (4). Estimates of genetic correlations among the type traits are consistently larger than in (10). Genetic correlations of type traits with MCC milk and fat are larger than those in reports and are consistently higher than corresponding estimates for NEAISC milk and fat. Estimates in Holsteins between type and production were - . 2 8 to - . 3 0 (4). Genetic correlations between type and stayability are of similar magnitude but opposite sign of estimates for Holsteins (4). Dairy character appears to have the highest genetic correlation of all the type traits with both production and stayability. Selection for production in Jersey cattle will increase desirable type as well as stayability. Individual type traits that may decline in selecting entirely on production include stature, back, rump and tail, and mammary system traits. However, as these traits decline, stayability of the breed will increase.

Journal of Dairy Science Vol. 66, No. 9, 1983

Table 3 contains genetic changes that have occurred in the breed for production and stayability. For those years with significant data (1969 to 1977), the breed has improved 293 kg of milk and 10.8 kg of fat from sire selection only. This indicates a negative genetic trend for fat percent. In the stayability traits, the improvement has been 1.3, 4.0, 4.5, and 3.1 units of improvement for 36-, 48-, 60-, and 72-mo stayability. For 48-mo stayability, this means 4% more cows are living to 48 mo of age in 1976 compared to 1969 as a result of genetic improvement due to sires used. This is substantial improvement compared to Holsteins (4). Environmental changes in Table 4 include genetic improvement from cow selection. The envirnmental change since 1969 has been positive for both milk and fat, indicating improvement of feeding and management systems. There has been an 8% environmental improvement of milk and 2% of fat, indicating percent fat in the Jersey breed is declining because of management. Environmental changes for all stayabilities in Table 4 show a decline with advancing time. Causes of a declining environment might include decreasing cow numbers, increasing cull cow prices, more intensive management, and decreases of breed popularity. CONCLUSIONS

Relationships among Jersey sire evaluations for production, stayability, and type were investigated. The data suggest: 1) production, stayability, and type sire evaluations are positively related; 2) of all the physical appearance traits recorded in Jersey type evaluations, dairy character is the best indicator of both production (r~.47) and stayability (r-~.27); 3) correlations between MCC and NEAISC sire evaluations for milk and fat were .77 and .71, indicating greater than expected differences of sire evaluations; 4) correlations between type trait evaluations and MCC were consistently .13 units higher than the corresponding correlations with the NEAISC evaluations for production; and 5) estimates of genetic changes from sires indicates substantial genetic improvement for both production and stayability in the Jersey breed.

GENETIC TRENDS OF JERSEYS

1953

TABLE 3. Genetic means for milk, fat, and stayability. Year

No.

of fresh

of cows

Milk

Mo. of stayability

1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977

67 89 132 161 165 266 385 414 414 3336 7595 8156 8515 8745 8685 9063 9154 5648

-43 -121 --82 --120 -73 --69 --138 --156 --148 --176 --182 --124 --61 26 66 104 133 114

Fat

36

48

60

72

.8 .4 --.4 --.6 -.6 --1.5 --2.2 --2.6 --2.9 --3.5 --3.8 --3.6 --3.2 --2.4 --2.0 --1.8 --2.0 --2.2

5.8 2.6 1.2 .2 .0 --1.3 --2.2 --2.7 --3.0 --4.1 --4.8 --4.3 --3.5 --1.9 --.9 --.3 --.1

6.6 2.6 1.5 .0 --.3 --1.3 --2.6 --3.1 --4.0 --4.5 --5.1 --4.4 --3.4 --1.6 --.6 .0

5.0 1.0 .7 .2 -.5 --.9 --1.6 --1.9 --2.5 --3.1 --3.7 -3.1 --2.1 -.6 .0

Fat

36

48

60

72

189 198 183 198 202 202 199 207 208 214 217 219 217 214 220 220 227 219

91.5 91.8 88.7 91.6 91.8 88.4 87.4 90.7 86.2 90.0 90.2 89.9 88.9 88.6 88.9 88.2 88.1 85.5

69.2 75.5 69.9 64.2 68.4 65.5 68.3 70.4 75.0 71.0 71.7 70.8 68.3 68.9 69.1 54.6 49.0

58.1 59.5 56.2 44.5 55.5 54.8 51.4 55.3 49.5 55.9 56.5 54.7 55.1 54.5 51.7 37.4

42.2 45.3 49.9 31.2 45.8 39.7 37.6 39.9 41.1 40.6 41.1 42.6 41.8 37.6 29.7

(kg) 2.3 -2.6 --.4 --1.9 .5 --.1 --3.5 --4.6 --5.3 --5.6 --5.9 --3.4 --1.2 1.2 2.8 4.9 5.6 4.9

TABLE 4. Environmental means 1 for milk, fat, and stayability. Year

No.

of fresh

of cows

Milk

Mo. of stayability

1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977

67 89 132 161 165 266 385 414 414 3336 7595 8156 8515 8745 8685 9063 9154 5648

3697 3835 3611 3905 3941 3934 3898 4079 4100 4192 4325 4377 4338 4305 4455 4472 4613 4534

(kg)

1 Environmental means are the differences between phenotypic and genetic means and contain one-half of the genetic component. Journal of Dairy Science Vol. 66, No. 9, 1983

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BLANCHARD ET AL. ACKNOWLEDGMENTS

The a u t h o r s e x p r e s s a p p r e c i a t i o n t o Eastern Artificial I n s e m i n a t i o n C o o p e r a t i v e for financial s u p p o r t f o r this research. The c o o p e r a t i o n o f the A m e r i c a n Jersey Cattle Club and the U S D A in providing data is greatly appreciated. REFERENCES 1 Bakker, J. J., R. W. Everett, and L. D. Van Vleck. 1980. A method for calculating the profitability index for sires. J. Dairy Sci. 63:1334. 2 Calo, L. L., R. E. McDowell, L. D. Van Vleck, and P. D. Miller. 1973. Genetic aspects of beef production among Holstein-Friesians pedigree selected for milk production. J. Anita. Sci. 37:676. 3 Dickinson, F. N., R. L. Powell, and H. D. Norman. 1976. An introduction to the USDA-DHIA Modified Contemporary Comparison. Prod. Res. Rep. 165, USDA. 4 Everett, R. W., J. F. Keown, and E. E. Clapp. 1976. Relationships among type, production and longevity in Holstein cattle. J. Dairy Sci. 59: 1505. 5 Everett, R. W., R. L. Quaas, and A. E. McClintock. 1979. Daughters' maternal grandsires in sire evaluation. J. Dairy Sci. 62:1304. 6 Grantham, J. A., J. M. White, W. E. Vinson, and R. H. Kliewer. 1974. Genetic relationships between milk production and type in Holsteins. J. Dairy Sci. 57:1483. 7 Henderson, C. R. 1975. Inverse of a matrix of relationships due to sires and maternal grandsires. J.

Journal of Dairy Science Vol. 66, No. 9, 1983

Dairy Sci. 58:1917. 8 Hudson, G.F.S., and L. D. Van Vleck. 1981. Relationship between production and stayability in Holstein cattle. J. Dairy Sci. 64:2246. 9 Miller, P., L. D. Van Vleck, and C. R. Henderson. 1967. Relationships among herd life, milk production and calving interval. J. Dairy Sci. 50:1283. 10 Norman, H. D., B. G. Cassell, and F. N. Dickinson. 1978. Phenotypic and genetic relationships between type classification traits in Jerseys. J. Dairy Sci. 61:1250. 11 Norman, H. D., B. G. Cassell, G. J. King, R. L. Powell, and E. E. Wright. 1979. Sire evaluation for conformation of Jersey cows. J. Dairy Sci. 62: 1914. 12 Parker, J. B., N. D. Bayley, M. H. Fobrman, and R. D. Plowman.. 1960. Factors influencing dairy cattle longevity. J. Dairy Sci. 43:401. 13 Quaas, R. L., R. W. Everett, and A. E. McClintock. 1979. A maternal grandsire model for sire evaluation. J. Dairy Sci. 62:1648. 14 Schaeffer, L. R., and E. B. Burnside. 1974. Survival rates of tested daughters of sires in artificial insemination. J. Dairy Sci. 57:1394. 15 Taylor, J. F. 1982. Assumptions required to approximate estimates of genetic (co)variance by the method of Calo et al. Page 256 in Genet. Res. 1982-1983 Rep. Eastern Artif. Insem. Coop. Dep. Anim. Sci., Cornell Univ., Ithaca, NV. 16 Van Vleck, L. D. 1964. First lactation performance and herd life. J. Dairy Sci. 47:1000. 17 White, J. M., and J. R. Nichols. 1965. Relationships between first lactation, later performance, and length of herd life in Holstein-Friesian cattle. J. Dairy Sci. 48:468.