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Estimation of Genetic Change in the Registered Holstein Cattle Population1
Estimation of Genetic Change in the Registered Holstein Cattle Population I K. L. LEE 2 and A. E. F R E E M A N
Department of Animal Science Iowa State University Ames 50011 L. P. JOHNSON
Holstein Association Brattleboro, VT 05301
ABSTRACT
change resulted from doubling the rate o f change in the bulls' average Predicted Differences. This method should yield more accurate estimates of genetic change when bulls with genetic evaluations are representative o f the population. Estimates of annual genetic change for 1971 to 1979 from the latter method were 84 kg milk, 2.6 kg milk fat, and - . 0 0 6 milk fat percentage.
We examined pedigrees of 440,702 males and 526,956 females born during 1960 through 1979 and registered in the herdbook of the Holstein Association. Annual trends in genetic merit o f sires, dams, maternal grandsires, and maternal granddams were determined from their estimated transmitting abilities. Genetic merit of parents changed substantially after more accurate procedures for estimating transmitting abilities were introduced in 1968. Rates of change for genetic merit of sires were larger than corresponding changes in dams. Annual genetic change for the male and female populations was estimated by two methods. The sum of annual changes in the average estimated transmitting abilities of sires and dams yielded estimates of annual genetic change. In addition, estimates of annual genetic change were obtained by doubling trends in the animals' average estimated transmitting abilities. Both methods yielded similar results in the female population. For 1971 to 1979, annual genetic change for females was approximately +55 kg milk, +1.5 kg milk fat, and - . 0 0 7 milk fat percentage. In the male population, estimates differed between methods. Smaller estimates of annual genetic
INTRODUCTION
The effectiveness o f dairy breeding programs is measured through annual genetic change. Several early genetic trend studies dealt with comparisons between milk production of artificial insemination (AI) and natural service progeny (7,12,20,21,22). Differences between the weighted averages of AI and non-AI daughters generally were slightly in favor o f AI progeny. Differences between AI and non-AI cows may underestimate the influence of AI bulls because non-Al bulls may themselves be sired by A[ bulls. Smith (19) proposed several methods for estimating annual genetic change (~) based on the regression of daughter performance on time. Three methods are:
Received December 3, 1984. 1Journal Paper No. J-11694 of the Iowa Agriculture and Home Economics Experiment Station, Ames. Project No. 1053. 2Department of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg 24061. 1985 J Dairy Sci 68:2629-2638
= 2(bp. T -- b p . T / S )
[11
= - 2 ( b ( p _ p ) . T/S )
[21
= bp. T -- bp. T/SD
[3]
Regression estimates are represented by b. P is performance of a cow, P is the average performance of her herdmates, S is sire of the cow, D is dam of the cow, and T is time of cow calving. Equations [1] and [2] were modified to account for age of dam or progressive culling
2629
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LEE ET AL.
of dams (5,19). In addition, adjustments made to Equations [1] and [2] accounted for older sires being mated to genetically superior cows (9,14). Burnside and Legates (2) developed a method to adjust first-lactation records of first-born fuI1 sisters for favorable environmental effects that bias estimates of g from Equation [3]. More recently, regressions of sires' average genetic merit on time have been used to estimate g (1,10,11,14,16,18). Yearly change in average sire transmitting ability, weighted by the number of offspring, represents half the change in genetic merit of the population. If genetic trends in the dams and sires are equal: = 2bsG. T
[41
where bSG .T is the regression of sires' average transmitting ability estimate (SG) on time (T). T was defined as year of daughter calving in most genetic trend studies. To simplify the review of previously reported estimates of g, the various modifications of Equations [1], [2], [3], and [4] will be referred to as Methods 1, 2, 3, and 4, respectively. Estimates of genetic change from the previous studies, however, are difficult to compare because different populations and time segments were investigated. An extensive investigation by Powell and Freeman (14) included ~ from modifications of Methods 1, 2, 3, and 4. Data were production records initiated between 1957 and 1969. Estimates of ~ from the various methods ranged from 49.2 -+ 2.4 kg milk (Method 4) to 133.4 -+ 54.7 kg milk (Method 3); and - . 4 7 -+ .09 kg milk fat (Method 4) to 5.07 ± 1.91 kg milk fat (Method 3). Powell et aI. (16) used more than 3 million records initiated in 1960 to 1975 to estimate by Method 4. Estimates of annual genetic change computed from trends in average estimated breeding value of sires were 18 kg milk, .3 kg milk fat, and - . 0 0 5 9 milk fat percentage for 1960 to 1975. For 1968 to 1975, ~ were 38 kg milk, .7 kg milk fat, and - . 0 1 0 6 milk fat percentage. Yearly genetic changes in milk estimated from changes in average estimated breeding values of the cows themselves were 8 kg in 1960 to 1975 and 21 kg in 1968 to 1975. Journal of Dairy Science Vol. 68, No. 10, 1985
Hintz et al. (10) reported estimates of genetic change in Holsteins in the northeastern US. When estimates of change in average genetic merit of cows were used in Method 4, was 26.1 kg milk for Al-sired cows compared with 35.8 kg milk when the trend in average sire transmitting ability was used to estimate ~. The latter approach is not valid when rates of genetic change for sires and dams are not equal. Several researchers have estimated genetic trend in Canadian Holsteins (1,11,18). Estimates of g ranged from 38 to 44 kg milk and 1.1 to 1.6 kg fat. Generation interval is an important component in the prediction and estimation of annual genetic change. The four described methods of estimating genetic change are dependent on the regressions of performance or genetic merit on time (year of birth, month of calving, etc.). Consequently, direct calculations of generation interval are unnecessary in estimating ~ from these methods. Current estimates of generation intervals, however, would be useful in chaxacterizing the dairy cattle population. The objectives of this study were 1) to estimate annual genetic change in milk, milk fat, and milk fat percentage and 2) to calculate generation intervals in the registered Holstein cattle population born in the United States during 1960 through 1979. MATERIALS AND METHODS
Pedigrees of 440,702 registered Holstein males and 526, 956 registered Holstein females were provided by the Holstein Association. The animals were born between 1960 and 1979, inclusive. The data comprised all registered males and a random 10% of the registered females born in the 20-yr span. Pedigree information consisted of estimated transmitting abilities (ETA) for milk, milk fat, and milk fat percentage of the animals and their sires, dams, maternal grandsires (MGS), and maternal granddams (MGD). Pedigrees contained the most current information available in August 1982. Availability of ETA for all pedigree members varied. Birth dates of all animals and their ancestors were included and used to calculate generation intervaJs over the 20-yr period. Male ETA for production traits were Predicted Differences (PD) computed by Modified
GENETIC CHANGE IN HOLSTEINS Contemporary Comparison (MCC) procedures (4). The PD computed before October 1974 were biased by genetic trend and could not be used to accurately estimate genetic change; PD were available for 87% of sires of males, 81% of sires of females, 68% of MGS of males, and 67% of MGS of females. Only 8% of the bulls born in 1960 through 1979 had PD. At the time of the study, PD were not available for bulls born in 1978 or later. ETA for female pedigree members were Cow Indexes (CI). The CI utilized in these analyses were calculated between January 1981 and July 1982 by the procedure described in (13); CI and PD were computed to the same genetic base. Pedigrees of 67% of the males and 59% of the females contained CI for dams. Fifty-three percent and 49% of the male and female offspring, respectively, had CI available for their MGD. Forty-seven percent of the cows born in the 20-yr had CI. Averages were computed within year of birth for PD of bulls, their sires and MGS, and CI of cows, dams, and MGD. (Bull and cow refer to the animals born during 1960 through 1979.) Averages of ancestor ETA, weighted by number of progeny, were calculated within sex of offspring. Trends in the yearly averages of PD and CI for production traits were determined by segmented regression techniques (6). Segmented regression analyses were utilized to account for two different linear time trends in the data. Plots of yearly averages for CI and PD of all pedigree members indicated when genetic merit of offspring and ancestors initially was affected by the availability of more accurate sire and cow evaluations. The year in which an increasing or decreasing trend in average ETA began was designated the join point. Join points for each segmented regression analysis were: I968 for bull PD, 1969 for cow CI, 1968 for sire PD, 1971 for dam CI, 1969 for MGS PD, and 1974 for MGD CI. The model for the segmented regression analyses of trends in each ancestor ETA was: ETAij = a + S i + b l i ( B Y j - 1 9 6 0 ) + b2i(Z)(BYj-JP) + eij
2631
where: ETAij is the weighted average of ancestor estimated transmitting ability for the ith sex of offspring in the jth birth year, a is the intercept, S i is the ith sex of offspring (male, female), BYj is the jth birth year of offspring (1960 to 1979),
Z
{1°
, within the ith sex,
for BYj~JP) was computed as (bli + b2i). The first regression coefficient, bli, is the trend over all 20 yr of birth while b2i represents the additional trend in the second time period. Separate segmented regression analyses estimated trends in cow CI and bull PD using the same model without S i. Annual genetic change (g) within sex of offspring was estimated as: ^
p,
= 3s + 3D
where flS is the regression of average sire PD(S) A
on birth year of son (daughter) and 3D is the regression of average dam CI (D) on birth year of son (daughter). The 3S and 3D were obtained
from the segmented regression analyses (Equation [5] ). This modification o f Method 4 made no assumption of equal change in genetic merit of sires and dams. Yearly trends in cow CI and bull PD were utilized to estimate annual genetic change within sex as: = 2fiETA
[51
[61
^
[7]
where flETA was the regression of average cow Journal of Dairy Science Vol. 68, No. 10, 1985
2632
LEE ET AL.
CI on birth year o f cow or regression of average bull PD on birth year of bull. Genetic trend estimates from Equations [6] and [71 were compared within sex of offspring.
.01 -a.,q \
~e
g
- .02
Trends in Genetic Merit of Ancestors
Trends in average PD milk (PDM) o f all sires of male and female progeny are illustrated in Figure 1. Genetic merit of sires remained relatively constant from 1960 to 1968. Steady increases in PDM of sires of male and female progeny occurced between 1968 and 1979 and were 5 39 and 450 kg, respectively. The positive change in genetic merit of sires was attributed primarily to changes in selection methods, which utilized the more accurate genetic evaluations available after 1967. Figure 1 also shows that modifications to sire evaluations in 1974 contributed additionally to the positive change in genetic merit of sires. As PDM of sires increased, a decline in sire PD fat percentage (PDF%) occurred (Figure 2). Relatively little change during the last 3 yr, however, m a y indicate a shift in emphasis on milk fat percentage. F r o m 1960 to 1979, overall changes in average PDF% of sires were only --.04 for male offspring and - . 0 6 for female progeny. If selection had been solely for milk production, the change in PDF% of sires, as the expected correlated response, would have
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RESULTS AND DISCUSSION
300
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68 70 72 74 76 78 BIRTHYEAR Figure 1. Trend in average Predicted Difference milk (PDM) of sires of male and female progeny (apD M = 270 kg). Journal of Dairy Science Vol. 68, No. 10, 1985
- .03 --
~>--~female
- o ~ ,
.04
60
62
64
66
68
70
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BIRTH YEAR
Figure 2. Trend in average Predicted Difference fat percent (PDF%) of sires of male and female progeny (aPDF% = .10).
been - . 1 3 and - . 1 2 for male and female progeny, respectively. Before 1971, average PDM of sires of females was larger than that of sires of bulls. Selection of male calves for registry may have been based on factors other than the apparent genetic merit for milk of their sires. Availability of accurate sire evaluations seemed to result in a modification of selection criteria for registry of males. Differences between average PDM of sires o f registered males and females, however, remained less than expected if registry of males had been based primarily on genetic merit for milk of parents. Average PD of sires of registered males could have been substantially larger because only 8 to 10% of all eligible males were registered annually. Figure 3 shows trends in CI milk (CIM) of dams. In contrast to sire PDM, average CIM of dams was larger for male progeny in all years. Yearly differences in CIM for dams of male and female offspring were approximately 45 kg before 1971. Larger genetic differences between dams of males and females in later years may have been influenced b y increased accuracy of CI computations and additional education on merits of using CI to select young bulls as service sires. As CIM of dams increased after 1971, average CI fat percentage (CIF%) of dams steadily declined (Figure 4). In all years, genetic merit for milk fat percentage was greater for dams of males. Dairy producers tended to register sons of dams with higher milk fat tests.
GENETIC CHANGE IN HOLSTEINS
2633
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Figure 4. Trend in average Cow Index fat percent (CIF%) of dams of male and female progeny (eCIF% = .07).
BIRTH YEAR Figure 3. Trend in average Cow Index milk (CIM) of dams of male and female progeny (aCl M = 190 kg).
TABLE 1. Estimates of annual change for estimated transmitting abilities of sires, dams, maternal grandsires (MGS), and maternal granddams (MGD) of male and female offspring. Ancestorsex of progeny
Period
Milk
Milk fat
Milk fat percent
(kg) SD Sires of mares
1960-- 1968 1968 - 1 9 7 9
Sires of females
1960--1968 1968--1979
Dams of males
X
SD
X
SD
1.18 49.86 ab
1.86 1.27
.14 1.44 ab
.09 .06
.0015 a --.0054 ab
.0005 .0003
3.55 38.59 ab
1.86 1.27
.00 1.10 ab
.09 .06
--.0020 a --.0044 ab
.0005 .0003
1960--1971 1971 --1979
--.01 19.32 ac
.45 .64
--.01 .55 ac
.01 .02
--.0001 --.0022 ac
.0001 .0001
Dams of females
1960--1971
--.91 a 16.18 ac
.45 .64
--.03 a .40 ac
.01 .02
.0000 --.0028 ac
.0001 .0001
MGS of males
1960--1969
1969--1979
--1.05 27.86 ad
1.41 1.27
--.08 a .84 ad
.04 .03
--.0006 --.0026 ad
.0003 .0003
MGS of females
1960--1969 1969--1979
1.27 23.55 ad
1.41 1.27
--.10 a .63 ad
.04 .03
--.0023 a --.0036 ad
.0003 .0003
MGD of males
1960--1974 1974--1979
--.91 a 10.09 ae
.18 .64
--.03 a .30 ae
.01 .02
.0000 a --.0010 ae
.0001 .0002
MGD of females
1960--1974
1.55 a 11.41 ae
.18 .64
.04 a .27 ae
.01 .02
.0003 a --.0022 ae
.0001 .0002
1971--1979
1974-- 1979
asignificantly different from zero (P<.05). bsignificantly different from estimates for 1960 to 1968 (P<.05). Csignificantly different from estimates for 1960 to 1971 (P<.05). dsignificantly different from estimates for 1960 to 1969 (P<.05). esignificantly different from estimates for 1960 to 1974 (P<.05).
Journal of Dairy Science Vol. 68, No. 10, 1985
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LEE ET AL.
Changes in average CI of dams lagged behind changes in average PD of sires. Similar results were reported by (17). Modifications in sire selection practices across years resulted in differences in rates of genetic change between the male and female population. Dentine and McDaniel (3) reported that several factors influence the time required to reach an equilibrium between genetic changes for sires and dams (i.e., flS =/3D)" Table 1 contains estimates of annual change in average PD of sires. Changes in sire PDF% during 1960 to 1968 were significant b u t in opposite directions for male and female progeny. Annual trends in PD of all three production traits were significant in the second time period. Estimates for rate of change in average CI of dams are in Table 1. Rates of change between 1 9 7 1 and 1979 were significantly different from zero for both sexes of offspring and for all production traits. Trends in CI of dams, however, were smaller than corresponding changes in PD of sires. Average PD of MGS changed little from 1960 to 1969 (Table 1). All estimates of change in MGS PD were significant in the second period. Table 1 contains rates of change for CI of MGD. The second time segment began in 1974. Length of generation interval and lag time for
changes in sire selection to affect genetic merit of females influenced trends in genetic merit of MGD. Trends in Estimated Transmitting Ability of Male and Female Offspring
Average PDM of bulls and average CIM of cows are in Figure 5. Average PDM of bulls born between 1960 and 1968 were approximately 100 kg. Genetic merit of bulls began to increase in 1968, whereas increases in average CIM began for cows born in 1969. Changes in genetic merit for milk production were larger for males. This was expected because of the larger selection differentials for parents of males. The decline in average PDF% of sons was largest from 1968 to 1 9 7 2 (Figure 6). After 1972, average PDF% of bulls generally fluctuated around -.03%. The CIF% of cows born after 1969 decreased more steadily and to a larger extent than PDF% of bulls born in corresponding years. Estimates of change in PD of bulls were nonsignificant for 1960 to 1968 (Table 2). Yearly trends in bull PD after 1968 were significantly different from zero. For the second period, average PDM of bulls increased 41.9 kg/yr while PDF% of males declined. Rates of change in the genetic merit of females were less than those of males (Table 2).
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68 70 72 BIRTH YEAR
7
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8
Figure 5. Trends in Predicted Difference Milk (PDM) of males and Cow Index milk (CIM) of females born in 1960 through 1979 (opDM = 225 kg, aCiM = 180 kg). Journal of Dairy Science Vol. 68, No. 10, 1985
--
.64
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Figure 6. Trends in Predicted Difference fat percent (PDF%) of males and Cow Index fat percent (CIF%) of females born in 1960 through 1979 (OPDF% = CrCiF%= .07).
J
GENETIC CHANGE IN HOLSTEINS
2635
TABLE 2. Estimates of annual change in Predicted Differences (PD) of bulls and Cow Indexes of cows. Sex
Period
Milk
Milk fat
Milk fat percent
(kg) R
SD
X
SD
Bulls
1960--1968 1968--1977'
1.27 1.14 41.86ab 1.00
--.02 1.31ab
.05 .05
--.0010 .0005 --.0031 ab .0004
Cows
1960--1969 1969--1979
25.77 ab
1.05 .95
--.01 .71ab
.04 .04
--.0006 .0002 --.0033 ab .0002
.77
~
SD
1No PD's for bulls born in 1978 and 1979. asignificantly different from zero (P<.05). bsignificantlydifferent from estimate in first period (P<.05).
Genetic Trend Estimates
Annual genetic change in the male and female populations of registered Holsteins was estimated by two methods (Equations [6] and [7] ). Yearly trends for PD of sires and CI of dams (Table 1) were utilized in Equation [6]. Estimates of change in average cow CI (Table 2) were used to compute ~ by Equation [7]. Both methods yielded similar results for the female population (Table 3). Estimates of genetic change for the male population (Equations [6] and [7]) are presented in Table 4. To compare the two methods more accurately, the same population needed to be used for both g estimation procedures. A substantially small percentage of registered bulls (approximately 8%) actually had sufficient production-tested daugbters for PD to be calculated. Consequently, trends for PD of sires
and CI of dams were recomputed only for bulls that had PD. Estimates of annual change in sire PD for bulls with PD were 4.79 + 2.27 kg PDM, .35 -+ .08 kg PDF, and .0026 ± .0007 PDF% in 1960 to 1968 and 66.12 -+ 1.98 kg PDM, 1.88 ± .07 kg PDF, and --.0076 -+ .0006 PDF% in 1968 to 1977. Trends in CI of dams (3D) were 1.35 ± 1.33 kg CIM, - . 0 4 ± .05 kg CIF, and - . 0 0 1 4 -+ .0002 CIF% in 1960 to 1971 and were 34.25 ± 2.64 kg CIM, 1.11 -+ .10 kg CIF, and - . 0 0 2 0 ± .0003 CIF% in 1971 to 1977. These values were used in Equation [6] and yielded the resuhs in the upper half of Table 4. Estimates of ~ for bulls did not agree closely between methods. The ~ for milk obtained by Equation [6] (i.e., /iS + 3D) for 1971 to 1977 was approximately 84% as large as the corresponding ~ for milk computed by doubling the
TABLE 3. Estimates of annual genetic change (~) in the female population. Period
Milk
Milk fat
Milk fat percent
X --.03 1.50 --.02 1.43
R -.0020 --.0072 --.0012 --,0066
(kg) 1960-19681 1971--1979 z 1960--19692
1969--1979 ~
R 2.64 54.77 1.55 51.55
SD 1.91 1.41 2.09 1.91
SD .09 .06 .08 .07
SD .0005 .0004 .0004 .0004
Computed by summing trends in sire Predicted Difference and dam Cow Index (CI) (Equation [6] ). 2Computed by doubling trend in cows' average CI (Equation 17] ). Journal of Dairy Science Vol. 68, No. 10, 1985
2636
LEE ET AL.
TABLE 4. Estimates of annual genetic change (g) in the male population. Period
Milk
Milk fat
Milk fat percent
(kg) 1960--19681 1971--19771 1960--19682 1968--19772
6,14 100.36 2.55 83.73
SD 2.63 3.30 2.27 2.00
X .31
SD .09 2.99 .12 --.04 .10 2.63 .09
X .0012 --.0096 --.0020 --.0062
SD .0007 .0007 .0010 .0008
1Computed by summing trends in sire Predicted Difference (PD) and dam Cow Index (Equation [6] ). 2Computed by doubling trend in bulls' average PD (Equation [7] ). trend in PDM of bulls (Equation [7]). The discrepancy might be explained by the less than perfect effect of pedigree information used to predict PDM (15). The ~ from Equation [6] also were computed for the male population regardless of the availability of PD of the bulls. Values from Table 1 yielded ~'s of 1.09 -+ 1.19 kg milk, .13 + .09 kg milk fat, and .0014 + .0005 milk fat percentage for 1 9 6 0 - 1 9 6 8 and o f 69.18 + 1.41 kg milk, 1.99 + .06 kg milk fat, and - . 0 0 7 6 + .0004 milk fat percentage for 1 9 7 1 - 1 9 7 9 . If each animal had a PD or CI, Equation [7] would be the more accurate method for estimating annual genetic change. Values from Equation [6] depend on the accuracy with which sire and dam information predicts genetic merit of offspring. Inaccuracies in ~ estimation by Equation [7] will occur if the animals that have PD or CI are not representative of the population being studied. Differences between trends in ETA of parents of all registered bulls and registered bulls with PD indicated that bulls with PD were not representative of the registered bull population. Larger annual trends in genetic merit of parents were found for registered bulls that had PD. The CI were available for a large proportion of the female population. Cows with CI were assumed representative of the female population. Substantial differences existed between ~ for the first and second periods. Powell et al. (16) also reported larger ~ for milk and milk fat percentage in the latter time period of their study. Their ~ for 1968 to 1975 were slightly less than ~ for the second time segment of this study. Journal of Dairy Science Vol. 68, No. 10, 1985
Estimates of ~ from this study indicated that little genetic change occurred in the registered Holstein population before 1968. Larger genetic trend estimates were reported by (2,8,9). Their data included only cows calving before 1968. Estimates of ~ in those studies were based on regressions of daughter performance on time of calving. Adjustments may not have completely accounted for environmental trend and biases due to nonrandom samples of dams. Annual genetic change in milk yield increased substantially after the availability of more accurate CI and PD in 1968 and later. The increase in ~ for milk, however, was considerably smaller than was possible. Dairy farmers evidently placed some emphasis on milk fat percentage and type. Less selection pressure on secondary traits may have been warranted economically, since the major source of income on most dairy farms is from the sale of milk. Generation Interval
In this study, ~ estimation accounted for generation interval by regression of ETA on birth year. Generation intervals, however, were calculated to determine if trends or changes occurred over the 20-yr span. Averages for ages of sires and dams at time of offspring birth are in Table 5. Generation intervals for sire-son increased steadily between 1960 and 1979. The regression of average sire-son generation interval on birth year was 1.93 + .09 mo. The trend toward registering sons of older sires suggests that dairy producers became more concerned about the repeatabilities of sires' proofs. Some genetic gain was sacrificed by requiring that PD
GENETIC CHANGE IN HOLSTEINS
26 37
TABLE 5. Means and standard deviations for ages of sires and dams at time of birth of their registered Holstein offspring. Generation interval (mo) Birth year
1960 1961 1962 1963 1964 1965 1966 1967
1968 1969 1970 1971 1972 1973 1974 1975 1976
1977 1978 1979
Sires Sons
75 77 79 80 81 83 84 86 85 88 89 92 97 100 100 104 107 110 111 107
Dams Daughters
Sons
Daughters
SD
X
SD
X
SD
X
SD
41 40 40 40 41 41 41 41 40 41 40 39 40 42 43 44 45 46 47 47
78 78 78 80 81 81 81 83 82 82 84 85 86 87 85 86 88 90 91 89
42 42 41 42 42 42 41 41 40 41 41 41 42 43 43 44 45 44 44 42
67 68 69
32 32 32 32 32 32 31 31 31 30 30 30 30 30 30 30 30 30 30 30
56 57 58 58 58 58 57 57 57 57 57 57 56 56 56 56 57 56 56 56
27 28 28 28 28 28 28 27 27 27 27 27 27 27 27 26 27 27 27 27
of sires reach a relatively high repeatability before their sons were registered and progeny tested. The annual trend in generation interval for sire-danghter (.65 + .03 mo) was smaller than for sires and sons. Average generation intervals for dam-son and dam-daughter r e m a i n e d relatively c o n s t a n t over the 20-yr. Average age o f dams o f registered sons was a p p r o x i m a t e l y 12 m o greater than the age o f dams o f registered daughters. Dairy producers t e n d e d to register bull calves o f cows t h a t had additional p r o d u c t i o n records. Summary The availability o f m o r e accurate genetic evaluations (PD and CI) in 1968 c o n t r i b u t e d substantially to the increase in E T A for milk yield o f males and females. A t i m e lag existed b e t w e e n the first change in PD of sires (1968) and the initial change in CI o f dams (1971). In addition, rates o f change in PD of sires and CI o f dams were n o t equal. The smaller rate of
69
68 68 68 67 68 68 67 67 67 66 67 67 66
67 66
65
increase for CIM o f dams p r o b a b l y was due to nonlinear genetic change in sires caused by changes in sire selection goals. A n n u a l genetic change (~) for the male and female populations was estimated by two methods. A n n u a l trends in PD o f sires and CI of dams were added t o g e t h e r to estimate g. In addition, ~ were o b t a i n e d by doubling the trends in the animals' o w n average ETA. Both m e t h o d s yielded similar results in the female p o p u l a t i o n (Table 3). In the male population, smaller estimates were o b t a i n e d by the m e t h o d of doubling the rate of change in the bulls' average PD (Table 4). This m e t h o d ( E q u a t i o n [7] ) should yield m o r e accurate ~ w h e n animals with E T A are k n o w n to be representative o f the entire population. ACKNOWLEDGMENTS The authors appreciate t h e support f r o m Grant I-3-79 of the US-Israel Binational Agricultural and D e v e l o p m e n t Project for partial support for c o m p u t i n g costs. Journal of Dairy Science Vol. 68, No. 10, 1985
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LEE ET AL.
REFERENCES 1 Batra, T. R. 1979. Genetic trends for milk and fat production in dairy cattle. Can. J. Anita. Sci. 59:203. 2 Burnside, E. B., and J. E. Legates. 1967. Estimation of genetic trends in dairy cattle populations. J. Dairy Sci. 50:1448. 3 Dentine, M. R., and B. T. McDaniel. 1982. A matrix approach to prediction of rates of genetic gain from selection for milk yield in dairy cattle. J. Dairy Sci. 65(Suppl. 1):96. (Abstr.) 4 Dickinson, F. N., R. L. Powell, and H. D. Norman. 1976. An introduction to the USDA-DHIA Modified Contemporary Comparison. Page 1 in The USDA-DHIA Modified Contemporary Comparison. US Dep. Agric. Prod. Res. Rep: 165. 5 Everett, R. W., C. E. Meadows, and J. L. Gill. 1967. Estimation of genetic trends in simulated data. J. Dairy Sci. 50:550. 6 Fuller, W. A. 1969. Grafted polynomials as approximating functions. Aust. J. Agric. Econ. 13:35. 7 Hahn, E. W., J. L. Cartoon, and W. J. Miller. 1958. An intraherd contemporary comparison of the production of artificially and naturally sired dairy cows in Georgia. J. Dairy Sci. 41:1061. 8 Hargrove, G. L., and J. E. Legates. 1971. Biases in dairy sire evaluation attributable to genetic trend and female selection. J. Dairy Sci. 54:1041. 9 Harville, D. A., and C. R. Henderson. 1967. Environmental and genetic trends in production and their effects on sire evaluation. J. Dairy Sci. 50: 870. 10 Hintz, R. L., R. W. Everett, and L. D. Van Vleck. 1978. Estimation of genetic trends from cow and sire evaluations. J. Dairy Sci. 61:607. 11 Kennedy, B. W., and J. E. Moxley. 1975. Genetic
Journal of Dairy Science Vol. 68, No. 10, 1985
12 13 14 15
16 17 18
19
20
21 22
trends among artificially bred Holsteins in Quebec. J. Dairy Sci. 58:1871. McDaniel, B. T., and G. J. King. 1974. Milk and fat differences for registered cows sired by artificial and natural insemination. J. Dairy Sci. 57:112. Powell, R. L. 1978. A procedure for including the dam and maternal grandsire in USDA-DHIA Cow Indexes. J. Dairy. Sci. 61:794. Powell, R. L., and A. E. Freeman. 1974. Genetic trend estimators. J. Dairy. Sci. 57:1067. Powell, R. L., H. D. Norman, and F. N. Dickinson. 1977. Relationships between bulls' pedigree indexes and daughters performance in the modified contemporary comparison. J. Dairy Sci. 60:961. Powell, R. L., H. D. Norman, and F. N. Dickinson. 1977. Trends in breeding value and production. J. Dairy Sci. 60:1316. Schaeffer, L. R. 1983. Effectiveness of model for cow evaluation intraherd. J. Dairy Sci. 66:874. Schaeffer, L. R., M. G. Freeman, and E. B. Burnside. 1975. Evaluation of Ontario Holstein dairy sires for milk and fat production. J. Dairy Sci. 58:109. Smith, C. 1962. Estimation of genetic change in farm livestock using field records. Anita. Prod. 4:239. Tucker, W. L., J. E. Legates, and B. R. Farthing. 1960. Genetic improvement in production attributable to sires used in artificial insemination in North Carolina. J. Dairy Sci. 43:982. Van Vleck, L. D., and C. R. Henderson. 1961. Measurement of genetic trend. J. Dairy Sci. 44: 1705. Wadell, L. H., and L. D. McGilliard. 1959. Influence of artificial breeding on production in Michigan dairy herds. J. Dairy Sci. 42:1079.
Department of Animal Science Iowa State University Ames 50011 L. P. JOHNSON
Holstein Association Brattleboro, VT 05301
ABSTRACT
change resulted from doubling the rate o f change in the bulls' average Predicted Differences. This method should yield more accurate estimates of genetic change when bulls with genetic evaluations are representative o f the population. Estimates of annual genetic change for 1971 to 1979 from the latter method were 84 kg milk, 2.6 kg milk fat, and - . 0 0 6 milk fat percentage.
We examined pedigrees of 440,702 males and 526,956 females born during 1960 through 1979 and registered in the herdbook of the Holstein Association. Annual trends in genetic merit o f sires, dams, maternal grandsires, and maternal granddams were determined from their estimated transmitting abilities. Genetic merit of parents changed substantially after more accurate procedures for estimating transmitting abilities were introduced in 1968. Rates of change for genetic merit of sires were larger than corresponding changes in dams. Annual genetic change for the male and female populations was estimated by two methods. The sum of annual changes in the average estimated transmitting abilities of sires and dams yielded estimates of annual genetic change. In addition, estimates of annual genetic change were obtained by doubling trends in the animals' average estimated transmitting abilities. Both methods yielded similar results in the female population. For 1971 to 1979, annual genetic change for females was approximately +55 kg milk, +1.5 kg milk fat, and - . 0 0 7 milk fat percentage. In the male population, estimates differed between methods. Smaller estimates of annual genetic
INTRODUCTION
The effectiveness o f dairy breeding programs is measured through annual genetic change. Several early genetic trend studies dealt with comparisons between milk production of artificial insemination (AI) and natural service progeny (7,12,20,21,22). Differences between the weighted averages of AI and non-AI daughters generally were slightly in favor o f AI progeny. Differences between AI and non-AI cows may underestimate the influence of AI bulls because non-Al bulls may themselves be sired by A[ bulls. Smith (19) proposed several methods for estimating annual genetic change (~) based on the regression of daughter performance on time. Three methods are:
Received December 3, 1984. 1Journal Paper No. J-11694 of the Iowa Agriculture and Home Economics Experiment Station, Ames. Project No. 1053. 2Department of Dairy Science, Virginia Polytechnic Institute and State University, Blacksburg 24061. 1985 J Dairy Sci 68:2629-2638
= 2(bp. T -- b p . T / S )
[11
= - 2 ( b ( p _ p ) . T/S )
[21
= bp. T -- bp. T/SD
[3]
Regression estimates are represented by b. P is performance of a cow, P is the average performance of her herdmates, S is sire of the cow, D is dam of the cow, and T is time of cow calving. Equations [1] and [2] were modified to account for age of dam or progressive culling
2629
2630
LEE ET AL.
of dams (5,19). In addition, adjustments made to Equations [1] and [2] accounted for older sires being mated to genetically superior cows (9,14). Burnside and Legates (2) developed a method to adjust first-lactation records of first-born fuI1 sisters for favorable environmental effects that bias estimates of g from Equation [3]. More recently, regressions of sires' average genetic merit on time have been used to estimate g (1,10,11,14,16,18). Yearly change in average sire transmitting ability, weighted by the number of offspring, represents half the change in genetic merit of the population. If genetic trends in the dams and sires are equal: = 2bsG. T
[41
where bSG .T is the regression of sires' average transmitting ability estimate (SG) on time (T). T was defined as year of daughter calving in most genetic trend studies. To simplify the review of previously reported estimates of g, the various modifications of Equations [1], [2], [3], and [4] will be referred to as Methods 1, 2, 3, and 4, respectively. Estimates of genetic change from the previous studies, however, are difficult to compare because different populations and time segments were investigated. An extensive investigation by Powell and Freeman (14) included ~ from modifications of Methods 1, 2, 3, and 4. Data were production records initiated between 1957 and 1969. Estimates of ~ from the various methods ranged from 49.2 -+ 2.4 kg milk (Method 4) to 133.4 -+ 54.7 kg milk (Method 3); and - . 4 7 -+ .09 kg milk fat (Method 4) to 5.07 ± 1.91 kg milk fat (Method 3). Powell et aI. (16) used more than 3 million records initiated in 1960 to 1975 to estimate by Method 4. Estimates of annual genetic change computed from trends in average estimated breeding value of sires were 18 kg milk, .3 kg milk fat, and - . 0 0 5 9 milk fat percentage for 1960 to 1975. For 1968 to 1975, ~ were 38 kg milk, .7 kg milk fat, and - . 0 1 0 6 milk fat percentage. Yearly genetic changes in milk estimated from changes in average estimated breeding values of the cows themselves were 8 kg in 1960 to 1975 and 21 kg in 1968 to 1975. Journal of Dairy Science Vol. 68, No. 10, 1985
Hintz et al. (10) reported estimates of genetic change in Holsteins in the northeastern US. When estimates of change in average genetic merit of cows were used in Method 4, was 26.1 kg milk for Al-sired cows compared with 35.8 kg milk when the trend in average sire transmitting ability was used to estimate ~. The latter approach is not valid when rates of genetic change for sires and dams are not equal. Several researchers have estimated genetic trend in Canadian Holsteins (1,11,18). Estimates of g ranged from 38 to 44 kg milk and 1.1 to 1.6 kg fat. Generation interval is an important component in the prediction and estimation of annual genetic change. The four described methods of estimating genetic change are dependent on the regressions of performance or genetic merit on time (year of birth, month of calving, etc.). Consequently, direct calculations of generation interval are unnecessary in estimating ~ from these methods. Current estimates of generation intervals, however, would be useful in chaxacterizing the dairy cattle population. The objectives of this study were 1) to estimate annual genetic change in milk, milk fat, and milk fat percentage and 2) to calculate generation intervals in the registered Holstein cattle population born in the United States during 1960 through 1979. MATERIALS AND METHODS
Pedigrees of 440,702 registered Holstein males and 526, 956 registered Holstein females were provided by the Holstein Association. The animals were born between 1960 and 1979, inclusive. The data comprised all registered males and a random 10% of the registered females born in the 20-yr span. Pedigree information consisted of estimated transmitting abilities (ETA) for milk, milk fat, and milk fat percentage of the animals and their sires, dams, maternal grandsires (MGS), and maternal granddams (MGD). Pedigrees contained the most current information available in August 1982. Availability of ETA for all pedigree members varied. Birth dates of all animals and their ancestors were included and used to calculate generation intervaJs over the 20-yr period. Male ETA for production traits were Predicted Differences (PD) computed by Modified
GENETIC CHANGE IN HOLSTEINS Contemporary Comparison (MCC) procedures (4). The PD computed before October 1974 were biased by genetic trend and could not be used to accurately estimate genetic change; PD were available for 87% of sires of males, 81% of sires of females, 68% of MGS of males, and 67% of MGS of females. Only 8% of the bulls born in 1960 through 1979 had PD. At the time of the study, PD were not available for bulls born in 1978 or later. ETA for female pedigree members were Cow Indexes (CI). The CI utilized in these analyses were calculated between January 1981 and July 1982 by the procedure described in (13); CI and PD were computed to the same genetic base. Pedigrees of 67% of the males and 59% of the females contained CI for dams. Fifty-three percent and 49% of the male and female offspring, respectively, had CI available for their MGD. Forty-seven percent of the cows born in the 20-yr had CI. Averages were computed within year of birth for PD of bulls, their sires and MGS, and CI of cows, dams, and MGD. (Bull and cow refer to the animals born during 1960 through 1979.) Averages of ancestor ETA, weighted by number of progeny, were calculated within sex of offspring. Trends in the yearly averages of PD and CI for production traits were determined by segmented regression techniques (6). Segmented regression analyses were utilized to account for two different linear time trends in the data. Plots of yearly averages for CI and PD of all pedigree members indicated when genetic merit of offspring and ancestors initially was affected by the availability of more accurate sire and cow evaluations. The year in which an increasing or decreasing trend in average ETA began was designated the join point. Join points for each segmented regression analysis were: I968 for bull PD, 1969 for cow CI, 1968 for sire PD, 1971 for dam CI, 1969 for MGS PD, and 1974 for MGD CI. The model for the segmented regression analyses of trends in each ancestor ETA was: ETAij = a + S i + b l i ( B Y j - 1 9 6 0 ) + b2i(Z)(BYj-JP) + eij
2631
where: ETAij is the weighted average of ancestor estimated transmitting ability for the ith sex of offspring in the jth birth year, a is the intercept, S i is the ith sex of offspring (male, female), BYj is the jth birth year of offspring (1960 to 1979),
Z
{1°
, within the ith sex,
for BYj~
p,
= 3s + 3D
where flS is the regression of average sire PD(S) A
on birth year of son (daughter) and 3D is the regression of average dam CI (D) on birth year of son (daughter). The 3S and 3D were obtained
from the segmented regression analyses (Equation [5] ). This modification o f Method 4 made no assumption of equal change in genetic merit of sires and dams. Yearly trends in cow CI and bull PD were utilized to estimate annual genetic change within sex as: = 2fiETA
[51
[61
^
[7]
where flETA was the regression of average cow Journal of Dairy Science Vol. 68, No. 10, 1985
2632
LEE ET AL.
CI on birth year o f cow or regression of average bull PD on birth year of bull. Genetic trend estimates from Equations [6] and [71 were compared within sex of offspring.
.01 -a.,q \
~e
g
- .02
Trends in Genetic Merit of Ancestors
Trends in average PD milk (PDM) o f all sires of male and female progeny are illustrated in Figure 1. Genetic merit of sires remained relatively constant from 1960 to 1968. Steady increases in PDM of sires of male and female progeny occurced between 1968 and 1979 and were 5 39 and 450 kg, respectively. The positive change in genetic merit of sires was attributed primarily to changes in selection methods, which utilized the more accurate genetic evaluations available after 1967. Figure 1 also shows that modifications to sire evaluations in 1974 contributed additionally to the positive change in genetic merit of sires. As PDM of sires increased, a decline in sire PD fat percentage (PDF%) occurred (Figure 2). Relatively little change during the last 3 yr, however, m a y indicate a shift in emphasis on milk fat percentage. F r o m 1960 to 1979, overall changes in average PDF% of sires were only --.04 for male offspring and - . 0 6 for female progeny. If selection had been solely for milk production, the change in PDF% of sires, as the expected correlated response, would have
# ~/d/~
500
400
~
,/
e---~o female
200
/ .
o ''~/
D...O~f .O...o--O--
-100 ~ , I 60 62
,
I
64
,
I
L
I
,
I
,
'~.~
~- - .01
RESULTS AND DISCUSSION
300
0
I
J
1
,
J
66
,
i
,
68 70 72 74 76 78 BIRTHYEAR Figure 1. Trend in average Predicted Difference milk (PDM) of sires of male and female progeny (apD M = 270 kg). Journal of Dairy Science Vol. 68, No. 10, 1985
- .03 --
~>--~female
- o ~ ,
.04
60
62
64
66
68
70
72
74
76
78
BIRTH YEAR
Figure 2. Trend in average Predicted Difference fat percent (PDF%) of sires of male and female progeny (aPDF% = .10).
been - . 1 3 and - . 1 2 for male and female progeny, respectively. Before 1971, average PDM of sires of females was larger than that of sires of bulls. Selection of male calves for registry may have been based on factors other than the apparent genetic merit for milk of their sires. Availability of accurate sire evaluations seemed to result in a modification of selection criteria for registry of males. Differences between average PDM of sires o f registered males and females, however, remained less than expected if registry of males had been based primarily on genetic merit for milk of parents. Average PD of sires of registered males could have been substantially larger because only 8 to 10% of all eligible males were registered annually. Figure 3 shows trends in CI milk (CIM) of dams. In contrast to sire PDM, average CIM of dams was larger for male progeny in all years. Yearly differences in CIM for dams of male and female offspring were approximately 45 kg before 1971. Larger genetic differences between dams of males and females in later years may have been influenced b y increased accuracy of CI computations and additional education on merits of using CI to select young bulls as service sires. As CIM of dams increased after 1971, average CI fat percentage (CIF%) of dams steadily declined (Figure 4). In all years, genetic merit for milk fat percentage was greater for dams of males. Dairy producers tended to register sons of dams with higher milk fat tests.
GENETIC CHANGE IN HOLSTEINS
2633
010 180
D----Dmale o----o female
150
/
/,/o,/
,005 ~
0
120 m
90
-
~"ta
.005
~ - .010 N -~
-
-
b
-.025
,
60 i
I
,
I
~
I
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t
60 62 64 66 68 70 72 74 76 78
q,
,020
-30 1
b"~"~"b'b~'%~
c----o female
..oy /o" 0 .o" "'~0-~0--0--0--0"-0..0.~0__0__0-~0" i
- - male
015
h
62
,
~
64
,
~
,
66
,
~
,
t
,
i
68 70 72 BIRTHYEAR
,
B
,
74
~
,
76
~
,
78
Figure 4. Trend in average Cow Index fat percent (CIF%) of dams of male and female progeny (eCIF% = .07).
BIRTH YEAR Figure 3. Trend in average Cow Index milk (CIM) of dams of male and female progeny (aCl M = 190 kg).
TABLE 1. Estimates of annual change for estimated transmitting abilities of sires, dams, maternal grandsires (MGS), and maternal granddams (MGD) of male and female offspring. Ancestorsex of progeny
Period
Milk
Milk fat
Milk fat percent
(kg) SD Sires of mares
1960-- 1968 1968 - 1 9 7 9
Sires of females
1960--1968 1968--1979
Dams of males
X
SD
X
SD
1.18 49.86 ab
1.86 1.27
.14 1.44 ab
.09 .06
.0015 a --.0054 ab
.0005 .0003
3.55 38.59 ab
1.86 1.27
.00 1.10 ab
.09 .06
--.0020 a --.0044 ab
.0005 .0003
1960--1971 1971 --1979
--.01 19.32 ac
.45 .64
--.01 .55 ac
.01 .02
--.0001 --.0022 ac
.0001 .0001
Dams of females
1960--1971
--.91 a 16.18 ac
.45 .64
--.03 a .40 ac
.01 .02
.0000 --.0028 ac
.0001 .0001
MGS of males
1960--1969
1969--1979
--1.05 27.86 ad
1.41 1.27
--.08 a .84 ad
.04 .03
--.0006 --.0026 ad
.0003 .0003
MGS of females
1960--1969 1969--1979
1.27 23.55 ad
1.41 1.27
--.10 a .63 ad
.04 .03
--.0023 a --.0036 ad
.0003 .0003
MGD of males
1960--1974 1974--1979
--.91 a 10.09 ae
.18 .64
--.03 a .30 ae
.01 .02
.0000 a --.0010 ae
.0001 .0002
MGD of females
1960--1974
1.55 a 11.41 ae
.18 .64
.04 a .27 ae
.01 .02
.0003 a --.0022 ae
.0001 .0002
1971--1979
1974-- 1979
asignificantly different from zero (P<.05). bsignificantly different from estimates for 1960 to 1968 (P<.05). Csignificantly different from estimates for 1960 to 1971 (P<.05). dsignificantly different from estimates for 1960 to 1969 (P<.05). esignificantly different from estimates for 1960 to 1974 (P<.05).
Journal of Dairy Science Vol. 68, No. 10, 1985
2634
LEE ET AL.
Changes in average CI of dams lagged behind changes in average PD of sires. Similar results were reported by (17). Modifications in sire selection practices across years resulted in differences in rates of genetic change between the male and female population. Dentine and McDaniel (3) reported that several factors influence the time required to reach an equilibrium between genetic changes for sires and dams (i.e., flS =/3D)" Table 1 contains estimates of annual change in average PD of sires. Changes in sire PDF% during 1960 to 1968 were significant b u t in opposite directions for male and female progeny. Annual trends in PD of all three production traits were significant in the second time period. Estimates for rate of change in average CI of dams are in Table 1. Rates of change between 1 9 7 1 and 1979 were significantly different from zero for both sexes of offspring and for all production traits. Trends in CI of dams, however, were smaller than corresponding changes in PD of sires. Average PD of MGS changed little from 1960 to 1969 (Table 1). All estimates of change in MGS PD were significant in the second period. Table 1 contains rates of change for CI of MGD. The second time segment began in 1974. Length of generation interval and lag time for
changes in sire selection to affect genetic merit of females influenced trends in genetic merit of MGD. Trends in Estimated Transmitting Ability of Male and Female Offspring
Average PDM of bulls and average CIM of cows are in Figure 5. Average PDM of bulls born between 1960 and 1968 were approximately 100 kg. Genetic merit of bulls began to increase in 1968, whereas increases in average CIM began for cows born in 1969. Changes in genetic merit for milk production were larger for males. This was expected because of the larger selection differentials for parents of males. The decline in average PDF% of sons was largest from 1968 to 1 9 7 2 (Figure 6). After 1972, average PDF% of bulls generally fluctuated around -.03%. The CIF% of cows born after 1969 decreased more steadily and to a larger extent than PDF% of bulls born in corresponding years. Estimates of change in PD of bulls were nonsignificant for 1960 to 1968 (Table 2). Yearly trends in bull PD after 1968 were significantly different from zero. For the second period, average PDM of bulls increased 41.9 kg/yr while PDF% of males declined. Rates of change in the genetic merit of females were less than those of males (Table 2).
400
.01
3OO
0
b--~'x~" = = mace c>--<~ female
2OO
]00
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01
\ V r ~ , ~ . .k --.~ . ~.~..
.02
~ - .03 o---~0 female
-1~
62
64
66
68 70 72 BIRTH YEAR
7
76
8
Figure 5. Trends in Predicted Difference Milk (PDM) of males and Cow Index milk (CIM) of females born in 1960 through 1979 (opDM = 225 kg, aCiM = 180 kg). Journal of Dairy Science Vol. 68, No. 10, 1985
--
.64
--
.05
,
60
I
62
,
k
64
i
I
66
L
I
68
i
I
i
70
t
72
i
I
74
i
L
76
i
I
78
BIRTHY[AR
Figure 6. Trends in Predicted Difference fat percent (PDF%) of males and Cow Index fat percent (CIF%) of females born in 1960 through 1979 (OPDF% = CrCiF%= .07).
J
GENETIC CHANGE IN HOLSTEINS
2635
TABLE 2. Estimates of annual change in Predicted Differences (PD) of bulls and Cow Indexes of cows. Sex
Period
Milk
Milk fat
Milk fat percent
(kg) R
SD
X
SD
Bulls
1960--1968 1968--1977'
1.27 1.14 41.86ab 1.00
--.02 1.31ab
.05 .05
--.0010 .0005 --.0031 ab .0004
Cows
1960--1969 1969--1979
25.77 ab
1.05 .95
--.01 .71ab
.04 .04
--.0006 .0002 --.0033 ab .0002
.77
~
SD
1No PD's for bulls born in 1978 and 1979. asignificantly different from zero (P<.05). bsignificantlydifferent from estimate in first period (P<.05).
Genetic Trend Estimates
Annual genetic change in the male and female populations of registered Holsteins was estimated by two methods (Equations [6] and [7] ). Yearly trends for PD of sires and CI of dams (Table 1) were utilized in Equation [6]. Estimates of change in average cow CI (Table 2) were used to compute ~ by Equation [7]. Both methods yielded similar results for the female population (Table 3). Estimates of genetic change for the male population (Equations [6] and [7]) are presented in Table 4. To compare the two methods more accurately, the same population needed to be used for both g estimation procedures. A substantially small percentage of registered bulls (approximately 8%) actually had sufficient production-tested daugbters for PD to be calculated. Consequently, trends for PD of sires
and CI of dams were recomputed only for bulls that had PD. Estimates of annual change in sire PD for bulls with PD were 4.79 + 2.27 kg PDM, .35 -+ .08 kg PDF, and .0026 ± .0007 PDF% in 1960 to 1968 and 66.12 -+ 1.98 kg PDM, 1.88 ± .07 kg PDF, and --.0076 -+ .0006 PDF% in 1968 to 1977. Trends in CI of dams (3D) were 1.35 ± 1.33 kg CIM, - . 0 4 ± .05 kg CIF, and - . 0 0 1 4 -+ .0002 CIF% in 1960 to 1971 and were 34.25 ± 2.64 kg CIM, 1.11 -+ .10 kg CIF, and - . 0 0 2 0 ± .0003 CIF% in 1971 to 1977. These values were used in Equation [6] and yielded the resuhs in the upper half of Table 4. Estimates of ~ for bulls did not agree closely between methods. The ~ for milk obtained by Equation [6] (i.e., /iS + 3D) for 1971 to 1977 was approximately 84% as large as the corresponding ~ for milk computed by doubling the
TABLE 3. Estimates of annual genetic change (~) in the female population. Period
Milk
Milk fat
Milk fat percent
X --.03 1.50 --.02 1.43
R -.0020 --.0072 --.0012 --,0066
(kg) 1960-19681 1971--1979 z 1960--19692
1969--1979 ~
R 2.64 54.77 1.55 51.55
SD 1.91 1.41 2.09 1.91
SD .09 .06 .08 .07
SD .0005 .0004 .0004 .0004
Computed by summing trends in sire Predicted Difference and dam Cow Index (CI) (Equation [6] ). 2Computed by doubling trend in cows' average CI (Equation 17] ). Journal of Dairy Science Vol. 68, No. 10, 1985
2636
LEE ET AL.
TABLE 4. Estimates of annual genetic change (g) in the male population. Period
Milk
Milk fat
Milk fat percent
(kg) 1960--19681 1971--19771 1960--19682 1968--19772
6,14 100.36 2.55 83.73
SD 2.63 3.30 2.27 2.00
X .31
SD .09 2.99 .12 --.04 .10 2.63 .09
X .0012 --.0096 --.0020 --.0062
SD .0007 .0007 .0010 .0008
1Computed by summing trends in sire Predicted Difference (PD) and dam Cow Index (Equation [6] ). 2Computed by doubling trend in bulls' average PD (Equation [7] ). trend in PDM of bulls (Equation [7]). The discrepancy might be explained by the less than perfect effect of pedigree information used to predict PDM (15). The ~ from Equation [6] also were computed for the male population regardless of the availability of PD of the bulls. Values from Table 1 yielded ~'s of 1.09 -+ 1.19 kg milk, .13 + .09 kg milk fat, and .0014 + .0005 milk fat percentage for 1 9 6 0 - 1 9 6 8 and o f 69.18 + 1.41 kg milk, 1.99 + .06 kg milk fat, and - . 0 0 7 6 + .0004 milk fat percentage for 1 9 7 1 - 1 9 7 9 . If each animal had a PD or CI, Equation [7] would be the more accurate method for estimating annual genetic change. Values from Equation [6] depend on the accuracy with which sire and dam information predicts genetic merit of offspring. Inaccuracies in ~ estimation by Equation [7] will occur if the animals that have PD or CI are not representative of the population being studied. Differences between trends in ETA of parents of all registered bulls and registered bulls with PD indicated that bulls with PD were not representative of the registered bull population. Larger annual trends in genetic merit of parents were found for registered bulls that had PD. The CI were available for a large proportion of the female population. Cows with CI were assumed representative of the female population. Substantial differences existed between ~ for the first and second periods. Powell et al. (16) also reported larger ~ for milk and milk fat percentage in the latter time period of their study. Their ~ for 1968 to 1975 were slightly less than ~ for the second time segment of this study. Journal of Dairy Science Vol. 68, No. 10, 1985
Estimates of ~ from this study indicated that little genetic change occurred in the registered Holstein population before 1968. Larger genetic trend estimates were reported by (2,8,9). Their data included only cows calving before 1968. Estimates of ~ in those studies were based on regressions of daughter performance on time of calving. Adjustments may not have completely accounted for environmental trend and biases due to nonrandom samples of dams. Annual genetic change in milk yield increased substantially after the availability of more accurate CI and PD in 1968 and later. The increase in ~ for milk, however, was considerably smaller than was possible. Dairy farmers evidently placed some emphasis on milk fat percentage and type. Less selection pressure on secondary traits may have been warranted economically, since the major source of income on most dairy farms is from the sale of milk. Generation Interval
In this study, ~ estimation accounted for generation interval by regression of ETA on birth year. Generation intervals, however, were calculated to determine if trends or changes occurred over the 20-yr span. Averages for ages of sires and dams at time of offspring birth are in Table 5. Generation intervals for sire-son increased steadily between 1960 and 1979. The regression of average sire-son generation interval on birth year was 1.93 + .09 mo. The trend toward registering sons of older sires suggests that dairy producers became more concerned about the repeatabilities of sires' proofs. Some genetic gain was sacrificed by requiring that PD
GENETIC CHANGE IN HOLSTEINS
26 37
TABLE 5. Means and standard deviations for ages of sires and dams at time of birth of their registered Holstein offspring. Generation interval (mo) Birth year
1960 1961 1962 1963 1964 1965 1966 1967
1968 1969 1970 1971 1972 1973 1974 1975 1976
1977 1978 1979
Sires Sons
75 77 79 80 81 83 84 86 85 88 89 92 97 100 100 104 107 110 111 107
Dams Daughters
Sons
Daughters
SD
X
SD
X
SD
X
SD
41 40 40 40 41 41 41 41 40 41 40 39 40 42 43 44 45 46 47 47
78 78 78 80 81 81 81 83 82 82 84 85 86 87 85 86 88 90 91 89
42 42 41 42 42 42 41 41 40 41 41 41 42 43 43 44 45 44 44 42
67 68 69
32 32 32 32 32 32 31 31 31 30 30 30 30 30 30 30 30 30 30 30
56 57 58 58 58 58 57 57 57 57 57 57 56 56 56 56 57 56 56 56
27 28 28 28 28 28 28 27 27 27 27 27 27 27 27 26 27 27 27 27
of sires reach a relatively high repeatability before their sons were registered and progeny tested. The annual trend in generation interval for sire-danghter (.65 + .03 mo) was smaller than for sires and sons. Average generation intervals for dam-son and dam-daughter r e m a i n e d relatively c o n s t a n t over the 20-yr. Average age o f dams o f registered sons was a p p r o x i m a t e l y 12 m o greater than the age o f dams o f registered daughters. Dairy producers t e n d e d to register bull calves o f cows t h a t had additional p r o d u c t i o n records. Summary The availability o f m o r e accurate genetic evaluations (PD and CI) in 1968 c o n t r i b u t e d substantially to the increase in E T A for milk yield o f males and females. A t i m e lag existed b e t w e e n the first change in PD of sires (1968) and the initial change in CI o f dams (1971). In addition, rates o f change in PD of sires and CI o f dams were n o t equal. The smaller rate of
69
68 68 68 67 68 68 67 67 67 66 67 67 66
67 66
65
increase for CIM o f dams p r o b a b l y was due to nonlinear genetic change in sires caused by changes in sire selection goals. A n n u a l genetic change (~) for the male and female populations was estimated by two methods. A n n u a l trends in PD o f sires and CI of dams were added t o g e t h e r to estimate g. In addition, ~ were o b t a i n e d by doubling the trends in the animals' o w n average ETA. Both m e t h o d s yielded similar results in the female p o p u l a t i o n (Table 3). In the male population, smaller estimates were o b t a i n e d by the m e t h o d of doubling the rate of change in the bulls' average PD (Table 4). This m e t h o d ( E q u a t i o n [7] ) should yield m o r e accurate ~ w h e n animals with E T A are k n o w n to be representative o f the entire population. ACKNOWLEDGMENTS The authors appreciate t h e support f r o m Grant I-3-79 of the US-Israel Binational Agricultural and D e v e l o p m e n t Project for partial support for c o m p u t i n g costs. Journal of Dairy Science Vol. 68, No. 10, 1985
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REFERENCES 1 Batra, T. R. 1979. Genetic trends for milk and fat production in dairy cattle. Can. J. Anita. Sci. 59:203. 2 Burnside, E. B., and J. E. Legates. 1967. Estimation of genetic trends in dairy cattle populations. J. Dairy Sci. 50:1448. 3 Dentine, M. R., and B. T. McDaniel. 1982. A matrix approach to prediction of rates of genetic gain from selection for milk yield in dairy cattle. J. Dairy Sci. 65(Suppl. 1):96. (Abstr.) 4 Dickinson, F. N., R. L. Powell, and H. D. Norman. 1976. An introduction to the USDA-DHIA Modified Contemporary Comparison. Page 1 in The USDA-DHIA Modified Contemporary Comparison. US Dep. Agric. Prod. Res. Rep: 165. 5 Everett, R. W., C. E. Meadows, and J. L. Gill. 1967. Estimation of genetic trends in simulated data. J. Dairy Sci. 50:550. 6 Fuller, W. A. 1969. Grafted polynomials as approximating functions. Aust. J. Agric. Econ. 13:35. 7 Hahn, E. W., J. L. Cartoon, and W. J. Miller. 1958. An intraherd contemporary comparison of the production of artificially and naturally sired dairy cows in Georgia. J. Dairy Sci. 41:1061. 8 Hargrove, G. L., and J. E. Legates. 1971. Biases in dairy sire evaluation attributable to genetic trend and female selection. J. Dairy Sci. 54:1041. 9 Harville, D. A., and C. R. Henderson. 1967. Environmental and genetic trends in production and their effects on sire evaluation. J. Dairy Sci. 50: 870. 10 Hintz, R. L., R. W. Everett, and L. D. Van Vleck. 1978. Estimation of genetic trends from cow and sire evaluations. J. Dairy Sci. 61:607. 11 Kennedy, B. W., and J. E. Moxley. 1975. Genetic
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16 17 18
19
20
21 22
trends among artificially bred Holsteins in Quebec. J. Dairy Sci. 58:1871. McDaniel, B. T., and G. J. King. 1974. Milk and fat differences for registered cows sired by artificial and natural insemination. J. Dairy Sci. 57:112. Powell, R. L. 1978. A procedure for including the dam and maternal grandsire in USDA-DHIA Cow Indexes. J. Dairy. Sci. 61:794. Powell, R. L., and A. E. Freeman. 1974. Genetic trend estimators. J. Dairy. Sci. 57:1067. Powell, R. L., H. D. Norman, and F. N. Dickinson. 1977. Relationships between bulls' pedigree indexes and daughters performance in the modified contemporary comparison. J. Dairy Sci. 60:961. Powell, R. L., H. D. Norman, and F. N. Dickinson. 1977. Trends in breeding value and production. J. Dairy Sci. 60:1316. Schaeffer, L. R. 1983. Effectiveness of model for cow evaluation intraherd. J. Dairy Sci. 66:874. Schaeffer, L. R., M. G. Freeman, and E. B. Burnside. 1975. Evaluation of Ontario Holstein dairy sires for milk and fat production. J. Dairy Sci. 58:109. Smith, C. 1962. Estimation of genetic change in farm livestock using field records. Anita. Prod. 4:239. Tucker, W. L., J. E. Legates, and B. R. Farthing. 1960. Genetic improvement in production attributable to sires used in artificial insemination in North Carolina. J. Dairy Sci. 43:982. Van Vleck, L. D., and C. R. Henderson. 1961. Measurement of genetic trend. J. Dairy Sci. 44: 1705. Wadell, L. H., and L. D. McGilliard. 1959. Influence of artificial breeding on production in Michigan dairy herds. J. Dairy Sci. 42:1079.