Association of Milk Protein Types with Growth and Reproductive Performance of Dairy Heifers1

Association of Milk Protein Types with Growth and Reproductive Performance of Dairy Heifers 1 C. Y. LIN and A. J. M c A L L I S T E R Animal Research Centre Agriculture Canada Ottawa, Ontario K 1 A 0C6 Canada K. F. N G - K W A I - H A N G and J. F. H A Y E S Department o f Animal Science Macdonald College of McGill University Quebec H9X 1C0 Canada T. R. B A T R A and A. J. LEE Animal Research Centre Agricultu re Canada Ottawa, Ontario K1A 0C6 Canada G. L. ROY Lennoxville Research Station Lennoxville, Quebec J 1M 1Z3 Canada J. A. V E S E L Y Lethbridge Research Station Lethbridge, Alberta T I J 4B1 Canada

J. M. W A U T H Y Normandin Research Station Normandin, Quebec G0W 2E0 Canada K. A. W I N T E R Charlottetown Research Station Charlottetown, Prince Edward Island, C1A 7M8 Canada ABSTRACT

Thus, /3-1actoglobulin locus shows overdominance, underdominance, or no dominance, depending upon the traits considered. The four milk protein loci contributed more dominance variance than additive variance to total phenotypic variance. This might account for the existence of milk protein polymorphism in the cattle population. The combined genotypes o f the four milk protein loci showed significant effects on 2 o f 14 traits studied.

A total of 890 heifers was used to study the effects o f four milk protein loci (asl-casein , /3-casein, K-casein, and /3-1actoglobulin) on heifer growth and reproduction. The additive effects of gene substitutions at the four milk protein loci were significant only in 4 of 56 cases for all traits studied. Dominance effects at asl-casein, /3-casein, and K-casein loci were not significant for any traits except /3-casein locus on b o d y weight at first calving. Heifers with AB t y p e of/3-1actoglobulin showed greater b o d y weights and measurements and gestation length than the AA or BB type, indicating an overdominance effect. Heifers with AB type o f 15-1actoglobulin were significantly younger at first conception and at first freshening and had fewer number of days from first service to conception than the A A or BB type, indicating underdominance effect.

INTRODUCTION

Possible associations between milk protein genotypes and lactation performance have been investigated in recent years because of the potential use of milk protein types as an aid for selection. Many research reports have studied the relationships of milk protein genotypes to milk and cheese production (1, 2, 4, 5, 7, 8, 9, 11, 12, 13, 17). However, there is a paucity of research in which the possible relationships of milk protein loci with other traits have been examined. Jairam and Nair (4) reported that cows with the genetic combinations of asl-Casein BC type, /3-casein A A type, K-casein AB type, ~-lactalbumin BB type, and/3-1actoglobu-

Received July 17, 1986. Accepted October 27, 1986. 1Animal Research Centre Contribution Number 1412. 1987 J Dairy Sci 70:29--39

29

30

LIN ET AL.

lin AB type had lower age at first calving than other combinations. Weights at birth to 12 mo were influenced by the B-casein and 3-1actoglobulin loci (15). Ronda and Perez-Beato (14) reported that K-casein locus had a significant effect on services per conception. The purpose of this study was to investigate the additive and dominance effects of %1-casein,/3-casein, K-casein, and/3-1actoglobulin loci on measures of heifer growth and reproduction. MATERIALS AND METHODS

of days from first service to conception, and gestation length. Statistical Analyses

Data were analyzed according to a gene substitution model, which simultaneously considers gene additive effect and gene interaction effect at each milk protein locus. The following model was used: Yijkl = M + T i + Bl + G k + b l rijkl + b2uijkl + b3vijkl + b4wijkl 4

Experimental Procedure

This study involved three genetic groups: a Holstein-based H line, an Ayrshire-based A line, and a C line of crossbreds between H and A lines, which were maintained by five research stations of Agriculture Canada. A detailed description of lines and management of animals are in McAllister et al. (10). Calves were separated from their dams within 24 h of birth and reared individually in calf stalls on a limited whole milk feeding program for 8 wk. Calf starter-grower was fed ad libitum up to a maximum of 2.5 kg/d for the first 34 wk. From 34 to 50 wk, the starter-grower was fed at 1.8 kg/d. Hay or silage was fed ad libitum until 2 wk before calving. Heifers were observed for estrus twice a day and bred at first estrus after reaching 350 d of age regardless of size. Any heifers not confirmed pregnant by rectal palpation by 574 d of age were culled. Milk samples of 920 heifers were used to determine the genetic variants of 0ql-casein, 3-casein, K-casein, ~-lactoglobulin, and a-lactalbumin (7). Milk samples from each cow were phenotyped by polyacrylamide gel electrophoresis methods (13) at three different times and the results were compared for consistency. Of 920 cows, 12 cows had erroneous eartags, resulting in 908 cows (377 H line, 158 A line, and 373 C line cows) for computing gene frequencies within lines. Body weights (kg) of the heifers at birth, at 350 d of age, and at first calving as welt as heart girth (cm) and withers height (cm) at 350 d of age and at first calving were examined. Heifer reproductive traits examined were age at first observed estrus, age at first breeding after 350 d of age, age at first conception, age at first calving, conception rate at first service, number Journal of Dairy Science Vol. 70, No. 1, 1987

+ m~=ldmXmijkl + eilkl

[1]

where M is the popuIation constant; T i is the effect of the ith station (i = 1, 2, 3, 4, or 5) ; Bj is the effect of the jth birth year-season (calendar season) combination; G k is the effect of the k tla genetic group (k = H, A or C); rilkb Uijkl, Vijkl , and W i j k l a r e the fractions of milk protein genes for asl-casein, 3-casein, K-casein, and 3-1actoglobulin, respectively; b l, bz, b3 and b4 are the corresponding partial regression coefficients which are also the gene substitution effects; Xmijk I is created by multiplying the fractions of milk protein genes at m th milk protein locus. The corresponding dm is the partial regression coefficient which measures the degree of dominance (allelic interaction) at the m th locus; and eijkl = random residual effect. All traits were analyzed according to the above model with the exception of body weight, heart girth and withers height at first calving, which were analyzed by adding age at first calving as a covariate. Because the fractions of milk protein genes for each milk protein locus sum to unity, there are dependencies among equations for each milk protein locus. To remove these dependencies, the fraction of gene contribution by the C allele of %l-casein, the A 2 allele of 3-casein, and the B allele of K-casein and/3-Iactoglobulin were set to zeros in obtaining solutions. As a consequence, bl is the average effect of substituting allele B for allele C in asl-casein, b2 is the average effect of substituting allele A 1 for allele .42 in 3-casein, and b3 and b4 are the average effects of substituting allele A for allele B in K-casein and /3-1actoglobulin, respectively. Alleles A 3 and B in 3-casein were excluded from estimation of gene substitution effects due to their extremely

EFFECTS OF MILK PROTEIN TYPES l o w f r e q u e n c y . G e n o t y p e and g e n e f r e q u e n c i e s o f t h e f o u r milk p r o t e i n t y p e s in o u r p o p u l a t i o n s have b e e n r e p o r t e d previously (7). In this s t u d y , o n l y 27 o f 890 heifers w e r e /3-1actoglobulin A A t y p e s . Precision o f e s t i m a t i o n of/3-1actoglobulin e f f e c t w o u l d increase if m o r e /3-1actoglobulin A A h e i f e r s w e r e available. G e n e t i c linkage a m o n g milk p r o t e i n loci d o e s n o t a f f e c t the e s t i m a t i o n o f t h e e f f e c t s o f t h e s e loci as long as t h e y are in linkage e q u i l i b r i u m . O t h e r w i s e c a u t i o n s h o u l d b e e x e r c i s e d in t h e i n t e r p r e t a t i o n o f t h e data. RESULTS AND DISCUSSION Heifer Body Weights and Measurements

A n a l y s e s o f variance f o r h e i f e r b o d y w e i g h t s a n d m e a s u r e m e n t s are in Table 1 and average

31

g e n e s u b s t i t u t i o n and d o m i n a n c e e f f e c t s are in Table 2. S t a t i o n and b r e e d w e r e significant s o u r c e s o f variation f o r h e i f e r b o d y w e i g h t s and m e a s u r e m e n t s as f o u n d previously (6). Age at first calving as a covariate had ( P < . 0 1 ) e f f e c t s o n b o d y weight, h e a r t girth, and w i t h e r s h e i g h t m e a s u r e d at first calving. The average e f f e c t s o f s u b s t i t u t i n g B f o r C allele at ~sl-Casein locus and A f o r B allele at /3-1actoglobulin locus w e r e n o t significant for t h e b o d y w e i g h t s and b o d y m e a s u r e m e n t s s t u d i e d (Table 1). S u b s t i t u t i o n o f A a for A 2 allele at /3-casein locus was significant ( P < . 0 5 ) f o r h e a r t girth at first calving b u t n o t significant f o r o t h e r b o d y w e i g h t s and m e a s u r e m e n t s . Similarly, Singh et al. (15) f o u n d n o significant e f f e c t s of/3-casein locus at b i r t h a n d at 1, 3, 6, a n d 12 m o o f age. S u b s t i t u t i n g A 1 f o r A 2 allele

TABLE 1. Residual mean squares and F statistics for heifer body weights and measurements. Body weight Effect

df

Age at first calving Year-season o f birth

1 35

Birth

350 d

. . . . . . 1.0

Heart girth First calving 269.9**

Withers height

350 d

First calving

350 d

...

311.3"*

...

First calving 44.5**

1.6"

1.5"

1.4"

1.1

2.0**

1.3

Station

4

9.1"*

58.5**

57.3**

32.8**

45.7**

70.9**

20.0**

38.6**

53.9**

26.1"*

29.1"*

73.9**

69.3**

Breed

2

51.5"*

B Allele of as 1-casein

1

.9

A a Allele of ~3-casein

1

0

A Allele of K-casein

1

3.9*

A Allele of 3-1actoglobulin

1

.4

B × C Interaction for aM-casein

1

1.2

A x × A 2 Interaction for 3-casein

1

2.6

.8

A X B Interaction for K-casein

1

1.6

A X B Interaction for ~3-1actoglobulin

1

4.6*

Residual mean squares df for residuals

23.6 839

0

.1

0

0

.2

2.3

0

4.3 *

2.9

.6

.4 0

0

.3

1.6

0

1.6

0

.1

.3 .1

0

0

1.5

.8

2.5

0

.1

.3

.3

5.8"

0

.3

.3

1.4

0

0

1.8

1.1

.7

1.6

4.2 *

2.7

6.1 *

4.7*

1.6

776.8 838

1693.2 829

31.3 839

35.6 814

14.1 839

0

20.8 816

*Significant at P<.05. * *Significant at P<.01. Journal of Dairy Science Vol. 70, No. 1, 1987

32

LIN ET AL.

at /3-casein locus decreases heart girth at first calving b y 1.31 cm (Table 2). Substituting A for B allele at n-casein locus was significant (P<.05) for birth weight and approached significance (P = .07) for b o d y weight at 350 d of age. Substitution of A for B allele at ~:-casein locus decreases the average effects for birth weight by 1.71 kg and b o d y weight at 350 d of age by 8.38 kg (Table 2). Interactions between B and C alleles at as~-casein locus and between A and B alleles at n-casein locus were not significant (P>.05) for b o d y weights and measurements (Table 1). Interaction between A ~ and A 2 alleles at /3-casein locus was significant for b o d y weight at first calving and not significant for other b o d y weights and measurements• Heifers with A ~ A ~ t y p e of ~-casein weighed 455.9 kg at first calving as compared with 445.7 kg for A~A a t y p e and 452.4 kg for AZA ~ t y p e (Table 3), indicating an overdominance effect at this locus. Interaction between A and B alleles at /3-1actoglobulin locus was significant (P<.05) for birth weight and body weight at first calving, heart girth at first calving, and withers height at 350 d (Table 1). As shown in Table 3, heifers with AB t y p e of ~-lactoglobulin consistently showed greater b o d y weights and measurements than either homozygotes (AA or BB type), indicating overdominance effect at the/3-1actoglobulin locus. If these traits have "selective values", both A and B alleles at/3-1actoglobulin locus will be maintained in the cattle population, exhibiting the phenomenon of milk protein polymorphism resulting from the overdominance effect•

~t~

Journal of Dairy Science Vol. 70, No. 1, 1987

e~

o~

~x

eq

t~ eq

[a~

,-~

7

i

t--

,.,

"7

~g

en

Heifer Reproduction Traits

The F statistics, average effect of gene substitution, and dominance effect are in Tables 4 and 5. The effects of station and year-season of birth were significant (P<.01) for heifer reproductive traits except for gestation length• Breed had a significant effect on age at first conception and at first calving, number of days from first service to conception, and conception rate at first service. The effect of breed approached significance (P = .06) for age at first estrus and gestation length. Effect of gene substitutions at asl-casein, /3-casein, and ~-casein loci was not significant (P>.05) for all heifer reproductive traits.

eq

~q

t~i

~

?

7

7

¢-1

t--

ox

7

EFFECTS OF MILK PROTEIN TYPES

"3

"3

m m

m m m

i

g ¢.

~g u~ .1 r~

<

~k

33

However, the average effect of substitution of A for B allele at j3-1actoglobulin locus was significant for age at first conception and gestation length and approached significance (P = .09) for age at first calving (Table 4). Substitution of A for B allele at fl-lactoglobulin locus increases the average effects for age at first conception b y 23.6 d and age at first calving b y 20 d, whereas it would reduce gestation length by 2.4 d (Table 5). Allelic interactions between B and C at a s l - c a s e i n locus and between A and B at K-casein locus were not significant for any heifer reproductive trait studied (Table 4). Interaction between A ~ and A 2 alleles at ~3-casein locus was not significant for heifer reproductive traits except for gestation length, which approached significance (P = .07). Heifers with A 1 A 2 type of/3-casein had gestation length of 278.7 d as compared with 279.6 d for A 1A 1 t y p e and 279.4 d for A 2 A 2 type, suggesting underdominance o f the /3-casein locus on gestation length. However, the reduction of gestation length due to underdominance effect at /3-casein locus does not appear large enough to have any economic impact in animal improvement. Although not statistically significant (P = .15), heifers with A1A 2 t y p e of/3-casein seems to have higher conception rate at first service (49%) as compared with 41% for A1A 1 t y p e and 46% for A 2 A 2 type. Interaction between A and B alleles at fl-lactoglobulin locus was significant for age at first conception and at first calving, days from first service to conception, and gestation length and was not significant for other heifer reproductive traits. Age at first conception, age at first calving, and number of days from first service to conception were 412.8, 694.2, and 28.6 d, respectively, for heifers with AB type of ~3-1actoglobulin. These were significantly lower than those for the AA type (440.4, 717.8, and 44.9 d) and the BB t y p e ( 4 1 6 . 7 , 6 9 7 . 8 and 32.3 d). This indicates that underdominance characterized the effect of/3-1actoglobulin locus on these three reproductive traits. Differences in gestation length among AB (280.2 d), A A (277.5 d), and BB (279.9 d) types of ~-lactoglobulin were small but statistically significant, suggesting an overdominance effect of/3-1actoglobulin locus on this trait. Therefore, a given locus may exhibit overdominance, under-

Journal of Dairy Science Vol. 70, No. 1, 1987

e.

.~" T A B L E 4. Residual m e a n squares and F statistics for heifer reproductive traits.

< o ,q

Age at first Effect

df

Estms

Breeding

Year-season o f birth

36

1.8"*

11.1"*

Station

4

14.7"*

3.4 * *

Breed

2

2.6

oo .q

Calving

Conception rate at first service

Gestation length

(d)

z o

Conception

Days from first service to conception

1.8

1.4"

8.1"*

7.9**

1.81 *

14.6"*

14.4"*

12.7"*

6.2"*

4.7"*

5.4"*

3.1"

3.2*

.7 .7 2.8

B Allele o f ~sl-Casein

1

2.3

.1

1.0

1.0

1.3

2.4

.0

A 1 Allele o f ~3-casein

1

2.1

.1

.2

.2

.1

.9

.0

A Allele o f g-casein

1

0

.2

.2

.3

.8

.1

A Allele o f B-lactoglobulin

1

.2

1.8

2.8

2.1

.3

3.7*

B × C Interaction for a s l -casein

1

1.0

.2

.7

.7

.9

.2

A t X A z Interaction for ~-casein

1

2.1

1.0

0

0

1.2

2.4

3.4

A X B Interaction f o r g-casein

1

.3

0

0

0

0

.1

.9

A X B I n t e r a c t i o n for ~-lactoglobulin

1

Residual m e a n squares d f for residual *Significant at P < . 0 5 . **Significant at P < . 0 1 .

0

4.2*

> .8

.7

1.7

2308.0

1558.5

3079.2

3131,4

1741.4

838

837

837

830

837

5.8*

4.1"

4.1"

1.3 .2 837

4.3* 35.6 829

EFFECTS OF MILK PROTEIN TYPES

I l l l

35

dominance, or no dominance gene action depending upon the traits considered (Tables 5 and 6). ~-Casein showed underdominance on gestation length, whereas ~3-1actoglobulin exhibited overdominance on this trait. When a trait is affected by multiple loci, some of them may exhibit overdominance, which is cancelled by other loci showing underdominance. This study confirmed the theoretical argument of Falconer (3) that the absence of heterosis in a trait does not necessarily mean the absence of heterosis in the individual loci which contribute to that trait.

I

Contribution of Milk Protein Loci to Additive and Dominance Variances

~

~7

~

~4

Ii e~ 0

~4

~ M M N d

"

~ "

~q

The proportion of total phenotypic variances of heifer b o d y weights, b o d y measurements, and heifer reproductive traits due to milk protein loci is in Table 7. The additive genetic variance ( 0 3 . ) due to each milk protein locus was estimated as o 3 , = 2 pq ¢x2 (3) where ~ is the average effect of gene substitution given in Tables 2 and 5. Dominance variance due to each milk protein locus was estimated as o~). = (2pqd) 2 (3) where d is the dominance effect in Tables 2 and 5. Phenotypic variances (o~) as shown in Table 7 were residual variances after adjusting for all fixed effects specified in the model. If overall phenotypic variances are used instead of residual variances, obviously, the ratios o 5 . : o ~ and o~*:o~ would be greatly reduced. Although the additive effects (a) of asl-Casein and K-casein loci were not significant for traits with the exception of birth weight, the contributions of o 3 . due to these two loci were greater than the other two protein loci. This is due mainly to sampling errors because the estimates o f additive effects (c0 at &sl-casein and K-casein loci had very large standard errors associated with them. In fact, estimates of a 3 . were greatly affected by estimation errors because the additive effects for the four milk protein loci were significant only in 4 of 56 cases (Tables 2 and 5). /~-Lactoglobulin locus showed significant dominance effect in 8 of 14 cases (Tables 2 and 5) and contributed greater amount of a ~ , than did the other three milk protein loci (Table 7). 2 The ratio of aD.:O p2 is generally greater than 2 2 the ratio o f aA.:O p, indicating the presence of Journal of Dairy Science Vol. 70, No. 1, 1987

36

LIN ET AL. allelic interactions at these loci. The conventional sire model used in the analysis of dairy cattle data does not permit the estimation of dominance variance which automatically becomes part of residual variance. The allelic interaction at these loci may account for milk protein polymorphism. If allelic interactions at the milk protein loci are associated with calf or heifer survival, different alleles at the milk protein loci will coexist in the cattle population regardless of the selective advantage of substituting one allele for the other in the economically important traits. However, the study on relationship between milk protein types and calf or heifer survival is not possible in the present circumstance since currently milk protein type may only be determined on lactating females. Smith (16) derived the relative efficiency of various selection methods in which known genetic loci were incorporated and compared to ignoring this genetic information. For example, using a~, and o~ in Table 7 for birth weight and assuming heritability and repeatability of .40 and .70, the additive variance from the four milk protein loci represents 13.8% of the total variance (a~,,:a~). Based on the method of Smith (16), index selection combining birth weight and information on the four milk protein loci is approximately 9% more efficient than mass selection on birth weight alone.

.2

.2 ~

~ °m

~

0

>

o

Joint Effects of Milk Protein Loci

2

e~

g.

e~

,d

e~

2 e~

< [Journal of Dairy Science Vol. 70, No. 1, 1987

e~ O

Of alI available combinations among the genotypes at the four milk protein loci, 21 genetic groups had occurred more than four times (Table 8). These 21 genetic groups were used to examine the joint effects of the milk protein loci on body weights, body measurements, and reproductive traits. The model for this genetic group analysis was the same as for gene substitution model except that genetic group effect replaced the additive and dominance effects in the gene substitution model. Of 14 traits studied, only two traits (withers height at 350 d and gestation length) were significantly (P<.05) affected by the genetic group of the four milk protein loci. As shown in Table 8, genetic group of BB a s 1-casein, A 1A 2/3-casein, BB K-casein, and BB /3-1actoglobulin had the greatest withers height at 350 d (118.0 +- 1.3) whereas genetic group of BC as l-casein, A2A 2

TABLE 7. Additive (o~k*) and dominance (of)*) variances of body weights and measurements and heifer reproductive traits due to milk protein loci. Body weight

Heart girth First

First Birth

350 d

calving

350 d

calving

Withers height First 350 d

calving

Age at first Estrus

Breeding

Conception

Calving

Days from first service to conception

Conception rate at first service

Gestation length (3

Additive variance ast-Casein p-Casein K-Casein g3-Lactoglobulin Total e~*

1.91 0 1.32 .08 3.31

,13 ,86 31.69 1.72 34.40

10.30 21.38 14.36 .25 46.29

.93 1.12 1.23 2.83 6.11

.34 11.85 36.19 32.91 81,29

.03 180.13 .65 191.60 372.41

.10 .12 .32 .54

.01 .83 .85 .18 1.87

0 0 0

.06 .O1 .00 2.18 2.25

.11 .23 2.15 6.01 8.50

.14 .08 .51 1.31 2,04

0

.53

6.96 .59 .10 18.00 25.65

281.2 3.3 9.5 81.5 375.5

280.2 3.8 6.9 58.5 349.4

197.2 1.1 7.6 23.2 229.1

.049 .000 ,005 .000 .054

0 0

.99

478.3 25.1 1.2 3.2 507.8

,20 ,00 .51 .90 1.61

75.8 90.3 23.3 43.6 234.0

9.21 28.96 1.07 68.29 107.53

85.2 .3 .3 459.6 545.4

73.1 1.7 .1 340.7 415.6

37.4 38.5 .0 182.6 258.5

.007 .009 .000 .008 .024

.3 2.1 1.1 4.0 7.5

1550.20

3079.2

3131.8

1743.8

.236

35.8

0 .46 .23 .23

0

© .1 .8 .9

Dominance variance asl-Casein /3-Casein K-Caseln j3-Lactoglobulin Total o~).

,< o~

< o ~q Z o

7~ 0o ,q

Phenotypic variance (o~)

23.9

776.8

1717.22

31.3

o~*:o~, % 2 % nD,:op,

13.8

4,4

2.0

1.7

25.5

10.5

21.6

7.1

35.9

14.1

20,8

2313.3

5.2

1.6

4.7

22.0

1.6

12.1

11.1

13.1

22.9

2.5

23.6

14.5

7.7

10.1

6.8

17.7

13.2

14.8

10.2

20.9

V" 70 © ~q

t~

38

LIN ET AL.

TABLE 8. Least squares means for the traits that showed significant (P<.05) differences among the genotypes of the four milk protein loci. Genetic group t C~sl-Cn

BB BB BB BB BB BB BB BB BB BB BB BB BB BB BB BC BC BC BC BC BC

3-Cn

AIA l Al A t AtA t AtA t A t A2 At A 2 A 1A 2 A 1A 2 At A 2 A 1A 2 A2A 2 A2A 2 A 2A 2 A 2A 2 A2A 2 A 1A 2 A1A 2 AtA 2 A2A 2 A2A 2 A2A 2

K-Cn

AA AA AB AB AA AA AB AB AB BB AA AA AB AB BB AA AB AB AA AB AB

13-Lg

AB BB AB BB AB BB AA AB BB BB AB BB AB BB BB BB AB BB BB AB BB

No. records

26 47 30 50 56 92 5 101 158 8 18 45 62 80 15 9 9 8 5 8 15

Withers height at 350 d

Gestation length

"X

SE

,X

SE

115.3 113.9 114.7 114.5 114.9 114.6 112.8 115.1 113.7 118.0 113.3 115.5 114.1 114.7 113.9 113.6 113.6 114.3 111.4 113.5 112.8

.8 .6 .7 .6 .6 .5 1.7 .5 .4 1.3 .9 .6 .5 .5 1.0 1.3 1.3 1.4 1.7 1.4 1.0

279.8 279.3 278.7 279.2 278.9 278.6 275.3 278.8 278.8 277.1 280.4 279.4 280.1 279.0 280.0 281.2 277.3 284.3 282.9 279.8 277.1

1.3 1.0 1.2 1.0 .9 .8 2.8 .8 .6 2.3 1.6 1.0 .9 .8 1.6 2.1 2.1 2.2 2.8 2.2 1.7

t See text for description. Cn = Casein, Lg = lactoglobulin.

3-casein, A A K-casein and BB 3-1actoglobulin had t h e smallest w i t h e r s height ( 1 1 1 . 4 + 1.7). G e n e t i c g r o u p o f BC a s 1 - c a s e i n , A 1 A 2/J-casein, AB K-casein and BB 3-1actoglobulin had t h e l o n g e s t g e s t a t i o n l e n g t h (284.3 + 2.2), w h e r e a s g e n e t i c g r o u p o f BB a s l - c a s e i n , A 1A 2 3-casein, AB K-casein, and A A l a c t o g l o b u l i n h a d the s h o r t e s t g e s t a t i o n l e n g t h (275.3 + 2.8).

CONCLUSIONS

P l e i o t r o p h i c e f f e c t s w e r e e x h i b i t e d b y milk p r o t e i n loci o n s o m e b o d y weights, b o d y m e a s u r e m e n t s , and r e p r o d u c t i v e traits o f h e i f e r s in a d d i t i o n to first l a c t a t i o n p r o d u c t i o n . H o w e v e r , t h e e f f e c t s o f milk p r o t e i n loci o n h e i f e r s ' g r o w t h a n d r e p r o d u c t i o n w e r e generally smaller t h a n their e f f e c t s o n h e i f e r s ' l a c t a t i o n yields. D o m i n a n c e at t h e milk p r o t e i n loci w a s an i m p o r t a n t c o n t r i b u t i n g f a c t o r for o c c u r r e n c e o f milk p r o t e i n p o l y m o r p h i s m . The additive g e n e t i c e f f e c t s o f milk p r o t e i n loci o n Journal of Dairy Science Vol. 70, No. 1, 1987

h e i f e r s ' g r o w t h and r e p r o d u c t i o n are n o t significant in m o s t cases and t h u s do n o t seem t o b e u s e f u l in c u r r e n t sire evaluation p r o g r a m . This is particularly t r u e since c u r r e n t l y milk p r o t e i n t y p e s m a y o n l y b e d e t e r m i n e d on lactating females. It m e r i t s f u r t h e r research to d e v e l o p a m e a n s o f i d e n t i f y i n g milk p r o t e i n t y p e s f o r b o t h sexes o f animals at an earlier age. With t h e a d v e n t o f D N A s e q u e n c i n g t e c h n i q u e , it m a y b e c o m e possible in t h e near f u t u r e to i d e n t i f y t h e milk p r o t e i n t y p e s for b o t h sexes o f animals at birth. This w o u l d u n d o u b t e d l y accelerate the s e l e c t i o n response b e y o n d t h a t possible w i t h t h e c o n v e n t i o n a l selection m e t h o d w h i c h ignores g e n e t i c i n f o r m a t i o n o n milk p r o t e i n loci. ACKNOWLEDGMENTS

The a u t h o r s wish t o t h a n k the technical a n d o p e r a t i o n a l s t a f f at all c o o p e r a t i n g research s t a t i o n s f o r collecting milk samples for milk protein typing.

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Journal of Dairy Science Vol. 70, No. 1, 1987