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Ability of Subjective Linear Scores to Represent Cow Differences in Objective Body Measurements1
Ability of Subjective Linear Scores to Represent Cow Differences in Objective Body Measurements 1 R. E. PEARSON, J. L. LUCAS, 2 and W. E. V I N S O N Department of Dairy Science Virginia Polytechnic Institute and State University Blacksburg 24061 ABSTRACT
INTRODUCTION
Body measurements, experimental linear descriptive scores, and Holstein descriptive codes on nearly 300 lactating cows in five herds were used 1) to compare the effectiveness of the later two subjective systems at depicting cow differences, and 2) to quantitate the loss of accuracy from reducing the 50 point linear scale to 25, 10, and 5 classes. Indirect selection differentials were calculated for various percentages saved (1 to 40%) when selection was based on linear scores and descriptive codes and compared with direct selection differentials. Indirect selection on linear stature and udder depth scores was nearly as useful as direct selection for stature and distance from rear udder to hock. Selection differential for linear scores for rear udder width were intermediate. For the measurements of chest depth, hip width, and thurl width, linear scores were only slightly better than descriptive scores. Reducing the number of linear classes to 25 had little impact on any of the traits studied. Reducing classes to 5 had a larger impact on explaining cow differences in chest depth (especially at the extreme), hip width, thurl width, and rear udder width. Where comparable, 10 to 25 classes of linear scores explain most of the cow differences that can be measured objectively.
Interest in improving components of conformation through selective breeding has existed for many years. The use of objective measurements as a basis for selection has been too costly, but previous descriptive coding systems have lacked accuracy in depicting cow differences. Linear scores resulting from the actions of the National Association of Animal Breeders and the breed associations during the late 1970's provide a possible basis for genetic change through selection with increased accuracy at little increase in cost. The objectives of this study were to compare the effectiveness with which cow differences in objective measures of conformation are depicted by 50-point subjective linear evaluations of conformation and by previous Holstein descriptive conformation codes; and to quantirate the loss of information from reducing linear scores from 50-point scale to 25, 10 or 5-point scales. We were unable to take objective measurements that corresponded closely to all linear scores or descriptive codes (2). Thus, this study was restricted to those traits for which there was a correspondence. MATERIALS AND METHODS Data
Data were body measurements, experimental linear descriptive scores of the Holstein-Friesian Association, and Holstein descriptive codes on approximately 300 lactating cows in five Virginia Institutional herds (1, 2). Scores and codes were assigned by one of two classifiers of the Holstein Association (2). Body measurements are described by Vinson et al. (2).
Received April 4, 1986. Accepted August 6, 1987. 1Supported by grants from the Holstein-Friesian Association of America and National Association of Animal Breeders. 2Tri-State Breeders, Baraboo, Wl 53913.
1987 J Dairy Sci 70:2610-2615
Method of Evaluation
Improving components of conformation requires that the measures used for selection represent true cow differences. Two approaches were used to quantitate the ability of subjective
2610
LINEAR SCORES FOR COW DIFFERENCES linear scores and descriptive codes to depict cow differences in objective measures of conformation. Indirect selection differentials for objective measures of conformation generated by selection on linear scores or descriptive codes were compared with selection differentials generated from direct selection on the objective measurement. That is, animals were ordered according to the criteria of selection: linear score, descriptive code, or objective measure, and the mean objective measurement of those selected under each criterion was calculated for percentages saved from 1 to 40%. These means were deviated from the overall mean and divided by the standard deviation of the objective measure to form indirect selection differentials for each selection criterion. When more than one individual had the same linear score or descriptive code, the mean objective measurement of all animals with the same score or code was used in calculating the selection differential. For example, if 40% of the animals were coded 1 for stature, selection differentials from 1 to 40% would be constant and would be calculated using the mean wither height of all animals coded 1 for stature. Selection differentials for each selection criterion in standard deviation units were graphed against percentage saved. This process was repeated for linear scores, descriptive codes, and objective measurements: stature scores, stature codes, and wither height; udder depth scores, udder support codes, and distance rear udder to hock; body strength scores, front end codes, and chest depth; rump width scores, rump codes, and hip width; rump width scores, rump codes, and thufl width; and rear udder width scores, rear udder codes, and rear udder width. The same approach was used to determine loss of sensitivity from combining 50 linear scores to 25, 10, or 5. Frequencies of the linear scores were presented previously (2). Again, when more than one individual had the same linear score or descriptive code all individuals assigned that score or code were assumed to have mean objective measurements of the group. These mean values were used to calculate the indirect selection differentials as described earlier. The second method used to determine the loss in sensitivity from collapsing the number of classes (combining classes) was to compare the percentage of the variation explained (R 2) by
2611
predicting the objective body measurement from the initial and collapsed linear scores. RESULTS AND DISCUSSION
Previous reports have suggested linear traits that could be improved by redefining the coding to utilize better the range in points (2) and demonstrated that classifiers generally ignored age or stage of lactation in assigning scores (1). This report quantitates the ability of linear scores to depict cow differences in physical components of conformation. Indirect selection differentials (in terms of objective measurement) for linear scores (50 points), descriptive codes (3 to 5 codes), and objective measurements are plotted against percentage saved (Figure 1). For any selection criterion where a large proportion of the individuals receive the most desirable score on code, there was no difference in the realized selection differential over the whole range that received that score. For example, 60.4% of the animals were coded 1 for udder support. Thus, the indirect selection differential when selecting on udder support code was constant for the top 1% to the top 40 or 60%. Frequency of animals assigned any one linear score generally was low. Frequency of animals assigned a single score exceeded 10% in only 10 cases, the highest being 17% for 78 points of body strength. This was in contrast to stature code 1 (38%) and udder support code 1 (60%). In all cases, the indirect selection differential from linear scores was intermediate to direct selection and selection on the descriptive code. The advantage of linear scores relative to descriptive codes was greatest at the most intense selection and decreased as selection intensity decreased. This is the area where insufficient classes to describe cow differences would have its greatest impact. Two other causes for differences between indirect and direct selection differentials were 1) errors in measurements and in assigning subjective linear scores or descriptive codes, and 2) lack of direct correspondence between the linear or descriptive trait and the objective measurement. Wither height and linear stature score, and distance rear udder to hock and udder depth score represent measurements and scores with direct correspondence. These two traits showed the closest agreement between direct (measurement) and indirect (linear score) selection Journal of Dairy Science Vol. 70, No. 12, 1987
2612
PEARSON ET AL.
A
D
WITHER HEIGHT
HIP WIDTH 3kO-
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Figure 1. Indirect selection differential versus percent saved for a) wither height, linear stature score (LST), and stature code (ST); b) chest depth, b o d y strength score (BS), and front end code (FE); c) distance rear udder to hock, udder depth score (UD), and udder support code (SU); d) hip width, rump width score (RW), and rump code (RP); e) thurl width, rump width score (RW) and rump code (RP); and f) rear udder width, rear udder width score (RUW); and rear udder code (RU).
Journal of Dairy Science Vol. 70, No. 12, 1987
LINEAR SCORES FOR COW DIFFERENCES differentials. The difference between direct and indirect selection differentials represents errors in assigning subjective scores and to a lesser extent errors in objective measurements. Rear udder width measurement and score have the same correspondence as the two traits discussed earlier; however, the indirect and direct selection differentials were more divergent than for wither height or distance rear udder to hock. This correspondence with the lower R 2 (.46 (rear udder width) vs. 58 (wither height) and .68 (udder depth)) from predicting linear scores from objective measurements (2) and is probably due to larger errors in assigning rear udder width scores or in measuring the width of rear udder. Hip width and thurl width are two component parts of rump width score and thus have less of the one-to-one correspondence of the previous three traits. In both cases, the indirect selection differentials are more nearly like those for the descriptive rump codes than those for the objective measurement. Chest depth may be a minor component of body strength. Except for the very high selection intensities, the realized selection differentials from use of 50 body strength scores were nearly identical to those for use of five front end codes. For subjective linear scores to be efficient substitutes for physical components of conformation, 1) there needs to be a direct correspondence between the trait described by the linear score and the component of conformation, and 2) the trait needs to be defined in ways to minimize errors in assigning scores (provide points of reference, provide actual definition of scores in metric terms). Loss of information from reducing the number of classes for each linear trait was evaluated in terms of the accuracy (R 2) of predicting objective measurements from linear scores and in terms of realized selection differentials. The R 2 from predicting body measurements from reduced number of classes of the linear scores are in Figure 2. Each is expressed as a percentage of the R 2 for 50 classes of the linear score. There was virtually no loss in R 2 from reducing from 50 to 25 classes. Reducing the number of classes to 10 for udder depth and body strength had virtually no effect. However, reducing to 10 classes for the remaining traits decreased R 2 2 to 4%, whereas
2613
scoring with 5 classes decreased R 2 from 4 to 14%. Indirect selection differentials using the 50, 10, and 5 classes for each trait are in Figure 3. As would be expected, the greatest advantage of the larger number of classes occurred at the most intense selection. At 40% saved there was little difference in indirect selection differentials for 5, 10, or 50 classes. Losses in indirect selection differential for wither height were small except for an area centered on 10% saved. The lack of reduction for this trait reflected the more uniform distribution o f score for this trait in the data set and the higher accuracy of assigning scores. Reducing to 5 classes of body strength caused a large decrease in selection differential for chest depth through 4% saved. Decreases were less from 6 to 30%. The reduced selection differential for distance rear udder to hock were noticeable from 4 to 30% for 5 classes. There was a very slight reduction from use of 10 rather than 50 classes for width at hips and a large reduction over most of the scale for 5 classes. Reducing from 10 to 5 classes produced a significant reduction in selection differentials of thurl width through 30%. For rear udder width, the reduction to 5 classes reduces the selection
100 (/3
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9o
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25 10 5 LST Withers
25 I0 5 BS CD
25 10 5 25 ~0 U9 RW RUTOH W-HP
25 I0 5 RW W-TH
25 10 5 LRUW RUW
Figure 2. The R 2 from predicting body measurements from linear scores: wither height (withers) from linear stature score (LST), chest depth (CD) from linear body strength score (BS), distance rear udder to hock (RUTOH) from linear udder depth score (UD), hip width (W-HP) from linear rump width score (RW), thud width (w-TH) from RW, and rear udder width (RUW) from linear rear udder width score (LRUW). Journal of Dairy Science Vol. 70, No. i2, 1987
2614
PEARSON ET AL.
D
WITHER HEIGHT
A
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Figure 3. Indirect selection differentials for a) wither height, b) chest depth, c) distance rear udder to hock, d) hip width, e) thurl width, and f) rear udder width vs. percent saved for 50, 10, and 5 classes of linearly coded stature (LST), body strength (BS), udder depth (UD), rump width (RW), or rear udder width (RUW).
Journal of Dairy Science Vol. 70, No. 12, 1987
LINEAR SCORES FOR COW DIFFERENCES differential through 18%. In general, these losses in accuracy would correspond to the theoretical increase in standard deviation associated with reducing the number of classes (A. E. Freeman, Iowa State University, personal communication). The effect of collapsing the 50 classes to 25, 10, or 5 varies with the distribution of observations across the point classes, the accuracy with which the scores are assigned and the degree of correspondence between the score descriptions and physical components of type which they are designed to measure. The greatest opportunity for decreased selection
2615
differential is at the most intense selection. Based on the results of this study there appears ample evidence to support 10 to 25 classes to be used in subjectively scoring cows for components of type.
REFERENCES
1 Lucas, J. L., R. E. Pearson, W. E. Vinson, and L. P. Johnson. 1984. Experimental linear descriptive type classification. J. Dairy Sci. 67:1767. 2 Vinson, W. E., R. E. Pearson, and L. P. Johnson. 1982. Relationships between linear descriptive type traits and body measurements. J. Dairy Sci. 65:995.
Journal of Dairy Science Vol. 70, No. 12, 1987
INTRODUCTION
Body measurements, experimental linear descriptive scores, and Holstein descriptive codes on nearly 300 lactating cows in five herds were used 1) to compare the effectiveness of the later two subjective systems at depicting cow differences, and 2) to quantitate the loss of accuracy from reducing the 50 point linear scale to 25, 10, and 5 classes. Indirect selection differentials were calculated for various percentages saved (1 to 40%) when selection was based on linear scores and descriptive codes and compared with direct selection differentials. Indirect selection on linear stature and udder depth scores was nearly as useful as direct selection for stature and distance from rear udder to hock. Selection differential for linear scores for rear udder width were intermediate. For the measurements of chest depth, hip width, and thurl width, linear scores were only slightly better than descriptive scores. Reducing the number of linear classes to 25 had little impact on any of the traits studied. Reducing classes to 5 had a larger impact on explaining cow differences in chest depth (especially at the extreme), hip width, thurl width, and rear udder width. Where comparable, 10 to 25 classes of linear scores explain most of the cow differences that can be measured objectively.
Interest in improving components of conformation through selective breeding has existed for many years. The use of objective measurements as a basis for selection has been too costly, but previous descriptive coding systems have lacked accuracy in depicting cow differences. Linear scores resulting from the actions of the National Association of Animal Breeders and the breed associations during the late 1970's provide a possible basis for genetic change through selection with increased accuracy at little increase in cost. The objectives of this study were to compare the effectiveness with which cow differences in objective measures of conformation are depicted by 50-point subjective linear evaluations of conformation and by previous Holstein descriptive conformation codes; and to quantirate the loss of information from reducing linear scores from 50-point scale to 25, 10 or 5-point scales. We were unable to take objective measurements that corresponded closely to all linear scores or descriptive codes (2). Thus, this study was restricted to those traits for which there was a correspondence. MATERIALS AND METHODS Data
Data were body measurements, experimental linear descriptive scores of the Holstein-Friesian Association, and Holstein descriptive codes on approximately 300 lactating cows in five Virginia Institutional herds (1, 2). Scores and codes were assigned by one of two classifiers of the Holstein Association (2). Body measurements are described by Vinson et al. (2).
Received April 4, 1986. Accepted August 6, 1987. 1Supported by grants from the Holstein-Friesian Association of America and National Association of Animal Breeders. 2Tri-State Breeders, Baraboo, Wl 53913.
1987 J Dairy Sci 70:2610-2615
Method of Evaluation
Improving components of conformation requires that the measures used for selection represent true cow differences. Two approaches were used to quantitate the ability of subjective
2610
LINEAR SCORES FOR COW DIFFERENCES linear scores and descriptive codes to depict cow differences in objective measures of conformation. Indirect selection differentials for objective measures of conformation generated by selection on linear scores or descriptive codes were compared with selection differentials generated from direct selection on the objective measurement. That is, animals were ordered according to the criteria of selection: linear score, descriptive code, or objective measure, and the mean objective measurement of those selected under each criterion was calculated for percentages saved from 1 to 40%. These means were deviated from the overall mean and divided by the standard deviation of the objective measure to form indirect selection differentials for each selection criterion. When more than one individual had the same linear score or descriptive code, the mean objective measurement of all animals with the same score or code was used in calculating the selection differential. For example, if 40% of the animals were coded 1 for stature, selection differentials from 1 to 40% would be constant and would be calculated using the mean wither height of all animals coded 1 for stature. Selection differentials for each selection criterion in standard deviation units were graphed against percentage saved. This process was repeated for linear scores, descriptive codes, and objective measurements: stature scores, stature codes, and wither height; udder depth scores, udder support codes, and distance rear udder to hock; body strength scores, front end codes, and chest depth; rump width scores, rump codes, and hip width; rump width scores, rump codes, and thufl width; and rear udder width scores, rear udder codes, and rear udder width. The same approach was used to determine loss of sensitivity from combining 50 linear scores to 25, 10, or 5. Frequencies of the linear scores were presented previously (2). Again, when more than one individual had the same linear score or descriptive code all individuals assigned that score or code were assumed to have mean objective measurements of the group. These mean values were used to calculate the indirect selection differentials as described earlier. The second method used to determine the loss in sensitivity from collapsing the number of classes (combining classes) was to compare the percentage of the variation explained (R 2) by
2611
predicting the objective body measurement from the initial and collapsed linear scores. RESULTS AND DISCUSSION
Previous reports have suggested linear traits that could be improved by redefining the coding to utilize better the range in points (2) and demonstrated that classifiers generally ignored age or stage of lactation in assigning scores (1). This report quantitates the ability of linear scores to depict cow differences in physical components of conformation. Indirect selection differentials (in terms of objective measurement) for linear scores (50 points), descriptive codes (3 to 5 codes), and objective measurements are plotted against percentage saved (Figure 1). For any selection criterion where a large proportion of the individuals receive the most desirable score on code, there was no difference in the realized selection differential over the whole range that received that score. For example, 60.4% of the animals were coded 1 for udder support. Thus, the indirect selection differential when selecting on udder support code was constant for the top 1% to the top 40 or 60%. Frequency of animals assigned any one linear score generally was low. Frequency of animals assigned a single score exceeded 10% in only 10 cases, the highest being 17% for 78 points of body strength. This was in contrast to stature code 1 (38%) and udder support code 1 (60%). In all cases, the indirect selection differential from linear scores was intermediate to direct selection and selection on the descriptive code. The advantage of linear scores relative to descriptive codes was greatest at the most intense selection and decreased as selection intensity decreased. This is the area where insufficient classes to describe cow differences would have its greatest impact. Two other causes for differences between indirect and direct selection differentials were 1) errors in measurements and in assigning subjective linear scores or descriptive codes, and 2) lack of direct correspondence between the linear or descriptive trait and the objective measurement. Wither height and linear stature score, and distance rear udder to hock and udder depth score represent measurements and scores with direct correspondence. These two traits showed the closest agreement between direct (measurement) and indirect (linear score) selection Journal of Dairy Science Vol. 70, No. 12, 1987
2612
PEARSON ET AL.
A
D
WITHER HEIGHT
HIP WIDTH 3kO-
3.0-
ST LST ACTUAL
-
--
2.5,
-
0 :~ o
2.0-
0 L~ o
"t:~
1.5.
"o
-
--
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.
.
.
.
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•
'
,
i
percent
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.
.
.
.
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.
.
.
.
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. . . .
40
CHEST DEPTH
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.... --
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1.0- \"',,,.. _ ....
. . . .
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. . . .
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,
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i 40
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. . . .
j 10
. . . . percent
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. . . .
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Figure 1. Indirect selection differential versus percent saved for a) wither height, linear stature score (LST), and stature code (ST); b) chest depth, b o d y strength score (BS), and front end code (FE); c) distance rear udder to hock, udder depth score (UD), and udder support code (SU); d) hip width, rump width score (RW), and rump code (RP); e) thurl width, rump width score (RW) and rump code (RP); and f) rear udder width, rear udder width score (RUW); and rear udder code (RU).
Journal of Dairy Science Vol. 70, No. 12, 1987
LINEAR SCORES FOR COW DIFFERENCES differentials. The difference between direct and indirect selection differentials represents errors in assigning subjective scores and to a lesser extent errors in objective measurements. Rear udder width measurement and score have the same correspondence as the two traits discussed earlier; however, the indirect and direct selection differentials were more divergent than for wither height or distance rear udder to hock. This correspondence with the lower R 2 (.46 (rear udder width) vs. 58 (wither height) and .68 (udder depth)) from predicting linear scores from objective measurements (2) and is probably due to larger errors in assigning rear udder width scores or in measuring the width of rear udder. Hip width and thurl width are two component parts of rump width score and thus have less of the one-to-one correspondence of the previous three traits. In both cases, the indirect selection differentials are more nearly like those for the descriptive rump codes than those for the objective measurement. Chest depth may be a minor component of body strength. Except for the very high selection intensities, the realized selection differentials from use of 50 body strength scores were nearly identical to those for use of five front end codes. For subjective linear scores to be efficient substitutes for physical components of conformation, 1) there needs to be a direct correspondence between the trait described by the linear score and the component of conformation, and 2) the trait needs to be defined in ways to minimize errors in assigning scores (provide points of reference, provide actual definition of scores in metric terms). Loss of information from reducing the number of classes for each linear trait was evaluated in terms of the accuracy (R 2) of predicting objective measurements from linear scores and in terms of realized selection differentials. The R 2 from predicting body measurements from reduced number of classes of the linear scores are in Figure 2. Each is expressed as a percentage of the R 2 for 50 classes of the linear score. There was virtually no loss in R 2 from reducing from 50 to 25 classes. Reducing the number of classes to 10 for udder depth and body strength had virtually no effect. However, reducing to 10 classes for the remaining traits decreased R 2 2 to 4%, whereas
2613
scoring with 5 classes decreased R 2 from 4 to 14%. Indirect selection differentials using the 50, 10, and 5 classes for each trait are in Figure 3. As would be expected, the greatest advantage of the larger number of classes occurred at the most intense selection. At 40% saved there was little difference in indirect selection differentials for 5, 10, or 50 classes. Losses in indirect selection differential for wither height were small except for an area centered on 10% saved. The lack of reduction for this trait reflected the more uniform distribution o f score for this trait in the data set and the higher accuracy of assigning scores. Reducing to 5 classes of body strength caused a large decrease in selection differential for chest depth through 4% saved. Decreases were less from 6 to 30%. The reduced selection differential for distance rear udder to hock were noticeable from 4 to 30% for 5 classes. There was a very slight reduction from use of 10 rather than 50 classes for width at hips and a large reduction over most of the scale for 5 classes. Reducing from 10 to 5 classes produced a significant reduction in selection differentials of thurl width through 30%. For rear udder width, the reduction to 5 classes reduces the selection
100 (/3
/ O
o nO U. eJ
9o
85
r~-
o~
8O
Closses
Troit Meosure
25 10 5 LST Withers
25 I0 5 BS CD
25 10 5 25 ~0 U9 RW RUTOH W-HP
25 I0 5 RW W-TH
25 10 5 LRUW RUW
Figure 2. The R 2 from predicting body measurements from linear scores: wither height (withers) from linear stature score (LST), chest depth (CD) from linear body strength score (BS), distance rear udder to hock (RUTOH) from linear udder depth score (UD), hip width (W-HP) from linear rump width score (RW), thud width (w-TH) from RW, and rear udder width (RUW) from linear rear udder width score (LRUW). Journal of Dairy Science Vol. 70, No. i2, 1987
2614
PEARSON ET AL.
D
WITHER HEIGHT
A
HiP WIDTH 3.0-
3•0-
2.5-
--
5 CLASSES LST
-....
10 CLASSES LST 5 0 CLASSES LST
o :,,u
2.0-
._o "0 "0
1.5-
@ c
L0-
--....
2.5-
0 .5-
5 CLASSES RW 10 CLASSES RW 5 0 CLASSES RW
2.0-
1.5-
1.0. -"Z[2-zz"~--_--~_.~. .5.
0
•
0
. . . .
1'0
'
,
. . . .
20
'
i
. . . .
,
30
10
40
20
CHEST DEPTH
B
.30
40
percent saved
percent saved
THURL WIDTH
E
3.0" --....
2.5-
2.0-
5 CLASSES BS ~0 CLASSES BS 5 0 CLASSES BS
2.5-
1.5-
1.0-
1.0-
.5-
i"~=-- . . . . . -L-z.:.Z.z.z.z.Z_:.:_:.:.:_2_:_2.:.:.:.:.:_=.=. a
.5-
O
•
0
10
.
.
p
20
•
. . . .
310 '
'
.
0
,
. . . .
40
0
i
10
percent saved
C
DISTANCE REAR UDDER TO HOCK
2.0-
"o
1.5-
.
.
.
20
.
.
.
.
.
.
.
4b
'0
.
F
---
5 CLASSES RUW 10 CLASSES RUW 5 0 CLASSES RUW
REAR UDDER WIDTH 3.0-
5 CLASSES UD I0 CLASSESUD .... 50 CLASSESUD
2.5-
._o
.
percent s a v e d
3.0-
o
5 CLASSES RW 10 CLASSES RW 5 0 CLASSES RW
2.0-
i'i
1,5-
--....
-
-
-
-
2.5-
0
o_
2.0-
1.5"0
g c
1,0-
o -,g
.5-
,5"
. . . . 20
30
percent saved
40
i 10
. . . .
I 20
. . . .
I 30
. . . .
I
40
percent saved
Figure 3. Indirect selection differentials for a) wither height, b) chest depth, c) distance rear udder to hock, d) hip width, e) thurl width, and f) rear udder width vs. percent saved for 50, 10, and 5 classes of linearly coded stature (LST), body strength (BS), udder depth (UD), rump width (RW), or rear udder width (RUW).
Journal of Dairy Science Vol. 70, No. 12, 1987
LINEAR SCORES FOR COW DIFFERENCES differential through 18%. In general, these losses in accuracy would correspond to the theoretical increase in standard deviation associated with reducing the number of classes (A. E. Freeman, Iowa State University, personal communication). The effect of collapsing the 50 classes to 25, 10, or 5 varies with the distribution of observations across the point classes, the accuracy with which the scores are assigned and the degree of correspondence between the score descriptions and physical components of type which they are designed to measure. The greatest opportunity for decreased selection
2615
differential is at the most intense selection. Based on the results of this study there appears ample evidence to support 10 to 25 classes to be used in subjectively scoring cows for components of type.
REFERENCES
1 Lucas, J. L., R. E. Pearson, W. E. Vinson, and L. P. Johnson. 1984. Experimental linear descriptive type classification. J. Dairy Sci. 67:1767. 2 Vinson, W. E., R. E. Pearson, and L. P. Johnson. 1982. Relationships between linear descriptive type traits and body measurements. J. Dairy Sci. 65:995.
Journal of Dairy Science Vol. 70, No. 12, 1987