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Relationships between growth parameters and scrotal circumference in young beef bulls
THERIOGENOLOGY
RELATIONSHIPS BETWEEN GROWTH PARAMETERS AND SCROTAL CIRCUMFERENCE IN YOUNG BEEF BULLS M. Makarechian, A. Department of University Edmonton, Alberta, Recetved for Pubiication:
June
Accepted:
Farid and R.T. Berg Animal Science of P.lberta Canada T6C 2H! 18, 1984
September
2, 1984
ABSTRACT Scrotal circumferences of 119 young bulls of four distinct breeding groups were measured at the end of a feedlot performance test and at the beginning of the breeding season when the bulls were approximately 14 months old, to study the relationships of weight and growth parameters with testes size. Scrotal circumference was positively correlated with body weight at the end of feedlot test in the four breeding groups. The association between scrotal circumference and body weight was much stronger m the breeding group which had been selected for low yeariing weighi than in the other three breeding groups which had been selected for high growth rate. The relationships between scrotal circumference and preweaning and postweaning gain differed among the four breeding groups. Preweaning gain was the most important factor in the association between hod! weight and scrotal circumference among the three beef breeding groups. The results indicated that the preweaninp stage was a critical period for testicular development and that the probability of finding beef bulls with smaller than average testes among the bulls selected for weaning weight would be relatively small. Scrotal circumference was reduced (2.5-11%) from the end of feedlot test until the beginning of the breeding season. Key words: scrotal circumference, growth, beef bulls. INTRODUCTION Recently, emphasis on natural service fertility of breeding beef bulls has increased. Scrotal circumference as a measure of testes size, has been associated with semen production (1). Scrotal circumference is an important component in examining beef bulls for breeding soundness (2, 3). Many factors such as breed, age. season and body weight influence testes size or scrotal circumference (4, 5). Body weight at the end of feeding test affects scrotal circumference, and is a function of birth weight, preweaning and feedlot growth rate and age, all of which may influence testes development. Information on the relationship between growth rate and scrotal circumference would be helpful when growth rate and testes size are both considered in selecting young beef bulls. The objective of this study was to evaluate the influence of age. body weight and weight gain on scrotal circumference of yearling bulls. Acknowledgements. This project was supported by a grant from Alberta Agricultural Research Council’s “Farming for the Future” Program. The authors also acknowledge the contribution of G. Minchau and his coworkers at the Kinsella ranch for managing the herd.
DECEMBER
1984 VOL. 22 NO. 6
667
THERIOGENOLOCY
MATER1 ALS AND METHODS The study involved 119 yeariing bulis which were given a feedlot performance test after weaning. The bulls belonged to four breeding groups; Hereford (HE), Beef Synthetic (SY), Dairy Synthetic (SD) and Pee Wee (PW), maintained at The University of Alberta Ranch at Kinsella. Alberta. The breeding and management practices of the herd have been described by Berg and Peebles (6). Brief)). the HE is a purebred nonulatton onen to the industry’s superior bulls. The Sj’ is a svntheric nopuiation composed of approximately l/3 Charolats. l/3 Angus and l/3 Galioway breeds. The SD 15 a synthetic popuiation composed of approximately 213 dairy breeds (Holstein, Brown Swiss and Simmental) and l/3 beef breeds (Charolais, Angus and Hereford). The HE, SY and SD populations have been selected based on an index composed of preweaning and feedlot daily gain. The PW is a composite population of the above beef breeds selected for low yearling weight. The calves were born between early April and early June 1980 and were weaned in the middle of October. After an adjustment period of three weeks, the male calves were put on a feedlot performance test for 143 days and were fed ad libitum a ration of 64% barley. 21% oats, 10% pelleted dehydrated alfalfa hay and 5% protein-mineral-vitamin supplement. The calves were weighed at birth, during weaning, and at the end of the feeding test. Scrotal circumference was measured at the largest diameter of scrotum twice with a flexible tape. The average of the two measurements was used for analysis. The bulls were from 320 LO 383 days old when their scrotal circumference was measured. Twenty- five bulls were selected for breeding from the HE, SY and SD groups as follows: 6 HE, 14 SY and 5 SD. Body weight and scrotal circumference of these 25 bulls were measured by the same technician before the breeding season started (last week of June) to relate the change in scrotal ctrcumference to the change m body weight from the end of feeding test to breeding. Body weight at the end of the feeding test, birth date (age), and their quadratic forms were used in stepwise regression analysis to explain variation in scrotal circumference and change in scrotal circumference from the end of feedlot to breeding. Scrotal circumference at the end of feedlot, body weight change from the end of feedlot to breeding and their quadratic forms were also used as explanatory variables to study the change in scrotal circumference between the two measurements. Tests for the homogeneityof regression coefficients among breeding groups were made using appropriate dumm! variables. Path-coefficient analysis (7) was used to compare the relative importance of the common causative factors which contribute to the association between scrotal circumference and body weight at the end of feeding test, RESULTS The mean scrotal circumference and body weight at the end of feeding test for the four breeding groups are presented in Table 1. The SY bulls had significantly larger scrotal circumference than the other breeding groups. The mean scrotal circumference in the HE was similar to that in the SD, but it was larger than the average scrotal circumference in the PW group. As mean body weight increased, the ratio of scrotal circumference to body weight decreased (Table 1). The scrotal circumference as percent of body- weight was maximum in the PW and minimum in the SD group. The relationship between scrotal circumference and bodv weight at the end of feeding test was linear, as the quadratic terms were not significantly different from zero The slopes of regression lines were not significantly different among the four breeding groups except between the PW and the SY. The explanatorv power of body. weight _ measured by the coefficient of determination (R’) increased- ai mean body weight decreased across the breeding groups (Table 2). Age did not significantly influence scrotal circumference over and above body weight.
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THERIOGENOLOGY
TABLE 1 Means and standard errors of scrotal circumference, body weight and the ratio of scrotal circumference to body weight in the four breeding groups at the end of feedlot test
Measurement
Hereford
Number of bulls SC (cm)l Body weight (kg) SC’100 kg body weight
17 37.4 I 0.67a 404.2 + ll.Oa 9.4 i
0.22a
Beef Synthetic
Pee Wee
66 39.0 ? 0.34b 444.4 + 5.5b
17 35.3 + 0.67~ 348.9 i 11.oc
8.8 I
0.12a
Means within each measurement followed by different (P
10.3 i
0.22c
Daq
Synthetic
19 36.6 rt 0.63ac 449.9 t 10.4b 8.2 Ifr 0.21b
letters are significantly different
TABLE 2 The regression
of scrotal cvcumference
on body weighi tn the jour breedtng groups at the end of feedlot test
Breeding group
I nrercept (cm)
Regression coefficient (b) (cm/kg)
Standard error of b (cm/kg)
R’
Hereford Beef Synthetic Pee Wee Dairy Synthetic
27.012 30.291 21.073 23.862
0.025. 0.019* 0.0410.028
0.011 0.007 0.008 0.018
0.252 11.107 0.601 0.074
* Significant at P
Figure 1. Path diagram depicting the relationship between scrotal circumference and body weight through birth weight, birth date and gain.
DECEMBER
1984 VOL. 22 NO. 6
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THERIOGENOLOGY
Body weight and scrotal circumference are correlated primarily because of their mutual association with age and growth traits. In such a causal system, path analysis is generally more descriptive than simple correlation (8). Fig. 1 was assumed to be the causal system leading to the association of scrotal circumference and body weight. Based on this causal system, the path values for the four breeding groups are presented in Table 3. The path values were considerably different among the breeding groups. All the path values had the same sign in the SY and PW groups. The path coefficients from feedlot daily gain, preweaning daily gain and birth weight to body weight were all comparable in the four breeding grcups. The rank of the breeding groups for the direct effect of age on body weight was the same as that of their average body weight, the highest was SD and the lowest was PW. In SY and PW groups, preweaning daily gain had a higher direct effect on scrotal circumference than the other variables, while in the SD group, birth weight had the highest direct effect followed by feedlot daily gain. The direct effect of preweaning daily gain on scrotal circumference was negative and small in the SD. Preweaning daily gain, birth weight and age all had considerable direct influences on scrotal circumference in the HE breeding group. The total variance of each system is shown in Table 4. The weight (over 94 percent) than in circumference. which suggests that the variation in body weight.
of the variables accounted for by the underlying causal system accounted for much more variation in body preweaning daily gain, feedlot daily gain and scrotal the variables considered in the system could explain only
The relationship between scrotal circumference and body weight via feedlot daily gain, preweaning daily gain, birth weight and age are shown in Table 5. The association via age-birth weight was negligible in all cases and is not shown in the table. The associations between body weight and scrotal circumference via feedlot daily gain were positive in all the breeding groups, but negligible in the SY and PW. The association between body weight and scrotal circumference via birth weight was negative and negligible in all the breeding groups, except in the SD which was positive and considerably higher than in the other groups. The average body weight and scrotal circumference were both lower at the start of breeding season than at the end of the feedlot performance test (Table 6). There was approximately 11%. 5.5% and 2.5% reduction in scrotal circumference for the HE, SY and SD bulls respectively. Neither scrotal circumference at the end of feedlot nor preweaning or postweaning gains had any influence on the amount of scrotal circumference change from feedlot to breeding. The reduction in scrotal circumference linearly decreased as body weight at the end of feedlot test increased (P
THERIOGENOLOGY TABLE 3 Path values of the causal system shown in Figure
I for the four breeding groups
Breeding Group Hereford
;,I P, P, P, p,
P6 P, P, P, P,, PI! P,: p::
- .1615 -0.1131 -0.4572 0.3410 0.0224 0.1286 0.5589 0.7336 0.0907 0.2408 0.2637 0.3729 0.4242 0.5157
Beef Synthetic
Pee Wee
Dair!- Synthetic
0.0624 0.4993 -0.1197 0.0713 0.4253 -0.1894 0.4775 0.6134 0.1517 0.3172 0.1373 0.3808 -0.1112 0.0102
0.1214 0.6746 -0.0229 0.2202 0.6456 -0.2463 0.4951 0.4715 0.1401 0.1446 0.1801 0.8018 -0.1431 0.0645
-0.0413 0.7285 0.0692 -0.2103 -0.1053 -0.4575 0.7328 0.7283 0.1037 0.7771 0.3050 -0.1223 0.5520 0.1509
‘r = correlation coefficient between birth date and birth weight. ‘Pi = the ith path coefficient in Figure 1.
TABLE 4 The proportion of variance of preweaning daily gain, feedlot daily gain, body weight and scrotal circumference accounted for by the causal system in the four breeding groups
Breeding group
Preweaning daily gain
Feedlot daily gain
Body weight
Scrotal circumference
Hereford Beef Synthetic Pee Wee Dairy Synthetic
0.205 0.256 0.452 0.531
0.093 0.244 0.672 0.301
0.974 0.979 0.992 0.943
0.454 0.151 0.691 0.229
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THERIOGENOLOGY
The of the four breeding groups generally the higher reported estimates the literature (9, IO). The ranks of the three beef breeding groups for body weight at the end of feeding test and for scrotal circumference were the same. The S‘I bulls were, on the average, 30% heavier than the PW bulls, but their average scrotal circumference was only 10.5% larger than that of the PW group. Considering that PW bulls have been selected for hght yearling weight. a minimum testes size is probably necessary for adequate fertility for a given body weight. The SD bulls were the heaviest and PW the lightest group at the end of feedlot performance test, but their scrotal circumferences were not significantly different. The SD group may not conform to the average body weights and scrotal circumferences of other breeding groups because growth pattern of testes may differ between dairy and beef breeds. Coulter et al. (11) reported that the mean scrotal circumference in young Angus bulls was greater than that in Hoisteins of comparable age and the situation was reversed as the animals reached maturity, concluding that Angus reached their maximum testicular size at a younger age than Holstein bulls. Lunstra et al. (12) found that Brown Swiss bulls had consistently larger scrotal circumferences than Angus, Hereford and their crosses when they were between 7 and 13 months old. In addition to breed differences in scrotal circumference, some factors such as station (11, 13). station by breed interaction (13), season and year (14) and breed by year interaction (5) are reported to have significant influence on measures of testtcular size. The scrotal circumference as percent of body weight ranked in reverse order of body weight among the four breeding groups. This observation suggested that the change in body weight as a result of selection would not necessarily result in proportional chaner in scrotal circumference. The absence of a significant difference between the HE and SY for this ratio rndicated that the difference in scrotal circumference between the two groups was mainly due to the difference in their body weights. The relatively smaller correlation coefficients between scrotal circumference and body weight in the SY. SD and HE compared to the PW population might have been resulted from selection for and against gain in these breeding groups. The correlation coefficients between body weight and scrotal circumference in the HE and PW groups were within the range of estimates reported in the literature for yearling bulls (10. 12), but the coefficients for the SY and SD groups were smaller. The in-significant effect of age on scrotal circumference was primarily due to the short range in age among the bulls (63 days) and, in addition, body weight and age are positively correlated in young animals. The results of the path analysis indicated that the relationships between the growth parameters and testicular development differed considerably among the breeding groups. Preweaning daily gain was the most important factor in the SY and PW and was an important factor in the HE group which influenced testicular growth and contributed to the association between body weight and scrotal circumference. Johnson et al. (15) also reported a high association between preweaning gain and testicular development in beef bulls. The results indicated that it would be unlikely for beef bulls with small testes to be selected for breeding when preweaning gain was considered in the selection program. In constrast to the beef breeding groups, preweaning gain did not have appreciable influence on testicular development in the SD. and its contribution to the association of body weight and scrotal circumference was also neghgible. In addition to the breeding group differences, the high level of milk production of SD cows might have contributed to the differences. Assuming that the rate of testicular growth is relatively slow in the SD breeding group, as reported by Coulter et al. (11) in Holstein bulls, the high level of milk production of SD cows would induce high preweaning gain without corresponding testicular development
SY and minimum in the SD which corresponds with the rate of fattening (maturity)
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THERIOGENOLOGY
these breeding groups (17). The extending of the scrotum as a result of further descent of the testes and seasonal temperature change might have contributed co the decrease in scrotal circumference.
TABLE 5 Cause: of
assoctatton
between
hod),
wetghi
and
scrotal
crrcumjerence
tn
tht
four
oreedmg
group
Hereford
Beef Synthetic
Pee Wee
Dairy Synthetic
0.2736 0.0711 0.3447
0.2336 0.0130 0.2466
0.3780 0.0874 0.4654
-0.0891 0.0188 -0.0703
Via birth weight Direct Indirect Total
0.0384 -0.0390 -0.0006
-0.0169 -0.0586 -0.0755
-0.0200 -0.1017 -0.1X7
0.0572 0.1883 0.2455
Via age Direct Indirect Total
0.1242 -0.1808 -0.0566
0.0032 -0.0016 0.0016
0.0093 -0.0087 0.0006
0.1173 -0.0446 0.0727
0.1474
0.0656
0.0891
0.2235
Pathways of association Via preweaning daily gain Direct’ Indirect ? Total
Via feedlot daily gain
‘The product of path values from the common source to scrotal circumference and body weight. ‘Sum of the products of path values leading to scrotal circumference and body weight from the common source via other variables.
TABLE 6 Means
and
standard
Number of bulls SC,-SC,. cm’ lOO(SC,-SC, )/SC, Daily body weight change, kg
errors
of
changes
in
scrotal circumference test to breedtng’
and
weight
from
the
end
of
feedlot
Hereford
Beef Synthetic
Dairy Synthetic
6 -3.91 f 1.20 -10.94 i- 3.63 -0.113 t 0.143
14 -2.21 z 0.58 -5.41 t 1.37 -0.135 I 0.112
5 -0.98 * 1.16 -2.52 i 3.07 -0.028 i 0.074
‘None of the differences between means were significant. ‘C, =Scrotal circumference at the end of feedlot period, SC,=Scrotal breeding season.
DECEMBER
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1984 VOL. 22 NO. 6
circumference at the start of the
673
THERIOGENOLOGY REFERENCES Amann, R.P. Sperm production rates. 12: Johnson, AD., Gomes, W.R. and Van Demark, N.L. (Ed.). The Testes. Vol. 1. Academic Press, New York, 1970, pp. 432-471. 2.
Carrol. E.J ., Ball, L. and Scot. J .A. Breeding soundness m bulls. A summar! of 10.940 examinations. J. Am. Vet. Med. Assoc. E: 1105-1111 (1963;.
3.
Cates. W .F. Observations on scrotal circumference and its relationship to classification of bulls. Proc. Ann. Meet. Sot. Theriogenology, Cheyenne, Wyoming, 1975, pp. l-18.
4.
Coulter, G .H. and Foote, R.H. Relationship of testicular weight to age and scrotal circumference ( Holstein bulls. J. Dairy Sci. 2: 730-732 (1976).
5.
Fields, M.J ., Burns, W.C. and Warnick, AC. Age, season and breed effects on testicular volume and semen traits in young beef bulls. J Anim. Sci. 48: 1299-1304 (1979).
6.
Berg, R.T. and Peebles. M. Updating The University of Alberta beef breeding project. 62nd Annuz Feeders’ Day Rep. Dept. Anim. Sci. Univ. of Alberta. Edmonton, Alberta, 1983, pp. 2-7. Li. C.C. Path Analysis - a primer. The Boxwood Press, Pacific Grove, California. 135-186.
1975, pp.
Tukey. J .W. Causation, regression, and path analysis. h: Statistics and Mathematics in Biology. The lowa State College Press, Ames, Iowa, 1954. pp. 35-65. 9.
Coulter, G.H. and Foote. R.H. Relationship of body weight to testicular size and consistency in growing Holstein bulls. J . Anim. Sci. 44: 1076-1079 (1977).
10.
Neely, J.D., Johnson. B.H., Dillard, E.U. and Robison, O.W. Genetic parameters for testes size and sperm number in Hereford bulls. J. Anim. Sci. 2: 1033-1040 (1982).
11.
Coulter, G.H.. Larson, L.L. and Foote, R.H. Effect of age on testicular growth and consistency c Holstein and Angus bulls. J. Anim. Sci. 41: 1383-1389 (1975).
12.
Lunstra, D.D., Ford, J.J. and Echternkamp, SE. Puberty in beef bulls: Hormone concentrations, growth, testicular development, sperm production and sel:ual aggressiveness in bulls of different breeds. J. Anim. Sci. 3: 1054.1062 (1978).
13.
Latimer, F.G., Wilson, L.L. and Cain. M.F. Scrotal measurements in beef bulls: Heritability estimates, breed and test station effects. J. Anim. Sci. 3: 473-479 (1982).
14.
Coulter, G.H. and Foote, R.H. Effect of season and year of measurement on testicular growth ar consistency of Holstein bulls. J Anim. Sci. 9: 434-438 (1976).
15.
Johnson, B.H.. Rolinson, O.W. and Dillard, E.U. Body growth and testicular development in yearling Hereford bulls. J. Anim. Sci. 2: 213 (Abstr.) (1974).
16.
Sitarz, N.E., Erb, R.E., Martin, T.G. and Singleton. W.L. Relationships between blood plasma testosterone, weaning treatment, daily gains and certain physical traits of young Angus bulls. J. Anim. Sci. 9: 342-349 (1977).
17.
Jones, S.D.M., Price, M.A.. and Berg, R.T. Effects of breed tvpe and slaughter weight on feedlo performance and carcass composition in bulls. Can. J. Anim. Sci. 3: 277-284 (1978).
674
DECEMBER
1984 VOL. 22 NO. 6
RELATIONSHIPS BETWEEN GROWTH PARAMETERS AND SCROTAL CIRCUMFERENCE IN YOUNG BEEF BULLS M. Makarechian, A. Department of University Edmonton, Alberta, Recetved for Pubiication:
June
Accepted:
Farid and R.T. Berg Animal Science of P.lberta Canada T6C 2H! 18, 1984
September
2, 1984
ABSTRACT Scrotal circumferences of 119 young bulls of four distinct breeding groups were measured at the end of a feedlot performance test and at the beginning of the breeding season when the bulls were approximately 14 months old, to study the relationships of weight and growth parameters with testes size. Scrotal circumference was positively correlated with body weight at the end of feedlot test in the four breeding groups. The association between scrotal circumference and body weight was much stronger m the breeding group which had been selected for low yeariing weighi than in the other three breeding groups which had been selected for high growth rate. The relationships between scrotal circumference and preweaning and postweaning gain differed among the four breeding groups. Preweaning gain was the most important factor in the association between hod! weight and scrotal circumference among the three beef breeding groups. The results indicated that the preweaninp stage was a critical period for testicular development and that the probability of finding beef bulls with smaller than average testes among the bulls selected for weaning weight would be relatively small. Scrotal circumference was reduced (2.5-11%) from the end of feedlot test until the beginning of the breeding season. Key words: scrotal circumference, growth, beef bulls. INTRODUCTION Recently, emphasis on natural service fertility of breeding beef bulls has increased. Scrotal circumference as a measure of testes size, has been associated with semen production (1). Scrotal circumference is an important component in examining beef bulls for breeding soundness (2, 3). Many factors such as breed, age. season and body weight influence testes size or scrotal circumference (4, 5). Body weight at the end of feeding test affects scrotal circumference, and is a function of birth weight, preweaning and feedlot growth rate and age, all of which may influence testes development. Information on the relationship between growth rate and scrotal circumference would be helpful when growth rate and testes size are both considered in selecting young beef bulls. The objective of this study was to evaluate the influence of age. body weight and weight gain on scrotal circumference of yearling bulls. Acknowledgements. This project was supported by a grant from Alberta Agricultural Research Council’s “Farming for the Future” Program. The authors also acknowledge the contribution of G. Minchau and his coworkers at the Kinsella ranch for managing the herd.
DECEMBER
1984 VOL. 22 NO. 6
667
THERIOGENOLOCY
MATER1 ALS AND METHODS The study involved 119 yeariing bulis which were given a feedlot performance test after weaning. The bulls belonged to four breeding groups; Hereford (HE), Beef Synthetic (SY), Dairy Synthetic (SD) and Pee Wee (PW), maintained at The University of Alberta Ranch at Kinsella. Alberta. The breeding and management practices of the herd have been described by Berg and Peebles (6). Brief)). the HE is a purebred nonulatton onen to the industry’s superior bulls. The Sj’ is a svntheric nopuiation composed of approximately l/3 Charolats. l/3 Angus and l/3 Galioway breeds. The SD 15 a synthetic popuiation composed of approximately 213 dairy breeds (Holstein, Brown Swiss and Simmental) and l/3 beef breeds (Charolais, Angus and Hereford). The HE, SY and SD populations have been selected based on an index composed of preweaning and feedlot daily gain. The PW is a composite population of the above beef breeds selected for low yearling weight. The calves were born between early April and early June 1980 and were weaned in the middle of October. After an adjustment period of three weeks, the male calves were put on a feedlot performance test for 143 days and were fed ad libitum a ration of 64% barley. 21% oats, 10% pelleted dehydrated alfalfa hay and 5% protein-mineral-vitamin supplement. The calves were weighed at birth, during weaning, and at the end of the feeding test. Scrotal circumference was measured at the largest diameter of scrotum twice with a flexible tape. The average of the two measurements was used for analysis. The bulls were from 320 LO 383 days old when their scrotal circumference was measured. Twenty- five bulls were selected for breeding from the HE, SY and SD groups as follows: 6 HE, 14 SY and 5 SD. Body weight and scrotal circumference of these 25 bulls were measured by the same technician before the breeding season started (last week of June) to relate the change in scrotal ctrcumference to the change m body weight from the end of feeding test to breeding. Body weight at the end of the feeding test, birth date (age), and their quadratic forms were used in stepwise regression analysis to explain variation in scrotal circumference and change in scrotal circumference from the end of feedlot to breeding. Scrotal circumference at the end of feedlot, body weight change from the end of feedlot to breeding and their quadratic forms were also used as explanatory variables to study the change in scrotal circumference between the two measurements. Tests for the homogeneityof regression coefficients among breeding groups were made using appropriate dumm! variables. Path-coefficient analysis (7) was used to compare the relative importance of the common causative factors which contribute to the association between scrotal circumference and body weight at the end of feeding test, RESULTS The mean scrotal circumference and body weight at the end of feeding test for the four breeding groups are presented in Table 1. The SY bulls had significantly larger scrotal circumference than the other breeding groups. The mean scrotal circumference in the HE was similar to that in the SD, but it was larger than the average scrotal circumference in the PW group. As mean body weight increased, the ratio of scrotal circumference to body weight decreased (Table 1). The scrotal circumference as percent of body- weight was maximum in the PW and minimum in the SD group. The relationship between scrotal circumference and bodv weight at the end of feeding test was linear, as the quadratic terms were not significantly different from zero The slopes of regression lines were not significantly different among the four breeding groups except between the PW and the SY. The explanatorv power of body. weight _ measured by the coefficient of determination (R’) increased- ai mean body weight decreased across the breeding groups (Table 2). Age did not significantly influence scrotal circumference over and above body weight.
668
DECEMBER
1984 VOL. 22 NO. 6
THERIOGENOLOGY
TABLE 1 Means and standard errors of scrotal circumference, body weight and the ratio of scrotal circumference to body weight in the four breeding groups at the end of feedlot test
Measurement
Hereford
Number of bulls SC (cm)l Body weight (kg) SC’100 kg body weight
17 37.4 I 0.67a 404.2 + ll.Oa 9.4 i
0.22a
Beef Synthetic
Pee Wee
66 39.0 ? 0.34b 444.4 + 5.5b
17 35.3 + 0.67~ 348.9 i 11.oc
8.8 I
0.12a
Means within each measurement followed by different (P
10.3 i
0.22c
Daq
Synthetic
19 36.6 rt 0.63ac 449.9 t 10.4b 8.2 Ifr 0.21b
letters are significantly different
TABLE 2 The regression
of scrotal cvcumference
on body weighi tn the jour breedtng groups at the end of feedlot test
Breeding group
I nrercept (cm)
Regression coefficient (b) (cm/kg)
Standard error of b (cm/kg)
R’
Hereford Beef Synthetic Pee Wee Dairy Synthetic
27.012 30.291 21.073 23.862
0.025. 0.019* 0.0410.028
0.011 0.007 0.008 0.018
0.252 11.107 0.601 0.074
* Significant at P
Figure 1. Path diagram depicting the relationship between scrotal circumference and body weight through birth weight, birth date and gain.
DECEMBER
1984 VOL. 22 NO. 6
669
THERIOGENOLOGY
Body weight and scrotal circumference are correlated primarily because of their mutual association with age and growth traits. In such a causal system, path analysis is generally more descriptive than simple correlation (8). Fig. 1 was assumed to be the causal system leading to the association of scrotal circumference and body weight. Based on this causal system, the path values for the four breeding groups are presented in Table 3. The path values were considerably different among the breeding groups. All the path values had the same sign in the SY and PW groups. The path coefficients from feedlot daily gain, preweaning daily gain and birth weight to body weight were all comparable in the four breeding grcups. The rank of the breeding groups for the direct effect of age on body weight was the same as that of their average body weight, the highest was SD and the lowest was PW. In SY and PW groups, preweaning daily gain had a higher direct effect on scrotal circumference than the other variables, while in the SD group, birth weight had the highest direct effect followed by feedlot daily gain. The direct effect of preweaning daily gain on scrotal circumference was negative and small in the SD. Preweaning daily gain, birth weight and age all had considerable direct influences on scrotal circumference in the HE breeding group. The total variance of each system is shown in Table 4. The weight (over 94 percent) than in circumference. which suggests that the variation in body weight.
of the variables accounted for by the underlying causal system accounted for much more variation in body preweaning daily gain, feedlot daily gain and scrotal the variables considered in the system could explain only
The relationship between scrotal circumference and body weight via feedlot daily gain, preweaning daily gain, birth weight and age are shown in Table 5. The association via age-birth weight was negligible in all cases and is not shown in the table. The associations between body weight and scrotal circumference via feedlot daily gain were positive in all the breeding groups, but negligible in the SY and PW. The association between body weight and scrotal circumference via birth weight was negative and negligible in all the breeding groups, except in the SD which was positive and considerably higher than in the other groups. The average body weight and scrotal circumference were both lower at the start of breeding season than at the end of the feedlot performance test (Table 6). There was approximately 11%. 5.5% and 2.5% reduction in scrotal circumference for the HE, SY and SD bulls respectively. Neither scrotal circumference at the end of feedlot nor preweaning or postweaning gains had any influence on the amount of scrotal circumference change from feedlot to breeding. The reduction in scrotal circumference linearly decreased as body weight at the end of feedlot test increased (P
THERIOGENOLOGY TABLE 3 Path values of the causal system shown in Figure
I for the four breeding groups
Breeding Group Hereford
;,I P, P, P, p,
P6 P, P, P, P,, PI! P,: p::
- .1615 -0.1131 -0.4572 0.3410 0.0224 0.1286 0.5589 0.7336 0.0907 0.2408 0.2637 0.3729 0.4242 0.5157
Beef Synthetic
Pee Wee
Dair!- Synthetic
0.0624 0.4993 -0.1197 0.0713 0.4253 -0.1894 0.4775 0.6134 0.1517 0.3172 0.1373 0.3808 -0.1112 0.0102
0.1214 0.6746 -0.0229 0.2202 0.6456 -0.2463 0.4951 0.4715 0.1401 0.1446 0.1801 0.8018 -0.1431 0.0645
-0.0413 0.7285 0.0692 -0.2103 -0.1053 -0.4575 0.7328 0.7283 0.1037 0.7771 0.3050 -0.1223 0.5520 0.1509
‘r = correlation coefficient between birth date and birth weight. ‘Pi = the ith path coefficient in Figure 1.
TABLE 4 The proportion of variance of preweaning daily gain, feedlot daily gain, body weight and scrotal circumference accounted for by the causal system in the four breeding groups
Breeding group
Preweaning daily gain
Feedlot daily gain
Body weight
Scrotal circumference
Hereford Beef Synthetic Pee Wee Dairy Synthetic
0.205 0.256 0.452 0.531
0.093 0.244 0.672 0.301
0.974 0.979 0.992 0.943
0.454 0.151 0.691 0.229
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The of the four breeding groups generally the higher reported estimates the literature (9, IO). The ranks of the three beef breeding groups for body weight at the end of feeding test and for scrotal circumference were the same. The S‘I bulls were, on the average, 30% heavier than the PW bulls, but their average scrotal circumference was only 10.5% larger than that of the PW group. Considering that PW bulls have been selected for hght yearling weight. a minimum testes size is probably necessary for adequate fertility for a given body weight. The SD bulls were the heaviest and PW the lightest group at the end of feedlot performance test, but their scrotal circumferences were not significantly different. The SD group may not conform to the average body weights and scrotal circumferences of other breeding groups because growth pattern of testes may differ between dairy and beef breeds. Coulter et al. (11) reported that the mean scrotal circumference in young Angus bulls was greater than that in Hoisteins of comparable age and the situation was reversed as the animals reached maturity, concluding that Angus reached their maximum testicular size at a younger age than Holstein bulls. Lunstra et al. (12) found that Brown Swiss bulls had consistently larger scrotal circumferences than Angus, Hereford and their crosses when they were between 7 and 13 months old. In addition to breed differences in scrotal circumference, some factors such as station (11, 13). station by breed interaction (13), season and year (14) and breed by year interaction (5) are reported to have significant influence on measures of testtcular size. The scrotal circumference as percent of body weight ranked in reverse order of body weight among the four breeding groups. This observation suggested that the change in body weight as a result of selection would not necessarily result in proportional chaner in scrotal circumference. The absence of a significant difference between the HE and SY for this ratio rndicated that the difference in scrotal circumference between the two groups was mainly due to the difference in their body weights. The relatively smaller correlation coefficients between scrotal circumference and body weight in the SY. SD and HE compared to the PW population might have been resulted from selection for and against gain in these breeding groups. The correlation coefficients between body weight and scrotal circumference in the HE and PW groups were within the range of estimates reported in the literature for yearling bulls (10. 12), but the coefficients for the SY and SD groups were smaller. The in-significant effect of age on scrotal circumference was primarily due to the short range in age among the bulls (63 days) and, in addition, body weight and age are positively correlated in young animals. The results of the path analysis indicated that the relationships between the growth parameters and testicular development differed considerably among the breeding groups. Preweaning daily gain was the most important factor in the SY and PW and was an important factor in the HE group which influenced testicular growth and contributed to the association between body weight and scrotal circumference. Johnson et al. (15) also reported a high association between preweaning gain and testicular development in beef bulls. The results indicated that it would be unlikely for beef bulls with small testes to be selected for breeding when preweaning gain was considered in the selection program. In constrast to the beef breeding groups, preweaning gain did not have appreciable influence on testicular development in the SD. and its contribution to the association of body weight and scrotal circumference was also neghgible. In addition to the breeding group differences, the high level of milk production of SD cows might have contributed to the differences. Assuming that the rate of testicular growth is relatively slow in the SD breeding group, as reported by Coulter et al. (11) in Holstein bulls, the high level of milk production of SD cows would induce high preweaning gain without corresponding testicular development
SY and minimum in the SD which corresponds with the rate of fattening (maturity)
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THERIOGENOLOGY
these breeding groups (17). The extending of the scrotum as a result of further descent of the testes and seasonal temperature change might have contributed co the decrease in scrotal circumference.
TABLE 5 Cause: of
assoctatton
between
hod),
wetghi
and
scrotal
crrcumjerence
tn
tht
four
oreedmg
group
Hereford
Beef Synthetic
Pee Wee
Dairy Synthetic
0.2736 0.0711 0.3447
0.2336 0.0130 0.2466
0.3780 0.0874 0.4654
-0.0891 0.0188 -0.0703
Via birth weight Direct Indirect Total
0.0384 -0.0390 -0.0006
-0.0169 -0.0586 -0.0755
-0.0200 -0.1017 -0.1X7
0.0572 0.1883 0.2455
Via age Direct Indirect Total
0.1242 -0.1808 -0.0566
0.0032 -0.0016 0.0016
0.0093 -0.0087 0.0006
0.1173 -0.0446 0.0727
0.1474
0.0656
0.0891
0.2235
Pathways of association Via preweaning daily gain Direct’ Indirect ? Total
Via feedlot daily gain
‘The product of path values from the common source to scrotal circumference and body weight. ‘Sum of the products of path values leading to scrotal circumference and body weight from the common source via other variables.
TABLE 6 Means
and
standard
Number of bulls SC,-SC,. cm’ lOO(SC,-SC, )/SC, Daily body weight change, kg
errors
of
changes
in
scrotal circumference test to breedtng’
and
weight
from
the
end
of
feedlot
Hereford
Beef Synthetic
Dairy Synthetic
6 -3.91 f 1.20 -10.94 i- 3.63 -0.113 t 0.143
14 -2.21 z 0.58 -5.41 t 1.37 -0.135 I 0.112
5 -0.98 * 1.16 -2.52 i 3.07 -0.028 i 0.074
‘None of the differences between means were significant. ‘C, =Scrotal circumference at the end of feedlot period, SC,=Scrotal breeding season.
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1984 VOL. 22 NO. 6
circumference at the start of the
673
THERIOGENOLOGY REFERENCES Amann, R.P. Sperm production rates. 12: Johnson, AD., Gomes, W.R. and Van Demark, N.L. (Ed.). The Testes. Vol. 1. Academic Press, New York, 1970, pp. 432-471. 2.
Carrol. E.J ., Ball, L. and Scot. J .A. Breeding soundness m bulls. A summar! of 10.940 examinations. J. Am. Vet. Med. Assoc. E: 1105-1111 (1963;.
3.
Cates. W .F. Observations on scrotal circumference and its relationship to classification of bulls. Proc. Ann. Meet. Sot. Theriogenology, Cheyenne, Wyoming, 1975, pp. l-18.
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Coulter, G .H. and Foote, R.H. Relationship of testicular weight to age and scrotal circumference ( Holstein bulls. J. Dairy Sci. 2: 730-732 (1976).
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Fields, M.J ., Burns, W.C. and Warnick, AC. Age, season and breed effects on testicular volume and semen traits in young beef bulls. J Anim. Sci. 48: 1299-1304 (1979).
6.
Berg, R.T. and Peebles. M. Updating The University of Alberta beef breeding project. 62nd Annuz Feeders’ Day Rep. Dept. Anim. Sci. Univ. of Alberta. Edmonton, Alberta, 1983, pp. 2-7. Li. C.C. Path Analysis - a primer. The Boxwood Press, Pacific Grove, California. 135-186.
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Tukey. J .W. Causation, regression, and path analysis. h: Statistics and Mathematics in Biology. The lowa State College Press, Ames, Iowa, 1954. pp. 35-65. 9.
Coulter, G.H. and Foote. R.H. Relationship of body weight to testicular size and consistency in growing Holstein bulls. J . Anim. Sci. 44: 1076-1079 (1977).
10.
Neely, J.D., Johnson. B.H., Dillard, E.U. and Robison, O.W. Genetic parameters for testes size and sperm number in Hereford bulls. J. Anim. Sci. 2: 1033-1040 (1982).
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Coulter, G.H.. Larson, L.L. and Foote, R.H. Effect of age on testicular growth and consistency c Holstein and Angus bulls. J. Anim. Sci. 41: 1383-1389 (1975).
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Lunstra, D.D., Ford, J.J. and Echternkamp, SE. Puberty in beef bulls: Hormone concentrations, growth, testicular development, sperm production and sel:ual aggressiveness in bulls of different breeds. J. Anim. Sci. 3: 1054.1062 (1978).
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Latimer, F.G., Wilson, L.L. and Cain. M.F. Scrotal measurements in beef bulls: Heritability estimates, breed and test station effects. J. Anim. Sci. 3: 473-479 (1982).
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Coulter, G.H. and Foote, R.H. Effect of season and year of measurement on testicular growth ar consistency of Holstein bulls. J Anim. Sci. 9: 434-438 (1976).
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Johnson, B.H.. Rolinson, O.W. and Dillard, E.U. Body growth and testicular development in yearling Hereford bulls. J. Anim. Sci. 2: 213 (Abstr.) (1974).
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Sitarz, N.E., Erb, R.E., Martin, T.G. and Singleton. W.L. Relationships between blood plasma testosterone, weaning treatment, daily gains and certain physical traits of young Angus bulls. J. Anim. Sci. 9: 342-349 (1977).
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Jones, S.D.M., Price, M.A.. and Berg, R.T. Effects of breed tvpe and slaughter weight on feedlo performance and carcass composition in bulls. Can. J. Anim. Sci. 3: 277-284 (1978).
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