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Associations Between Service Interval, Interval from First Service to Conception, Number of Services per Conception, and Level of Butterfat Production
ASSOCIATIONS BETWEEN SERVICE INTERVAL, INTERVAL FROM F I R S T S E R V I C E TO C O N C E P T I O N , N U M B E R O F S E R V I C E S P E R CONCEPTION, AND LEVEL OF BUTTERFAT PRODUCTION R. W. TOUCHBERRY Department of Dairy Science, University of Illinois, Urbana AI'~D
K. ROTTENSTEN A~D It. Alk~DERSEN
Institute of Artificial Ins'emination, Royal College of Agriculture and Veterinary Medicine, Copenhagen, Denmark SUMMARY The interval from first service to conception decreases as the service interval increases from 0 to 127 days, at which it reaches a minimum. After a service interval of 127 days, the interval from first service to conception increases slightly as the service interval increases. The length of the service interval accounts for only 1.4% of the variance of the interval from first service to conception; thus, even though its effect on the interval from first service to conception is significant, its effect is small and of little importance. Service interval does not significantly affect the number of services per conception and accounts for only 0.3% of the variance o2 services per conception. I n general, the number of services per conception (Y) decreases linearly as the service interval (X) increases, the partial regression of :g on X for a constant butterfat production being --0.0034 - - 0.0023. The partial regressions of the number of services per conception and the interval from first service to conception on butterfat production for a constant service interval are +0.003 ± 0.001 and +0.091 ± 0.046, respectively, and are significant at the 0.05 level of probability. The partial regression of the number of services per conception on butterfat production for a constszlt service interval and constant interval from first service to conception is --0.00004 ± 0.0007 and is not significant. I t is concluded that there is no real biological relationship between services per conception and level of butterfat production. The service intem.al alone accounts for 16.8% of the variance of the interval from calving to conception. The interval from calving to conception increases at an increasing rate as selwice interval increases to approxintately 50 days. A f t e r a service intelwal of 50 days, the interval from calving to conception increases almost linearly and in an approximate 1 to I ratio as the service inte~wal increases. Calving interval has approximately the same relationship to service interval as has the interval from first service to conception. With the present data, the average service interval must be from 36 to 49 days to maintain an average calving interval of approximately 365 days. I f the conception rate was increased to 60%, the average service interval allowed could be increased approximately 11.5 days, and a calving interval of approximately 365 days could still be nmintained. T h e a s s o c i a t i o n b e t w e e n t h e i n t e r v a l f r o m c a l v i n g to t h e first service (service i n t e r v a l ) a n d t h e n u m b e r of services p e r c o n c e p t i o n has been i n v e s t i g a t e d b y a n u m b e r of w o r k e r s (6, 11, 12, 19, 21, 23, 24, 25). I n g e n e r a l , these w o r k e r s rep o r t e d t h a t t h e services r e q u i r e d p e r c o n c e p t i o n d e c r e a s e d as t h e s e r v i c e i n t e r v a l i n c r e a s e d to 100-120 d a y s , w h e n t h e n u m b e r of services p e r co'nception r e a c h e d a m i n i m u m . V a n D e m a r k a n d S a l i s b u r y (25), r e c o g n i z i n g the i m p o r t a n c e of the l e n g t h of t h e c a l v i n g i n t e r v a l , s u g g e s t e d t h a t cows s h o u l d first be s e r v i c e d 60 to 80 d a y s a f t e r p a r t u r i t i o n , so as to r e q u i r e a r e l a t i v e l y s m a l l n u m b e r of services p e r c o n c e p t i o n a n d s t i l l m a i n t a i n a c a l v i n g i n t e r v a l of a p p r o x i m a t e l y 12 mo. Received for publication February 2, ]959. 1157
115~
R.W.
T O U C t t B E R R Y , K. R O T T E N S T E N , AND I-I, A N D E R S E N
T r i m b e r g e r (24) recommended that cows should not be serviced until 50 days or more a f t e r parturition. Gaines (13) found a correlation of 0.039 ± .010 between the service period (the interval f r o m calving to conception) and the first m o n t h ' s milk production; thus, it is evident that the first m o n t h ' s milk yield and service period are practically independent. I n another study, Gaines and P a l f r e y (14) fomld a correlation of 0.053 ± 0.049 between the average length of the first nine calving intervals and the average yield per day over the nine calving intervals of 186 cows. On the basis of this correlation, the authors concluded that calving interval has no effect on yield and that high-yielding cows are not bred so as to freshen less frequently t h a n lo~v-yielding cows. Several workers (1, 3, 18, 20, 22) have reported that higher-yielding cows require more services per conception, whereas others (2, 4, 10, 13) have reported that there is no relationship between level of production and services per conception. The associations between the service interval and the interval f r o m first service to conception, and between the immber of services required per conception and the interval from first service to conception, have not been well established. The objectives of the present s t u d y are to determine the associations between the service interval, the interval f r o m first service to conception, the n u m b e r of services per conception, and the level of b u t t e r f a t production; and to demonstrate some of the consequences of these associations. S O U R C E A N D D E S C R I P T I O N OF DATA
The data for this study were taken f r o m the Red Danish Milkrace progeny groups at the Danish progeny testing stations during the testing year 1955-56. All heifers were in their first lactations when the data involved in this s t u d y were recorded. The observations consist of 2,056 artificial services on 902 first-calf heifers by 51 Red Danish Milkrace sires. The n u m b e r of daughters per sire ranged f r o m 14 to 20, with a meau of 17.7. E i g h t hundred fifty-three of the 902 heifers conceived and these heifers received a total of 1,782 services. Some of the 49 heifers returned to the f a r m e r s as n o n p r e g n a n t m a y have been p r e g n a n t as a result of their last services, but no info'rmation on p r e g n a n c y examinations was available after the heifers left the testing stations. The variables involved in this s t u d y are days f r o m first service to conception (W), n u m b e r of services per conception (Y), service interval (days f r o m calving to first service) (X), services per cow (Y'), and first lactation f a t production in kilograms (Z). The herdsmen at the varimis testing stations recorded the calving dates and service dates on b a r n sheets, and the heifers were diagnosed for p r e g n a n c y by veterinarians or technicians, who indicated the service at which conception most likely occurred. The herdsmen then recorded the date of pregn a n c y examination and the service at which p r e g n a n c y most likely occurred. The data in this study on days f r o m calving to first service (X), days f r o m first service to conception (W), services per conception (Y), and all services (Y') were taken f r o m the b a r n sheets. The first lactation b u t t e r f a t records are those given in (15) for the Red Danish Milkrace sires.
ASSOCIATIONS B E T W E E N SERVICE I N T E R V A L S , CONCEPTION AND P R O D U C T I O N 1 1 5 9
All cows included in this s t u d y were first-calf heifers, all were given ample o p p o r t u n i t y to conceive, there was no' selection for production among the daughters of a sire, and there was no tendency to give high producers more of a chance to conceive than low producers; therefore, the data should provide unbiased estimates of the relationships between days from first service to conception, services per conception, total n u m b e r of services, service interval, and b u t t e r f a t production. A N A L Y S I S OF DATA A N D t~ESULTS
The service intervals ranged f r o m 44 to 202 days; there were only 11 below 50 days and only 13 above 150 days, of which four were above 160 days. I n Table 1, the mean days f r o m first service to conception, the services per conception, and the total n u m b e r of services are listed according to the length of the service interval. The means listed u n d e r the service intervals f r o m 44-49 days a p p e a r to be slightly larger than those listed under the longer intervals, but other t h a n this there seems to be no consistent trend of the means with respect to the length of service interval. I n Table 2, the analyses of variance for days f r o m first service to conception, for services per Cmlception, and for all services are shown. I n no case is the variation between ten-day service interval groups significant at as low a probability as 0.05. I n spite of the general lack of significance of the between-group mean squares, one should not conclude that the length of the service interval is not associated with the length of the interval f r o m first service to' conception, with services per conception, or with total number of services per cow. The differences associated with the different lengths of service interval could be confounded with test station differences in m a n a g e m e n t and with service sire differences in fertility. Such differences are likely to be small in the data, because a concerted effort is exerted to standardize m a n a g e m e n t practices at the various statio'ns and the field nonr e t u r n rates for the various service sires used were approxinlately equal. I n addition to the possibility o'f confounded effects, it is possible that the linear or a curvilinear component or both components of the length of service interval could be significant, as was shown by V a n D e m a r k and Salisbury (25). To avoid the possibility of confounded variables and to more precisely determine the associations of the length of service interval (X) with the interval from first service to conception ( W ) , services per conception (Y), total services (Y'), and b u t t e r f a t production (Z), the within-sire variations were studied. I n addition, the square root of the service interval ~ / ( X ) was included, to help aecount for a curvilinear effect. On observing the b u t t e r f a t records published in (15), it was obvious that there were variations in days in production and age at calving. The age at calving ranged f r o m 658 to 1,341 days, with a mean of 899 days, and the days in milk ranged f r o m 232 to 370 days, with a mean of 304 days. To adjust the individual records for these variations in age at calving and days in milk, the within-sire
TABLE I Mean n u m b e r of days f r o m first service to conception, n u m b e r of services per conception, and mmfl)er of services as distribute([ by ten-day periods of the service interval L e n g t h of service interval
Heifers t h a t conceived Days f r o m first service to conception Services per COlleeption
44-49
50-59
60-69
70-79
~0-$9
90-99
100-109
110-119
120-129
130-139
140-149
150 and up
Tot'~ls
W N a~
72.1 10 56.09
43.29 39 39.85
41.14 127 48.79
35.77 177 48.~9
36.35 178 47.33
30.04 134 42.31
31.07 86 39.59
38.96 45 46.(18
37.00 23 41.14
41.54 13 44.71
40.00 9 42.00
12.42 12 24,52
36.08 853 45.78
Y N a~
2.50 10 1.43
2.15 39 1.22
2.16 127 1.37
2.10 177 1.45
2.13 178 1.47
2.04 134 1.39
1.95 86 1.19
2.20 45 1.39
2.] 7 23 1.11
1.92 13 0.95
2.00 9 1.12
1.50 12 0.80
2.09 853 1.37
P e r cent conceived on first service Heifers f a i l i n g to conceive No. services All heifers All services
Per cent conceived on first service
30.0
38.5
40.2
49.7
46.1
44.8
47.7
40.0
30.4
46.2
44.4
66.7
44.9
Y" N
7.00 1
6.60 5
5.78 9
5.75 8
5.80 10
5.50 4
4.33 3
4.0o 2
5,50 2
4.67 3
3.00 1
4.00 1
5.53 49
Y' N ~,,
2.91 11 1.92
2.66 44 1.16
2.40 136 1.68
'2.26 185 1.61
2.32 188 1.68
2.14 138 1.50
2.03 89 1.26
2.28 47 1.41
2.44 25 1.42
2.44 16 1.46
2.10 10 1.10
1.69 13 1.03
2.28 902 1.58
27.2
34.1
37,5
47.6
43.6
4.3.5
46.1
38.3
28.0
37.5
40.0
61.5
42.5
1161
ASSOCIATIONS B E T W E E N SERVICE I N T E R V A L S , CONCEPTION AND P R O D U C T I O N
TABLE 2 Analyses of variance for days from first service to conception, services per conception, and all services Heifers that conceived
Source Between ten-day service intervals Within ten-day service intervals
All heifers
Days from first service to conception (W)
Services per conception (Y)
All services (Y') per cow
D.F.
M.S.
D.F.
M.S.
D.F.
M.S.
11 841
2,996 2,084
11 841
0.91 1.88
11 890
2.46 2.50
m u l t i p l e r e g r e s s i o n of b u t t e r f a t p r o d u c t i o n ( i n k i l o g r a m s ) on a g e a t c a l v i n g a n d d a y s in m i l k (D) was d e r i v e d a n d is E q u a t i o n ( 1 ) . Z'=--18.59
+ 0.075 A + 0.510 D
(A) (1)
B y u s i n g E q u a t i o n ( 1 ) , the b u t t e r f a t r e c o r d s w e r e a d j u s t e d to a n a v e r a g e l e n g t h of l a c t a t i o n of 304 d a y s a n d a n a v e r a g e age of c a l v i n g of 899 d a y s . Since t h e r a n g e s in age a t c a l v i n g a n d d a y s in m i l k w e r e r e l a t i v e l y n a r r o w , i t was a s s u m e d t h a t a l i n e a r e q u a t i o n w o u l d be sufficient to a d j u s t t h e b u t t e r f a t r e c o r d s . The w i t h i n - s i r e c o r r e l a t i o n s involving service i n t e r v a l (X), the s q u a r e r o o t of service i n t e r v a l x/(X), d a y s f r o m first service to c o n c e p t i o n (iV), services p e r c o n c e p t i o n (Y), a n d a d j u s t e d b u t t e r f a t p r o d u c t i o n (Z) a r e l i s t e d i n T a b l e 3. T h e c o r r e l a t i o n b e t w e e n services p e r c o n c e p t i o n a n d d a y s f r o m first service to c o n c e p t i o n is l a r g e (0.859), as w o u l d be expected, b u t i t i n d i c a t e s t h a t t h e two v a r i a b l e s a r e n o t i d e n t i c a l . B y h o l d i n g one of t h e two c o n s t a n t , the o t h e r w o u l d r e t a i n a p p r o x i m a t e l y 2 6 % of its o r i g i n a l v a r i a n c e . I t is r e c o g n i z e d t h a t services p e r c o n c e p t i o n (Y) a n d d a y s f r o m first service to c o n c e p t i o n (W) a r e n o t n o r m a l l y d i s t r i b u t e d l o t a g i v e n service i n t e r v a l . H o w e v e r , j u d g i n g f r o m T a b l e 1 i t is a p p a r e n t t h a t the v a r i a n c e s of ( W ) a n d (Y) a r e a p p r o x i m a t e l y e q u a l f o r t h e v a r i o u s i n t e r v a l s of (X). Thus, f o r e s t i m a t i n g t h e r e g r e s s i o n coefficients t h e req u i r e m e n t s t h a t t h e e r r o r s be no n e o r r e l a t e d a n d h a v e the same v a r i a n c e a r e satisfied, b u t f o r m a k i n g the u s u a l tests of significance t h e r e q u i r e m e n t s t h a t t h e e r r o r s be n o r m a l l y a n d i n d e p e n d e n t l y d i s t r i b u t e d a r e n o t satisfied. C o n s e q u e n t l y , one s h o u l d c o n s i d e r t h e s t a n d a r d e r r o r s of r e g r e s s i o n coefficients i n v o l v i n g (Y) a n d (IV) as r o u g h a p p r o x i m a t i o n s . TABLE 3 Within-sire correlations between serviee interval (X), square root of service interval (VX), days from firs~ service to conception (IV), services per conception (Y), and adjuste4 butterfat production (Z) for those heifers that conceived Vx
X __ gX W y
0.952
w --0.042 --0.073
Y --0.047 --0.045 0.859
z 0.082 0.045 0.078 0.065
The above correlations are based on 801 degrees of freedom and the 0.05 and 0.01 critical values are 0.069 and 0.091, respectively.
1162
R. W. TOUCHBERRY, K. ROTTENSTEN, AND H. A~DERSEN
The i.t~terval from first service to co~tception. The multiple regression of the interval from first service to conception (W) on the service interval (X) and ~/(X) was derived on a within-sire basis from the correlations, standard deviations, and means given in Tables 3 and 4. The resulting regression equation is Equation (2). W~-- 110.99 + 0.62 (-+ 0.24) X - - 13.88 (+_ 4.49) V ~
R w . x ~ - - 0.116
(2)
D.F. ~ 800
Equation (2) is plotted in Figure 1. In Equation (2), the partial regression coeefficints are significant at the 0.01 level of probability. Although the above multiple correlation coefficient (Rw.x¢~ ~ 0.116) is significant at the 0.01 level of probability, X and ~/X have accounted for only 1.4% of the variance of the interval from first service to conception (W). F r o m Equation (2), it is apparent that only a minor part of the variance of the interval from first service to conceptio~ is associated with variation in the service interval. VanDemark and SalisTABLE 4 M e a n s a n d w i t h i n - s i r e s t a n d a r d d e v i a t i o n s for X Mean a
86.7 21.4
VX 9.24 1.14
(X), (VX), (W), (Y), W 36.08 44.84
Y 2.09 1.39
and
(Z) Z 204.0 34.7
b u r y (25) found that only 1.0% of the variance of services per conception was associated with variation in the service interval. One would expect this rather close agreement, since the correlation between services per conception and the days from first service to conception as given in Table 3 is 0.859. On observing F i g u r e 1, it is apparent that the average interval from first service to conception decreases at a decreasing rate as the service interval increases from 36 to 127 days. As the service interval increases from 127 to 169 days there is practically no change in the average interval from first service to conception. If the derivative of ~'V with respect to X for Equation 2 is set equal to zero, the minimum average interval from first service to conception is found to occur at a service interval of 127 days. As the service interval increases from 36 to 127, 49 to 127, and 64 to 127 days, the average interval from first service to conception decreases 17.1, 11.2, and 6.6 days, respectively. Thus, it is apparent that relatively long average intervals from first service to conception are associated with service intervals below 60 days. Since the interval from first service to conception and the service interval are both significantly correlated with butterfat production, as shown in Table 3, one should, perhaps, include Z as an independent variable along with X and ~/X when predicting W. This was done, and the resulting equation is Equation 3. ][A"~ 88.51 ~- 0.55(--+ 0.24) X - - 12.88 ( ± 4.50) v / X + 0.09 (___ 0.05) Z.
Rw.zxv~i~ 0.136
D.F. ~ 799
(3)
ASSOCIATIONS BETWEEN SERVICE INTERVALS, CONCEPTION AND PRODUCTION
1163
220
200
180
160
140 W and (W+X) 120
220
I00 -
Z 80-
~'10
^
60 -
~00
I
3030 40
I
60
I
80 SERVIGE
I
LO0
~'
120
i~
140
I-"
160
INTERVAL (X)
Fro. 1. The r e g r e s s i o n s of the i n t e r v a l f r o m first service to c o n c e p t i o n (W), t he i n t e r v a l f r o m c a l v i n g to c o n c e p t i o n ( W + X ) , a n d b u t t e r f a t p r o d u c t i o n (Z) on the service i n t e r v s l
(X). Z is e x p r e s s e d in k i l o g r a m s a n d W, X, a n d W + X a re e x p r e s s e d i n days. l~z = 110.99 + 0.62 X -- 13.88 VX--~ W ÷ X = 110.99 ÷ 1.6') X -- 13.88 V X .
= ~045.2~+ o.~o x - 1o.oo vx--. I f Equation (3) is plotted for a constant Z of 204 kg., the resulting g r a p h coincides almost exactly with the graph of Equation (2) in F i g u r e 1. F o r corresponding regression values to differ more than a day, the service interval must exceed 160 days. Shown in Table 5 are the mean squares of the interval from first service to conceptio]b accounted for independently by the service interval and the level of butterfat production. I n general, only a small p a r t of the variance of the interval from first service to conception is associated with variation in the level of butterfat production.
The interval from calving to co tzccption. F r o m tile correlations, means, and standard deviations in Tables 3 and 4, the regression of the interval from calving to conception ( W + X ) on the service interval ( X ) and ~/X can be derived. This regression was derived and is Equation 4.
R . W. T O U C H B E R R Y ,
1164
f
K. I ~ O T T E N S T E N , A N D I-I. A N D E R S E N
A
IV + X = W -~ X ~--- 110.99 + 1.62 (___ 0.24) X -- 13.88 (+__4.49) N/~ R( w+x).x¢~ --- 0.41
D.F. == 800
(4)
Since service interval is a part of the interval from calving to conception, the regression of ( W + X ) on X would be expected to be large and the multiple correlati(m R (w+x),x ¢-~ would be expected to be large. Equation (4) is plotted in F i g u r e 1. The interval from calving to conception increases markedly as the service interval increases, as would be expected. After a service interval of 49 days, there is au increase in the interval from calving to conception of almost a day for each day the service interval increases. The graph of Equation (4) clearly indicates ho~v prolonging the service interval increases the interval from calving to conception. The regression of ( W + X ) on X, ~/X, and Z was derived and is Equation (5) W ~- X ~
88.51 ~- 1.55 (--+0.24)X --12.88 (_+4.50) v/X -~ 0.09(+_0.05)Z R~ w+x).x v~z ~ 0.42
D.F. ~- 799
(5)
If Equation (5) is graphed for Z = 204, the resulting curve coincides almost exactly with that of Equation (4) in F i g u r e 1. The service interval must exceed 160 days for the difference between corresponding regression values to exceed one day. The level of butterfat prod~¢ction. F r o m the correlations, means, and standard deviations in Tables 3 and 4, the regression of the adjusted butterfat production (iu kilograms) on service interval ( X ) and k / X was derived and is Equation (6). A
Z ~ 245.26 + 0.69 (-+0.18) X - - 10.90 ± (3.46) ~/X
Rz.xv~--- 0.137
(6)
D.F. -~ 800
Equation (6) is plotted in F i g u r e 1 aud, as is evident from F i g u r e 1, the derivative of Z with respect to X gives a minimum butterfat production for X ----63. As the service interval increases from 63 to 169 days, there is a general increase in the level of butterfat production. As the service interval increases from 36 to 63 days there is a small, but probably not real, decrease in butterfat pr(~duction. F o r service intervals from 49 to 90 days there is little difference in the levels of associated b u t t e r f a t production. The increase in the level of butterfat production associated with an increase in the length of the service interval could be brought about in several ways: (1) There could be a tendency to breed low-producing cows earlier after calving than high-producing cows. (2) High-producing cows may tend to show estrus at a larger interval postpartum than low-producing cows and, thus, d r i f t into the classes with longer service intervals. (3) With the longer service intervals, conception occurs later after calving; thus, there is less interference of pregnancy with production. The first possible cause is not likely to be of importance, since the management practices were standard for cows at the testing stations. Clapp
ASSOCIATIONS BETWEEN SERVICE INTERVALS, CONCEPTION AND PRODUCTION
1165
TABLE 5 Tests of significance of the within-sire variance of the i n t e r w d f r o m first service to conception Source
Service interval (Rw.x
D.F.
-- r~wz) S w2
Butterfat production ( R ~w.xv~
_
s Rw.xv,"~) S w2
Error ( 1 - R2w.xv~z ) S w 2
5I.S.
2
9,991.8"*
1
7,895.7* 1,978.8
799
++ Significant at the 0.01 level of probability. Significant at the 0.05 level of probability.
(8) has presented information, though not conclusive, that the second cause may be real. Gaines (13), on the other hand, has presented data which indicate that the first full month's milk yield and the service period (the interval from calving to conception) are independent ; thus, the second cause is not likely. It is probable that the third possibility is the real cause of the association. Services per conception. Previous studies (6, 11, 12, 19, 21, 23, 24, 25) have dealt with the number of services per conception in relation to' the length of the srvice interval. In the present data, the regression of services per conception on service interval, as shown in Equation (7) and in Table 6, was not significant. J ( = 1.67 -- 0.005 (±0.007) Z + 0.03 (+_0.14) v/X ~- 0.0028 (--+0.0014)Z R r . x ¢ ~ 0.084
D.F. ~ 799
(7)
If Equation (7) is plotted for Z = 204 kg., the resulting graph shows that there is a decrease in services per conceptio~ as the service interval increases; thus agreeing with the results in (6, 11, 12, 19, 21, 23, 24, 25). Since most of the sum of squares of services per conception associated with service interval was associated with the linear component, the regression of services per conception on the first power of service interval and butterfat production was derived, and the two partial regression coefficients bJ~x.z = --0.0034 +_ 0.0023 and brz.x -= 0.0028 ±_ 0.0014 were found. With 800 degrees of freedom, the partial regression -0.0034 is significant (0.05 < P < 0.10). This result again substantiates previous conclusions that there is a general decrease in the services required per eonception as the service interval increases. TABLE 6 Tests of significance of the within-sire variances of services p e r conception associated ~dth b u t t e r f a t production and se~wice interval Source
D.F.
M.S.
Butterfat production (R~.x¢-~ z-- R~.x¢-~) (St2)
1
7.52*
Service interval ( R2r. x v~ z - r~z) (Sy2)
2
2.18
Error (1 - R~..xv~z) (St2) + Significant at the 0.05 level of probability.
799
1.93
R. ~,V. TOUCHBERRY, K. ROTTENSTEN, AND H. ANDERSEN
1166
TABLE
7
T e s t s of significance of t h e within-sire v a r i a n c e of total n u m b e r of services a s s o c i a t e d with service interval a n d b u t t e r f a t p r o d u c t i o n
Source Service i n t e r v a l
(R2:r,.xz)
D.F. Sz,2 -- r~z S r,2
B u t t e r f a t production (R-r,.xz") S z 2 - rrx2 S t , 2 :Error ( 1 - R~.xz ) S t 2
M.S.
1
12.269"
l
9.875*
849
2.32
+ Significant at the 0.05 ]eve] of probability. The partial regression of services per conceptiou on b u t t e r f a t production of 0.0028 is significant (0.01 < P < 0.05) and suggests that higher-producing cows require more services per conception. This relationship is probably brought about by the fact that cows requiring more services per conception are not pregn a n t as soon a f t e r calving and, thus, that their current production is less affected by p r e g n a n c y than is that of cows requirino" fewer services per conception. To provide evidence oll this point, the regression of services per conception (Y) on service interval (X). the interval from first service to conception (W), and the level of b u t t e r f a t production (Z) was derived and is Equation (8). I 1 = 1.19 - - 0.0007 _ (0.001) X - t - 0.0267 (__0.0005) W - - 0.00004
(+_0 0007)z.
(8)
The partial regression bYZ.X~g~- - 0 . 0 0 0 0 4 is n o t significant and is practically zero. Thus, it a p p e a r s that there is no real biological basis for concluding that high-producing cows are less fertile than low-producing cows. This is in agreement with the conclusions of (2, 4, 10, 13) and in disagreement with the findings of (1, 3, 18, 20, 22). The relationships reported in (1, 3, 18, 20, 22) are, possibly, confounded with other facto'rs. All services. B y studying services per conception, one includes only those cows that conceived. This, in effect, truncates the data to include only fertile cows. Equation (9) is the regression of all services (Y'), whether or not the co~s conceived, on service interval (X) and b u t t e r f a t production (Z). A
: Y ' = 2.13 - - 0.0056 (+--.0024) X + 0.0031 (+-.0015) Z
Rr,.xz ~ .104
(9)
D.F. mE 849
A prediction equatiou including x / X was first derived, but the partial regression of (Y') on V X was extremely small and not significant; consequently, it was not included and Equation (9) was used. The regression co'efficients in Equation (9) are based on 849 degrees of freedom and are significant at the 0.05 level of probability. The variances independently accounted for by service interval and b u t t e r f a t production are shown in Table 7. I n spite of their significance, the two together account for only 1.1% of the within-sire variance of n u m b e r of services. F o r a constant b u t t e r f a t production, the n u m b e r of services goes up 0.56 of a service for each 100 days the service interval increases.
ASSOCIATIONS B E T W E E N SERVICE I N T E R V A L S , CONCEPTION AND P R O D U C T I O N
1167
D I S C U S S I O N OF R E S U L T S
The results indicate that the number of services per conception (Y) decreases a small, though not significant, amount for each day the service interval (X) increases, the equation for a coustaut level of butterfat production (Z) being = 2.38 - .0034 X. The results also indicate that the interval from first service to conception (IV) decreases at a decreasing rate as the service interval (X) increases to 127 days. With a service interval of 127 days, the interval from first service to conception reaches a nlinimum of 32.8 days. Both a small number of services per eo'nception and a short interval from first service to conception are important items and are to be desired by a dairynlan. However, the attaimnent of a minimum number of services per conception and a minimum interval from first service to conception are not the only items to be considered in recommending an optimunl iuterval from calving to first service. The interval from calving to conception should definitely be considered, as it and the length of the gestation period determine the length of the calving interval. Most of the variation in calving interval is brought about by variation in the interval from calving to conception, because the variation in the length of the gestation period is relatively small. F r o m results in (9) and Tables 3 and 4 of this study, it appears that about 99% of the variation in calving interval is caused by variation in the interval from calving to conception. The important thing to remember concerning the service interval, services per conception, the interval from first service to (.onceptiou, and the interval from calving to conception is: that variation in service interval a(.comlts for approximately 1% of the variation in servic'es per conception and the interval from first service to conception; whereas, variation in the service interval accounts for approximately 17% of the variation in the interval from calving to conception and, thus, of calving interval. Thus, it would seem that the length of the calving interval desired would be the p r i m a r y consideration in recommending a short or long service interval. Equation (10) expresses the relationship between the interval from calving to conception and service interval for the mean Z and, as meutioned previously, almost coincides with that plotted in F i g u r e 1. A
( W -t- X) --~ 107.13 +1.55"* X --12.88"* v / X
(10)
If one assumes that the length of the service interval has little or no effect on the length of the gestation period, then the relationship between service interval and calving interval is expressed by adding the gestation period to Equatiml (10). Assuming a gestation period of 280 days, the calving interval for a mean Z may be expressed as shown in Equation (11). C.I. ~ 387.13 -t- 1.55 X - 12.88 \ / X (11) B y taking the partial derivative of C.I. in Equation (11) with respect to (X), setting the derivative equal to 0, and solving for X, it is found that the average minimum calving interval occurs when the average service interval (X) is
11.68
R. W. T O U C H B E R R Y , K. R O T T E N S T E N , AND 14. A N D E R S E N
].7.2 days. This value of X is outside the range of service intervals in these data and is one that is unrealistic. F o r service intervals of 36, 49, 64, 81, and 100 days, which are more nearly within the range of the present data, one would expect calving intervals of 366, 373, 383, 397, and 414 days, respectively. If the expected b u t t e r f a t production values in F i g u r e l for Equation (6) are divided by the corresponding expected calving intervals derived above, the production per calving interval day decreases as the calving interval increases. F o r example, for service intervals of 36, 49, 64, 81, and 100 days, the expected calving intervals are 366, 373, 383, 397, and 414 days, the expected b u t t e r f a t production is 205, 203, 202, 203, and 205 kg., and the expected production per calving interval day is 0.56, 0.54, 0.53, 0.51, and 0.50 kg., respectively. The present study agrees with the results of Gaines (14) in regard to current calving interval. Gaines (14) obtained a correlation of -0.134 ± 0.018 between current calving interval and current production per calving interval day, and a correlation of 0.142 ± 0.018 between the previous calving interval and current production per calving interval day. The correlation, 0.142, between previous calving interval and current production per calving interval day would be partially confounded with age effects, since cows having longer previous calving intervals are likely to be older at calving; thus, the correlation of 0.142 is probably too large. Consequently, the conclusion of Gaines (14) that what is gained in the current lactation by having a short calving interval is lost in the next lactation is questionable. Chapman and Casida (7) concluded that there was no marked reduction in production per calving interval day in the second of two consecutive short calving intervals. Johansson and Hansson (17) concluded that the optimum length of the first calving interval would be about 410 to 430 days, and about 400 days for the second. F o r the third and, probably, later lactations any length between 310 and 430 days gives about the same yield per calving interval day. On a basis of these references, calving intervals of 12 to 13 mo. seem to be desirable. The real solution for the optimum length of calving interval is, within biological limits, an economic one and depends on the relative economic values of calving intervals of varying lengths, including yield per calving interval day, seasonal variations in the price of milk, number of calves bovn, and varying number of services required per conception. Under present economic conditions, with the seasonal variations in milk prices, an average calving interval of approximately 12 too. would seem to be desirable. It should be remembered that at present under practical conditions nmn's control over calving interval is limited to that period between the time when the cow first comes into heat after calving and the time when the first service is made. There is more freedom to make this period long than there is to make it short. Artificial breeding organizations would be interested in a low number of services per conception, as this would be less expensive to the organizations where second and third services are given free of charge, when the first or first and second services fail to get the cow in calf. Thus, it is likely that artificial breeding organizations would recommend a relatively long service interval. On the
ASSOCIATIONS BETWEEN SERVICE INTERVALS, CONCEPTION AND PRODUCTION ] ] ( ) 9
other hand, a shorter service interval might result in more first services per cow, since the calving intervals would be shorter, thus producing more income for artificial breeding organizations. I f one reduces the number of services required per conception f r o m that found in these data, it would probably be possible to allow a longer service interval and still maintain a 365-day or shorter calving interval. I n the present data, the linear regressio'n of the interval f r o m first service to conception (W) on services per conception (Y) is 27.64; thus, if the average n u m b e r of services per conception was reduced f r o m 2.09, as found in these data, to 1.67, which would correspond to a 60% conception rate, the interval f r o m first service to conceptio~q would be reduced b y a p p r o x i m a t e l y 11.6 days. Assuming t h a t this effect is the same for service intervals of different lengths, one could allow an average service interval of from 45 to 60 days and still maintain a 365-day calving interval. I f one reduced the length of normal gestations by 15 to 30 days, it would be possible to allow a 15- to 30-day lo~ger service interval and still maintain a 365-day calving interval. Although it has not been conclusively established that the gestation period can be shortened without associated deleterious effects, work by DeFries et al. (9) indicates that selection for a shorter gestation is feasible. Other workers (1, 3, 18, 20, 22) have presented data which indicate that highproducing cows require a considerably increased n u m b e r of services per conceptio~. These data could be biased, in that there would be a tendency to be more patient with high-producing cows and allow them more chance to conceive than would be the case with low-producing cows. I t is also possible that service sire differences are confounded with production levels. In addition, it is probable that those cows requiring more services have a longer period f r o m calving to conception and, thus, less effect o'f p r e g n a n c y on lactations. t~y utilizing the present data, the partial regression of services per conception on b u t t e r f a t production for a constant service interval and a constant interval f r o m first service to conception is -0.00004 ± 0.0007. F r o m the eorrelatiolls, standard deviations, and means in Tables 3 and 4, the partial regressions br~w+x).z~-0.022 and brz.~w+~i)~---0.0007 can be derived. This suggests that the association between the level of b u t t e r f a t production and services per conception is largely a result of the corresponding relationship between the interval f r o m calving to conception and b u t t e r f a t production; thus, there is no evidence in these data to suggest t h a t the biological relationship between services per conception and the level of b u t t e r f a t production is different f r o m zero. Artificial breeding associations would p r o b a b l y be more interested in total n u m b e r of services as related to service interval r a t h e r t h a n services per conception because, at the time of first sorvice, ~o one really knows which cows will conceive and which will uot. As shown in Eqliation (5), 0.56 less services cau be expected for each 100 days of increase in the length of the service interval. REFERENCES (1) ANONYMOT:S. S e v e n t e e n t h A n n u a l R e p o r t of t h e N e w Zeala1~d D a i r y Board. House, Wellillgton, N e w Zealand. pp. 32-39. 1940-41.
Invicta
1170
It. W. TOUCHBERRY, K. ROTTENSTEN. AND It. ANDERSEN
(2) BOYD~ L. J., SEATH, D. M., AND OLDS, DU~WAaD. Relationship Between Level of Milk Production and Breeding Efficiency in Dairy Cattle. J. Animal Sci., 13 : 89. 1954. (3) BRANTON, CECIL, GRIFFITH, W. S., NORTON, H. W., AND HALL, J. G. The Influence of Heredity and Environment on the Fertility of Dairy Cattle. J. Dairy Sci., 39 : 933. 1956. (4) CARMAN,G. M. Interrelations of Milk Production and Breeding Efficiency in Dairy Cows. J. Animal Sci., 14: 753. 1955. (5) CASmA, L. E., A~D VENZKR, W. G. Observations on Reproductive Processes in Dairy Cattle and Their Relation to Breeding Efficiency. Proc. Am. Soc. Animal Production, pp. 221-223. 1936. (6) CHAPMAN, A. B., AND CASIDA, L. E. Factors Associated with Breeding Efficiency in Dairy Cattle. Proc. Am. Soe. Animal Production, pp. 57-59. 1934. (7) CHAPMAN, A. B., AND CASIDA, L. E. Length of Service Period in Relation to Productive and Reproductive Efficiency in Dairy Cows. Proc. Am. Soe. Animal Production, pp. 66-70. 1935. (8) CLAPP, HOwAP,~. A Factor in Breeding Efficiency of Dairy Cattle. Proc. Am. Soc. Animal Production, pp. 259-264. 1937. (9) DRFR'mS, J. C., TOVCHB~Y, R. W., AND HAYS, R. L. Heritability of Gestation Length in Dairy Cattle. J. Dairy Sei., 41: 745. 1958. (10) ECKLRS, C. H. A. Study of Breeding Records of Dairy Herds. Minnesota Agr. Expt. Sta., Bull. 258. 1929. (11) EL~NG, E. C., AND LA MASTEa, J. P. The Relation Between the Interval Elapsing After Calving and the First Service to the Number of Services Required for Conception. South Carolina Agr. Expt. Sta., Ann. Rept., 46: 64. 1933. (12) Er~, R. E., AND SHAW, A. O. Breeding Failure Survey in Washington. A Summary. Proc. West Div. Am. Dairy Sei. Assoc. 1948. (13) GAINRS, W. L. Milk Yield in Relation to Recurrence of Conception. J. Dairy Sci., 10: 117. 1927. (14) GAINES, W. L., AND PAL~'R~¥, J. R. Length of Calving Interval and Average Milk Yield. J. Dairy Sci., 14: 294. 1931. (15) HANSEN, K. Afkomspr~ver med tyre X. 286. beretning fra fors0gslaboratoriet. 1956. (16) HOFSTAg, M. S. A Study of Breeding Records of One Large Herd of Dairy Cattle. (Postpartum Breeding and Removal of the Corpus Luteum.) Cornell Vet., 31: 379. 1941. (17) JOHANSSON, IVAR, AND HA~SSON, AttTHUR. Causes of Variation in Milk and B u t t e r f a t Yieeld of Dairy Cows. Kgl. Lantbru~'sakad. Tidskr., Yr. 6 1/2. 1940. (18) JON~S, L R., DOUGHI,~KTY, R. W., AND HAAG, J. R. Reproductive Performance in Dairy Cattle. Oregon Agr. Expt. Sta., Bull. 395. 1941. (19) LASLRY, J. F,, AND BOGAltT, RALPH. Some Factors Influencing Reproductive Efficiency of Range Cattle Under Artificial and Natural Breeding Conditions. Missouri Agr. Expt. Sta., Research Bull. 376. 1943. (20) LgWIS, R. C., AND HORW0OD, R. E. The Influence of Age, Level of Production and Management on the Calving Interval. Michigan Agr. Expt. Sta., Quart. Bull., 32: 4. 1950. (21) OLDS, DURWAPm. Effect of the Post-Partum Interval on Fertility and on Percentage of Normal Returns. Proc. Assoc. So~thern Agr. Workers, 47: 78. 1950. (22) ROT~NS~_Ar, K., AND AIVDRI~SRRr, H. ForelObig undersCgelse over sammenhaeng mellem ydelse og frugtbarhed i malkekvaegavlen. 280. beretning fra forsOgslaboratoriet. 1955. (23) SHANNON, 1~. P., S.~LISBURY, G. W., A N D ¥ANDRMARK, N. L. The Fertility of Cows Inseminated at Various Intervals After Calving. J. Animal Sci., 11: 355. 1952. (24) TttIMBRRGEI~, GEORGE W. Conception Rates in Dairy Cattle from Services at Various Intervals A f t e r Parturition. J. Dairy Sci., 37: 1042. 1954. (2'5) VA~TDR]~IARI~,N. L., AND SALISR~IRY, G. W. The Relation of the Post-Partum Breeding Interval to the Reproductive Efficiency in the Dairy Cow. J. Animal Sei., 9 : 307. 1950.
K. ROTTENSTEN A~D It. Alk~DERSEN
Institute of Artificial Ins'emination, Royal College of Agriculture and Veterinary Medicine, Copenhagen, Denmark SUMMARY The interval from first service to conception decreases as the service interval increases from 0 to 127 days, at which it reaches a minimum. After a service interval of 127 days, the interval from first service to conception increases slightly as the service interval increases. The length of the service interval accounts for only 1.4% of the variance of the interval from first service to conception; thus, even though its effect on the interval from first service to conception is significant, its effect is small and of little importance. Service interval does not significantly affect the number of services per conception and accounts for only 0.3% of the variance o2 services per conception. I n general, the number of services per conception (Y) decreases linearly as the service interval (X) increases, the partial regression of :g on X for a constant butterfat production being --0.0034 - - 0.0023. The partial regressions of the number of services per conception and the interval from first service to conception on butterfat production for a constant service interval are +0.003 ± 0.001 and +0.091 ± 0.046, respectively, and are significant at the 0.05 level of probability. The partial regression of the number of services per conception on butterfat production for a constszlt service interval and constant interval from first service to conception is --0.00004 ± 0.0007 and is not significant. I t is concluded that there is no real biological relationship between services per conception and level of butterfat production. The service intem.al alone accounts for 16.8% of the variance of the interval from calving to conception. The interval from calving to conception increases at an increasing rate as selwice interval increases to approxintately 50 days. A f t e r a service intelwal of 50 days, the interval from calving to conception increases almost linearly and in an approximate 1 to I ratio as the service inte~wal increases. Calving interval has approximately the same relationship to service interval as has the interval from first service to conception. With the present data, the average service interval must be from 36 to 49 days to maintain an average calving interval of approximately 365 days. I f the conception rate was increased to 60%, the average service interval allowed could be increased approximately 11.5 days, and a calving interval of approximately 365 days could still be nmintained. T h e a s s o c i a t i o n b e t w e e n t h e i n t e r v a l f r o m c a l v i n g to t h e first service (service i n t e r v a l ) a n d t h e n u m b e r of services p e r c o n c e p t i o n has been i n v e s t i g a t e d b y a n u m b e r of w o r k e r s (6, 11, 12, 19, 21, 23, 24, 25). I n g e n e r a l , these w o r k e r s rep o r t e d t h a t t h e services r e q u i r e d p e r c o n c e p t i o n d e c r e a s e d as t h e s e r v i c e i n t e r v a l i n c r e a s e d to 100-120 d a y s , w h e n t h e n u m b e r of services p e r co'nception r e a c h e d a m i n i m u m . V a n D e m a r k a n d S a l i s b u r y (25), r e c o g n i z i n g the i m p o r t a n c e of the l e n g t h of t h e c a l v i n g i n t e r v a l , s u g g e s t e d t h a t cows s h o u l d first be s e r v i c e d 60 to 80 d a y s a f t e r p a r t u r i t i o n , so as to r e q u i r e a r e l a t i v e l y s m a l l n u m b e r of services p e r c o n c e p t i o n a n d s t i l l m a i n t a i n a c a l v i n g i n t e r v a l of a p p r o x i m a t e l y 12 mo. Received for publication February 2, ]959. 1157
115~
R.W.
T O U C t t B E R R Y , K. R O T T E N S T E N , AND I-I, A N D E R S E N
T r i m b e r g e r (24) recommended that cows should not be serviced until 50 days or more a f t e r parturition. Gaines (13) found a correlation of 0.039 ± .010 between the service period (the interval f r o m calving to conception) and the first m o n t h ' s milk production; thus, it is evident that the first m o n t h ' s milk yield and service period are practically independent. I n another study, Gaines and P a l f r e y (14) fomld a correlation of 0.053 ± 0.049 between the average length of the first nine calving intervals and the average yield per day over the nine calving intervals of 186 cows. On the basis of this correlation, the authors concluded that calving interval has no effect on yield and that high-yielding cows are not bred so as to freshen less frequently t h a n lo~v-yielding cows. Several workers (1, 3, 18, 20, 22) have reported that higher-yielding cows require more services per conception, whereas others (2, 4, 10, 13) have reported that there is no relationship between level of production and services per conception. The associations between the service interval and the interval f r o m first service to conception, and between the immber of services required per conception and the interval from first service to conception, have not been well established. The objectives of the present s t u d y are to determine the associations between the service interval, the interval f r o m first service to conception, the n u m b e r of services per conception, and the level of b u t t e r f a t production; and to demonstrate some of the consequences of these associations. S O U R C E A N D D E S C R I P T I O N OF DATA
The data for this study were taken f r o m the Red Danish Milkrace progeny groups at the Danish progeny testing stations during the testing year 1955-56. All heifers were in their first lactations when the data involved in this s t u d y were recorded. The observations consist of 2,056 artificial services on 902 first-calf heifers by 51 Red Danish Milkrace sires. The n u m b e r of daughters per sire ranged f r o m 14 to 20, with a meau of 17.7. E i g h t hundred fifty-three of the 902 heifers conceived and these heifers received a total of 1,782 services. Some of the 49 heifers returned to the f a r m e r s as n o n p r e g n a n t m a y have been p r e g n a n t as a result of their last services, but no info'rmation on p r e g n a n c y examinations was available after the heifers left the testing stations. The variables involved in this s t u d y are days f r o m first service to conception (W), n u m b e r of services per conception (Y), service interval (days f r o m calving to first service) (X), services per cow (Y'), and first lactation f a t production in kilograms (Z). The herdsmen at the varimis testing stations recorded the calving dates and service dates on b a r n sheets, and the heifers were diagnosed for p r e g n a n c y by veterinarians or technicians, who indicated the service at which conception most likely occurred. The herdsmen then recorded the date of pregn a n c y examination and the service at which p r e g n a n c y most likely occurred. The data in this study on days f r o m calving to first service (X), days f r o m first service to conception (W), services per conception (Y), and all services (Y') were taken f r o m the b a r n sheets. The first lactation b u t t e r f a t records are those given in (15) for the Red Danish Milkrace sires.
ASSOCIATIONS B E T W E E N SERVICE I N T E R V A L S , CONCEPTION AND P R O D U C T I O N 1 1 5 9
All cows included in this s t u d y were first-calf heifers, all were given ample o p p o r t u n i t y to conceive, there was no' selection for production among the daughters of a sire, and there was no tendency to give high producers more of a chance to conceive than low producers; therefore, the data should provide unbiased estimates of the relationships between days from first service to conception, services per conception, total n u m b e r of services, service interval, and b u t t e r f a t production. A N A L Y S I S OF DATA A N D t~ESULTS
The service intervals ranged f r o m 44 to 202 days; there were only 11 below 50 days and only 13 above 150 days, of which four were above 160 days. I n Table 1, the mean days f r o m first service to conception, the services per conception, and the total n u m b e r of services are listed according to the length of the service interval. The means listed u n d e r the service intervals f r o m 44-49 days a p p e a r to be slightly larger than those listed under the longer intervals, but other t h a n this there seems to be no consistent trend of the means with respect to the length of service interval. I n Table 2, the analyses of variance for days f r o m first service to conception, for services per Cmlception, and for all services are shown. I n no case is the variation between ten-day service interval groups significant at as low a probability as 0.05. I n spite of the general lack of significance of the between-group mean squares, one should not conclude that the length of the service interval is not associated with the length of the interval f r o m first service to' conception, with services per conception, or with total number of services per cow. The differences associated with the different lengths of service interval could be confounded with test station differences in m a n a g e m e n t and with service sire differences in fertility. Such differences are likely to be small in the data, because a concerted effort is exerted to standardize m a n a g e m e n t practices at the various statio'ns and the field nonr e t u r n rates for the various service sires used were approxinlately equal. I n addition to the possibility o'f confounded effects, it is possible that the linear or a curvilinear component or both components of the length of service interval could be significant, as was shown by V a n D e m a r k and Salisbury (25). To avoid the possibility of confounded variables and to more precisely determine the associations of the length of service interval (X) with the interval from first service to conception ( W ) , services per conception (Y), total services (Y'), and b u t t e r f a t production (Z), the within-sire variations were studied. I n addition, the square root of the service interval ~ / ( X ) was included, to help aecount for a curvilinear effect. On observing the b u t t e r f a t records published in (15), it was obvious that there were variations in days in production and age at calving. The age at calving ranged f r o m 658 to 1,341 days, with a mean of 899 days, and the days in milk ranged f r o m 232 to 370 days, with a mean of 304 days. To adjust the individual records for these variations in age at calving and days in milk, the within-sire
TABLE I Mean n u m b e r of days f r o m first service to conception, n u m b e r of services per conception, and mmfl)er of services as distribute([ by ten-day periods of the service interval L e n g t h of service interval
Heifers t h a t conceived Days f r o m first service to conception Services per COlleeption
44-49
50-59
60-69
70-79
~0-$9
90-99
100-109
110-119
120-129
130-139
140-149
150 and up
Tot'~ls
W N a~
72.1 10 56.09
43.29 39 39.85
41.14 127 48.79
35.77 177 48.~9
36.35 178 47.33
30.04 134 42.31
31.07 86 39.59
38.96 45 46.(18
37.00 23 41.14
41.54 13 44.71
40.00 9 42.00
12.42 12 24,52
36.08 853 45.78
Y N a~
2.50 10 1.43
2.15 39 1.22
2.16 127 1.37
2.10 177 1.45
2.13 178 1.47
2.04 134 1.39
1.95 86 1.19
2.20 45 1.39
2.] 7 23 1.11
1.92 13 0.95
2.00 9 1.12
1.50 12 0.80
2.09 853 1.37
P e r cent conceived on first service Heifers f a i l i n g to conceive No. services All heifers All services
Per cent conceived on first service
30.0
38.5
40.2
49.7
46.1
44.8
47.7
40.0
30.4
46.2
44.4
66.7
44.9
Y" N
7.00 1
6.60 5
5.78 9
5.75 8
5.80 10
5.50 4
4.33 3
4.0o 2
5,50 2
4.67 3
3.00 1
4.00 1
5.53 49
Y' N ~,,
2.91 11 1.92
2.66 44 1.16
2.40 136 1.68
'2.26 185 1.61
2.32 188 1.68
2.14 138 1.50
2.03 89 1.26
2.28 47 1.41
2.44 25 1.42
2.44 16 1.46
2.10 10 1.10
1.69 13 1.03
2.28 902 1.58
27.2
34.1
37,5
47.6
43.6
4.3.5
46.1
38.3
28.0
37.5
40.0
61.5
42.5
1161
ASSOCIATIONS B E T W E E N SERVICE I N T E R V A L S , CONCEPTION AND P R O D U C T I O N
TABLE 2 Analyses of variance for days from first service to conception, services per conception, and all services Heifers that conceived
Source Between ten-day service intervals Within ten-day service intervals
All heifers
Days from first service to conception (W)
Services per conception (Y)
All services (Y') per cow
D.F.
M.S.
D.F.
M.S.
D.F.
M.S.
11 841
2,996 2,084
11 841
0.91 1.88
11 890
2.46 2.50
m u l t i p l e r e g r e s s i o n of b u t t e r f a t p r o d u c t i o n ( i n k i l o g r a m s ) on a g e a t c a l v i n g a n d d a y s in m i l k (D) was d e r i v e d a n d is E q u a t i o n ( 1 ) . Z'=--18.59
+ 0.075 A + 0.510 D
(A) (1)
B y u s i n g E q u a t i o n ( 1 ) , the b u t t e r f a t r e c o r d s w e r e a d j u s t e d to a n a v e r a g e l e n g t h of l a c t a t i o n of 304 d a y s a n d a n a v e r a g e age of c a l v i n g of 899 d a y s . Since t h e r a n g e s in age a t c a l v i n g a n d d a y s in m i l k w e r e r e l a t i v e l y n a r r o w , i t was a s s u m e d t h a t a l i n e a r e q u a t i o n w o u l d be sufficient to a d j u s t t h e b u t t e r f a t r e c o r d s . The w i t h i n - s i r e c o r r e l a t i o n s involving service i n t e r v a l (X), the s q u a r e r o o t of service i n t e r v a l x/(X), d a y s f r o m first service to c o n c e p t i o n (iV), services p e r c o n c e p t i o n (Y), a n d a d j u s t e d b u t t e r f a t p r o d u c t i o n (Z) a r e l i s t e d i n T a b l e 3. T h e c o r r e l a t i o n b e t w e e n services p e r c o n c e p t i o n a n d d a y s f r o m first service to c o n c e p t i o n is l a r g e (0.859), as w o u l d be expected, b u t i t i n d i c a t e s t h a t t h e two v a r i a b l e s a r e n o t i d e n t i c a l . B y h o l d i n g one of t h e two c o n s t a n t , the o t h e r w o u l d r e t a i n a p p r o x i m a t e l y 2 6 % of its o r i g i n a l v a r i a n c e . I t is r e c o g n i z e d t h a t services p e r c o n c e p t i o n (Y) a n d d a y s f r o m first service to c o n c e p t i o n (W) a r e n o t n o r m a l l y d i s t r i b u t e d l o t a g i v e n service i n t e r v a l . H o w e v e r , j u d g i n g f r o m T a b l e 1 i t is a p p a r e n t t h a t the v a r i a n c e s of ( W ) a n d (Y) a r e a p p r o x i m a t e l y e q u a l f o r t h e v a r i o u s i n t e r v a l s of (X). Thus, f o r e s t i m a t i n g t h e r e g r e s s i o n coefficients t h e req u i r e m e n t s t h a t t h e e r r o r s be no n e o r r e l a t e d a n d h a v e the same v a r i a n c e a r e satisfied, b u t f o r m a k i n g the u s u a l tests of significance t h e r e q u i r e m e n t s t h a t t h e e r r o r s be n o r m a l l y a n d i n d e p e n d e n t l y d i s t r i b u t e d a r e n o t satisfied. C o n s e q u e n t l y , one s h o u l d c o n s i d e r t h e s t a n d a r d e r r o r s of r e g r e s s i o n coefficients i n v o l v i n g (Y) a n d (IV) as r o u g h a p p r o x i m a t i o n s . TABLE 3 Within-sire correlations between serviee interval (X), square root of service interval (VX), days from firs~ service to conception (IV), services per conception (Y), and adjuste4 butterfat production (Z) for those heifers that conceived Vx
X __ gX W y
0.952
w --0.042 --0.073
Y --0.047 --0.045 0.859
z 0.082 0.045 0.078 0.065
The above correlations are based on 801 degrees of freedom and the 0.05 and 0.01 critical values are 0.069 and 0.091, respectively.
1162
R. W. TOUCHBERRY, K. ROTTENSTEN, AND H. A~DERSEN
The i.t~terval from first service to co~tception. The multiple regression of the interval from first service to conception (W) on the service interval (X) and ~/(X) was derived on a within-sire basis from the correlations, standard deviations, and means given in Tables 3 and 4. The resulting regression equation is Equation (2). W~-- 110.99 + 0.62 (-+ 0.24) X - - 13.88 (+_ 4.49) V ~
R w . x ~ - - 0.116
(2)
D.F. ~ 800
Equation (2) is plotted in Figure 1. In Equation (2), the partial regression coeefficints are significant at the 0.01 level of probability. Although the above multiple correlation coefficient (Rw.x¢~ ~ 0.116) is significant at the 0.01 level of probability, X and ~/X have accounted for only 1.4% of the variance of the interval from first service to conception (W). F r o m Equation (2), it is apparent that only a minor part of the variance of the interval from first service to conceptio~ is associated with variation in the service interval. VanDemark and SalisTABLE 4 M e a n s a n d w i t h i n - s i r e s t a n d a r d d e v i a t i o n s for X Mean a
86.7 21.4
VX 9.24 1.14
(X), (VX), (W), (Y), W 36.08 44.84
Y 2.09 1.39
and
(Z) Z 204.0 34.7
b u r y (25) found that only 1.0% of the variance of services per conception was associated with variation in the service interval. One would expect this rather close agreement, since the correlation between services per conception and the days from first service to conception as given in Table 3 is 0.859. On observing F i g u r e 1, it is apparent that the average interval from first service to conception decreases at a decreasing rate as the service interval increases from 36 to 127 days. As the service interval increases from 127 to 169 days there is practically no change in the average interval from first service to conception. If the derivative of ~'V with respect to X for Equation 2 is set equal to zero, the minimum average interval from first service to conception is found to occur at a service interval of 127 days. As the service interval increases from 36 to 127, 49 to 127, and 64 to 127 days, the average interval from first service to conception decreases 17.1, 11.2, and 6.6 days, respectively. Thus, it is apparent that relatively long average intervals from first service to conception are associated with service intervals below 60 days. Since the interval from first service to conception and the service interval are both significantly correlated with butterfat production, as shown in Table 3, one should, perhaps, include Z as an independent variable along with X and ~/X when predicting W. This was done, and the resulting equation is Equation 3. ][A"~ 88.51 ~- 0.55(--+ 0.24) X - - 12.88 ( ± 4.50) v / X + 0.09 (___ 0.05) Z.
Rw.zxv~i~ 0.136
D.F. ~ 799
(3)
ASSOCIATIONS BETWEEN SERVICE INTERVALS, CONCEPTION AND PRODUCTION
1163
220
200
180
160
140 W and (W+X) 120
220
I00 -
Z 80-
~'10
^
60 -
~00
I
3030 40
I
60
I
80 SERVIGE
I
LO0
~'
120
i~
140
I-"
160
INTERVAL (X)
Fro. 1. The r e g r e s s i o n s of the i n t e r v a l f r o m first service to c o n c e p t i o n (W), t he i n t e r v a l f r o m c a l v i n g to c o n c e p t i o n ( W + X ) , a n d b u t t e r f a t p r o d u c t i o n (Z) on the service i n t e r v s l
(X). Z is e x p r e s s e d in k i l o g r a m s a n d W, X, a n d W + X a re e x p r e s s e d i n days. l~z = 110.99 + 0.62 X -- 13.88 VX--~ W ÷ X = 110.99 ÷ 1.6') X -- 13.88 V X .
= ~045.2~+ o.~o x - 1o.oo vx--. I f Equation (3) is plotted for a constant Z of 204 kg., the resulting g r a p h coincides almost exactly with the graph of Equation (2) in F i g u r e 1. F o r corresponding regression values to differ more than a day, the service interval must exceed 160 days. Shown in Table 5 are the mean squares of the interval from first service to conceptio]b accounted for independently by the service interval and the level of butterfat production. I n general, only a small p a r t of the variance of the interval from first service to conception is associated with variation in the level of butterfat production.
The interval from calving to co tzccption. F r o m tile correlations, means, and standard deviations in Tables 3 and 4, the regression of the interval from calving to conception ( W + X ) on the service interval ( X ) and ~/X can be derived. This regression was derived and is Equation 4.
R . W. T O U C H B E R R Y ,
1164
f
K. I ~ O T T E N S T E N , A N D I-I. A N D E R S E N
A
IV + X = W -~ X ~--- 110.99 + 1.62 (___ 0.24) X -- 13.88 (+__4.49) N/~ R( w+x).x¢~ --- 0.41
D.F. == 800
(4)
Since service interval is a part of the interval from calving to conception, the regression of ( W + X ) on X would be expected to be large and the multiple correlati(m R (w+x),x ¢-~ would be expected to be large. Equation (4) is plotted in F i g u r e 1. The interval from calving to conception increases markedly as the service interval increases, as would be expected. After a service interval of 49 days, there is au increase in the interval from calving to conception of almost a day for each day the service interval increases. The graph of Equation (4) clearly indicates ho~v prolonging the service interval increases the interval from calving to conception. The regression of ( W + X ) on X, ~/X, and Z was derived and is Equation (5) W ~- X ~
88.51 ~- 1.55 (--+0.24)X --12.88 (_+4.50) v/X -~ 0.09(+_0.05)Z R~ w+x).x v~z ~ 0.42
D.F. ~- 799
(5)
If Equation (5) is graphed for Z = 204, the resulting curve coincides almost exactly with that of Equation (4) in F i g u r e 1. The service interval must exceed 160 days for the difference between corresponding regression values to exceed one day. The level of butterfat prod~¢ction. F r o m the correlations, means, and standard deviations in Tables 3 and 4, the regression of the adjusted butterfat production (iu kilograms) on service interval ( X ) and k / X was derived and is Equation (6). A
Z ~ 245.26 + 0.69 (-+0.18) X - - 10.90 ± (3.46) ~/X
Rz.xv~--- 0.137
(6)
D.F. -~ 800
Equation (6) is plotted in F i g u r e 1 aud, as is evident from F i g u r e 1, the derivative of Z with respect to X gives a minimum butterfat production for X ----63. As the service interval increases from 63 to 169 days, there is a general increase in the level of butterfat production. As the service interval increases from 36 to 63 days there is a small, but probably not real, decrease in butterfat pr(~duction. F o r service intervals from 49 to 90 days there is little difference in the levels of associated b u t t e r f a t production. The increase in the level of butterfat production associated with an increase in the length of the service interval could be brought about in several ways: (1) There could be a tendency to breed low-producing cows earlier after calving than high-producing cows. (2) High-producing cows may tend to show estrus at a larger interval postpartum than low-producing cows and, thus, d r i f t into the classes with longer service intervals. (3) With the longer service intervals, conception occurs later after calving; thus, there is less interference of pregnancy with production. The first possible cause is not likely to be of importance, since the management practices were standard for cows at the testing stations. Clapp
ASSOCIATIONS BETWEEN SERVICE INTERVALS, CONCEPTION AND PRODUCTION
1165
TABLE 5 Tests of significance of the within-sire variance of the i n t e r w d f r o m first service to conception Source
Service interval (Rw.x
D.F.
-- r~wz) S w2
Butterfat production ( R ~w.xv~
_
s Rw.xv,"~) S w2
Error ( 1 - R2w.xv~z ) S w 2
5I.S.
2
9,991.8"*
1
7,895.7* 1,978.8
799
++ Significant at the 0.01 level of probability. Significant at the 0.05 level of probability.
(8) has presented information, though not conclusive, that the second cause may be real. Gaines (13), on the other hand, has presented data which indicate that the first full month's milk yield and the service period (the interval from calving to conception) are independent ; thus, the second cause is not likely. It is probable that the third possibility is the real cause of the association. Services per conception. Previous studies (6, 11, 12, 19, 21, 23, 24, 25) have dealt with the number of services per conception in relation to' the length of the srvice interval. In the present data, the regression of services per conception on service interval, as shown in Equation (7) and in Table 6, was not significant. J ( = 1.67 -- 0.005 (±0.007) Z + 0.03 (+_0.14) v/X ~- 0.0028 (--+0.0014)Z R r . x ¢ ~ 0.084
D.F. ~ 799
(7)
If Equation (7) is plotted for Z = 204 kg., the resulting graph shows that there is a decrease in services per conceptio~ as the service interval increases; thus agreeing with the results in (6, 11, 12, 19, 21, 23, 24, 25). Since most of the sum of squares of services per conception associated with service interval was associated with the linear component, the regression of services per conception on the first power of service interval and butterfat production was derived, and the two partial regression coefficients bJ~x.z = --0.0034 +_ 0.0023 and brz.x -= 0.0028 ±_ 0.0014 were found. With 800 degrees of freedom, the partial regression -0.0034 is significant (0.05 < P < 0.10). This result again substantiates previous conclusions that there is a general decrease in the services required per eonception as the service interval increases. TABLE 6 Tests of significance of the within-sire variances of services p e r conception associated ~dth b u t t e r f a t production and se~wice interval Source
D.F.
M.S.
Butterfat production (R~.x¢-~ z-- R~.x¢-~) (St2)
1
7.52*
Service interval ( R2r. x v~ z - r~z) (Sy2)
2
2.18
Error (1 - R~..xv~z) (St2) + Significant at the 0.05 level of probability.
799
1.93
R. ~,V. TOUCHBERRY, K. ROTTENSTEN, AND H. ANDERSEN
1166
TABLE
7
T e s t s of significance of t h e within-sire v a r i a n c e of total n u m b e r of services a s s o c i a t e d with service interval a n d b u t t e r f a t p r o d u c t i o n
Source Service i n t e r v a l
(R2:r,.xz)
D.F. Sz,2 -- r~z S r,2
B u t t e r f a t production (R-r,.xz") S z 2 - rrx2 S t , 2 :Error ( 1 - R~.xz ) S t 2
M.S.
1
12.269"
l
9.875*
849
2.32
+ Significant at the 0.05 ]eve] of probability. The partial regression of services per conceptiou on b u t t e r f a t production of 0.0028 is significant (0.01 < P < 0.05) and suggests that higher-producing cows require more services per conception. This relationship is probably brought about by the fact that cows requiring more services per conception are not pregn a n t as soon a f t e r calving and, thus, that their current production is less affected by p r e g n a n c y than is that of cows requirino" fewer services per conception. To provide evidence oll this point, the regression of services per conception (Y) on service interval (X). the interval from first service to conception (W), and the level of b u t t e r f a t production (Z) was derived and is Equation (8). I 1 = 1.19 - - 0.0007 _ (0.001) X - t - 0.0267 (__0.0005) W - - 0.00004
(+_0 0007)z.
(8)
The partial regression bYZ.X~g~- - 0 . 0 0 0 0 4 is n o t significant and is practically zero. Thus, it a p p e a r s that there is no real biological basis for concluding that high-producing cows are less fertile than low-producing cows. This is in agreement with the conclusions of (2, 4, 10, 13) and in disagreement with the findings of (1, 3, 18, 20, 22). The relationships reported in (1, 3, 18, 20, 22) are, possibly, confounded with other facto'rs. All services. B y studying services per conception, one includes only those cows that conceived. This, in effect, truncates the data to include only fertile cows. Equation (9) is the regression of all services (Y'), whether or not the co~s conceived, on service interval (X) and b u t t e r f a t production (Z). A
: Y ' = 2.13 - - 0.0056 (+--.0024) X + 0.0031 (+-.0015) Z
Rr,.xz ~ .104
(9)
D.F. mE 849
A prediction equatiou including x / X was first derived, but the partial regression of (Y') on V X was extremely small and not significant; consequently, it was not included and Equation (9) was used. The regression co'efficients in Equation (9) are based on 849 degrees of freedom and are significant at the 0.05 level of probability. The variances independently accounted for by service interval and b u t t e r f a t production are shown in Table 7. I n spite of their significance, the two together account for only 1.1% of the within-sire variance of n u m b e r of services. F o r a constant b u t t e r f a t production, the n u m b e r of services goes up 0.56 of a service for each 100 days the service interval increases.
ASSOCIATIONS B E T W E E N SERVICE I N T E R V A L S , CONCEPTION AND P R O D U C T I O N
1167
D I S C U S S I O N OF R E S U L T S
The results indicate that the number of services per conception (Y) decreases a small, though not significant, amount for each day the service interval (X) increases, the equation for a coustaut level of butterfat production (Z) being = 2.38 - .0034 X. The results also indicate that the interval from first service to conception (IV) decreases at a decreasing rate as the service interval (X) increases to 127 days. With a service interval of 127 days, the interval from first service to conception reaches a nlinimum of 32.8 days. Both a small number of services per eo'nception and a short interval from first service to conception are important items and are to be desired by a dairynlan. However, the attaimnent of a minimum number of services per conception and a minimum interval from first service to conception are not the only items to be considered in recommending an optimunl iuterval from calving to first service. The interval from calving to conception should definitely be considered, as it and the length of the gestation period determine the length of the calving interval. Most of the variation in calving interval is brought about by variation in the interval from calving to conception, because the variation in the length of the gestation period is relatively small. F r o m results in (9) and Tables 3 and 4 of this study, it appears that about 99% of the variation in calving interval is caused by variation in the interval from calving to conception. The important thing to remember concerning the service interval, services per conception, the interval from first service to (.onceptiou, and the interval from calving to conception is: that variation in service interval a(.comlts for approximately 1% of the variation in servic'es per conception and the interval from first service to conception; whereas, variation in the service interval accounts for approximately 17% of the variation in the interval from calving to conception and, thus, of calving interval. Thus, it would seem that the length of the calving interval desired would be the p r i m a r y consideration in recommending a short or long service interval. Equation (10) expresses the relationship between the interval from calving to conception and service interval for the mean Z and, as meutioned previously, almost coincides with that plotted in F i g u r e 1. A
( W -t- X) --~ 107.13 +1.55"* X --12.88"* v / X
(10)
If one assumes that the length of the service interval has little or no effect on the length of the gestation period, then the relationship between service interval and calving interval is expressed by adding the gestation period to Equatiml (10). Assuming a gestation period of 280 days, the calving interval for a mean Z may be expressed as shown in Equation (11). C.I. ~ 387.13 -t- 1.55 X - 12.88 \ / X (11) B y taking the partial derivative of C.I. in Equation (11) with respect to (X), setting the derivative equal to 0, and solving for X, it is found that the average minimum calving interval occurs when the average service interval (X) is
11.68
R. W. T O U C H B E R R Y , K. R O T T E N S T E N , AND 14. A N D E R S E N
].7.2 days. This value of X is outside the range of service intervals in these data and is one that is unrealistic. F o r service intervals of 36, 49, 64, 81, and 100 days, which are more nearly within the range of the present data, one would expect calving intervals of 366, 373, 383, 397, and 414 days, respectively. If the expected b u t t e r f a t production values in F i g u r e l for Equation (6) are divided by the corresponding expected calving intervals derived above, the production per calving interval day decreases as the calving interval increases. F o r example, for service intervals of 36, 49, 64, 81, and 100 days, the expected calving intervals are 366, 373, 383, 397, and 414 days, the expected b u t t e r f a t production is 205, 203, 202, 203, and 205 kg., and the expected production per calving interval day is 0.56, 0.54, 0.53, 0.51, and 0.50 kg., respectively. The present study agrees with the results of Gaines (14) in regard to current calving interval. Gaines (14) obtained a correlation of -0.134 ± 0.018 between current calving interval and current production per calving interval day, and a correlation of 0.142 ± 0.018 between the previous calving interval and current production per calving interval day. The correlation, 0.142, between previous calving interval and current production per calving interval day would be partially confounded with age effects, since cows having longer previous calving intervals are likely to be older at calving; thus, the correlation of 0.142 is probably too large. Consequently, the conclusion of Gaines (14) that what is gained in the current lactation by having a short calving interval is lost in the next lactation is questionable. Chapman and Casida (7) concluded that there was no marked reduction in production per calving interval day in the second of two consecutive short calving intervals. Johansson and Hansson (17) concluded that the optimum length of the first calving interval would be about 410 to 430 days, and about 400 days for the second. F o r the third and, probably, later lactations any length between 310 and 430 days gives about the same yield per calving interval day. On a basis of these references, calving intervals of 12 to 13 mo. seem to be desirable. The real solution for the optimum length of calving interval is, within biological limits, an economic one and depends on the relative economic values of calving intervals of varying lengths, including yield per calving interval day, seasonal variations in the price of milk, number of calves bovn, and varying number of services required per conception. Under present economic conditions, with the seasonal variations in milk prices, an average calving interval of approximately 12 too. would seem to be desirable. It should be remembered that at present under practical conditions nmn's control over calving interval is limited to that period between the time when the cow first comes into heat after calving and the time when the first service is made. There is more freedom to make this period long than there is to make it short. Artificial breeding organizations would be interested in a low number of services per conception, as this would be less expensive to the organizations where second and third services are given free of charge, when the first or first and second services fail to get the cow in calf. Thus, it is likely that artificial breeding organizations would recommend a relatively long service interval. On the
ASSOCIATIONS BETWEEN SERVICE INTERVALS, CONCEPTION AND PRODUCTION ] ] ( ) 9
other hand, a shorter service interval might result in more first services per cow, since the calving intervals would be shorter, thus producing more income for artificial breeding organizations. I f one reduces the number of services required per conception f r o m that found in these data, it would probably be possible to allow a longer service interval and still maintain a 365-day or shorter calving interval. I n the present data, the linear regressio'n of the interval f r o m first service to conception (W) on services per conception (Y) is 27.64; thus, if the average n u m b e r of services per conception was reduced f r o m 2.09, as found in these data, to 1.67, which would correspond to a 60% conception rate, the interval f r o m first service to conceptio~q would be reduced b y a p p r o x i m a t e l y 11.6 days. Assuming t h a t this effect is the same for service intervals of different lengths, one could allow an average service interval of from 45 to 60 days and still maintain a 365-day calving interval. I f one reduced the length of normal gestations by 15 to 30 days, it would be possible to allow a 15- to 30-day lo~ger service interval and still maintain a 365-day calving interval. Although it has not been conclusively established that the gestation period can be shortened without associated deleterious effects, work by DeFries et al. (9) indicates that selection for a shorter gestation is feasible. Other workers (1, 3, 18, 20, 22) have presented data which indicate that highproducing cows require a considerably increased n u m b e r of services per conceptio~. These data could be biased, in that there would be a tendency to be more patient with high-producing cows and allow them more chance to conceive than would be the case with low-producing cows. I t is also possible that service sire differences are confounded with production levels. In addition, it is probable that those cows requiring more services have a longer period f r o m calving to conception and, thus, less effect o'f p r e g n a n c y on lactations. t~y utilizing the present data, the partial regression of services per conception on b u t t e r f a t production for a constant service interval and a constant interval f r o m first service to conception is -0.00004 ± 0.0007. F r o m the eorrelatiolls, standard deviations, and means in Tables 3 and 4, the partial regressions br~w+x).z~-0.022 and brz.~w+~i)~---0.0007 can be derived. This suggests that the association between the level of b u t t e r f a t production and services per conception is largely a result of the corresponding relationship between the interval f r o m calving to conception and b u t t e r f a t production; thus, there is no evidence in these data to suggest t h a t the biological relationship between services per conception and the level of b u t t e r f a t production is different f r o m zero. Artificial breeding associations would p r o b a b l y be more interested in total n u m b e r of services as related to service interval r a t h e r t h a n services per conception because, at the time of first sorvice, ~o one really knows which cows will conceive and which will uot. As shown in Eqliation (5), 0.56 less services cau be expected for each 100 days of increase in the length of the service interval. REFERENCES (1) ANONYMOT:S. S e v e n t e e n t h A n n u a l R e p o r t of t h e N e w Zeala1~d D a i r y Board. House, Wellillgton, N e w Zealand. pp. 32-39. 1940-41.
Invicta
1170
It. W. TOUCHBERRY, K. ROTTENSTEN. AND It. ANDERSEN
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