Controlling First Service and Calving Interval by Prostaglandin F2α, Gonadotropin-Releasing Hormone, and Timed Insemination1

Controlling First Service and Calving Interval by Prostaglandin F2e, Gonadotropin-Releasing Hormone, and Timed Insemination 1 M A T T H E W C. LUCY, 2 JE F F R E Y S. STEVENSON, 3 and E. P. C A L L Department of Animal Sciences and Industry Kansas State University Manhattan 66506

ABSTRACT

rate approached that of controls (51%). Methods of timed insemination utilized and their resulting low conception rates prevented a true test of the utility of estrous synchronization to reduce duration and variability of calving intervals, even though variability of intervals to first service was reduced.

Our objective was to determine if calving intervals could be shortened and made less variable by using prostaglandin F2~ to control the occurrence of first services. Holstein cows (n=348) were assigned at calving to four treatment groups. Control cows (n=88) were inseminated at their first observed estrus after 40 d postpartum. Estrous cycles of the remaining cows were synchronized with prostaglandin F2a to allow insemination (first services) 80 h after the second injection (n=86), insemination at 80 h preceded by gonadotropin-releasing hormone at 72 h (n=86), or insemination at 72 and 96 h (n=88) after the second injection (51 to 57 d postpartum). By design, interval to first service was reduced to 57 d for treated cows (63 d for controls) and was less variable (12% of that for controls). Conception rate at first service was lower after timed inseminations than that of controls. Intervals to conception and subsequent calving were similar in all treated cows and controls. Of cows sampled, 23 of 176 (13%) failed to respond with luteolysis when progesterone in serum exceeded 1 ng/ml, and 26 of 176 (15%) had low concentrations of progesterone in serum and could not respond to prostaglandin F2~. Poor response to timed inseminations may have occurred because only 72% of 176 cows sampled responded with luteolysis. When only cows observed in estrus were considered, conception

INTRODUCTION

Received February 7, 1986. 1Contribution Number 86-283-J, Kansas Agricultural Experiment Station. Department of Animal Science, North Carolina State University, Raleigh. 3 Reprint requests. 1986 J Dairy Sci 69:2186-2194

Calving interval has profound economic effects in the dairy herd (5, 14). Numerous studies have demonstrated the benefit of a 12 to 13-too calving interval (1, 5, 8, 14, 19). Inte~als longer than optimum result when cows spend a greater proportion of their lactation in the less profitable part of their lactation curve (5). Thus, economic losses associated with prolonged calving intervals often go undetected by dairy producers, because they represent lost potential income and not cash outlay (5). Estrous detection and interval to first service are the most important factors affecting calving interval (15). Poor estrous detection has been linked to reproductive inefficiency in many studies, and its effect on calving interval is well-documented (2, 3, 15). Interval to first service has received less attention than estrous detection as a measure of reproductive performance. Furthermore, calving interval and interval to first service are highly correlated (4, 10, 18), and optimal calving intervals may be unattainable without decreasing intervals to first service (5). Unfortunately, reducing calving intervals to an optimal average on a herd basis does not optimize necessarily individual calving intervals for all cows in the herd. The herd average can include cows with extremely short or extended intervals. Reducing interval to first services will shorten calving intervals (10, 20), but postpartum inseminations earlier than 60 d m a y lower conception rates (4). An option through estrous

21 86

CONTROLLING FIRST SERVICE AND CALVING INTERVAL synchronization is to schedule all first services at approximately 60 d in an attempt to shorten the duration and variability of calving intervals. Our objectives were 1) to determine if duration and variability of calving intervals could be reduced by controlling time of first services with prostaglandin F2a (PGF); 2) to evaluate the reproductive performance of dairy cows subjected to three treatments using timed insemination after PGF and gonadotropinreleasing hormone (GnRH); and 3) to assess efficacy of PGF-induced luteolysis and estrus and its subsequent effect on the success of timed inseminations. MATERIALS A N D METHODS Experimental Design

Lactating Holstein cows (n=348) from the Kansas State University dairy herd that calved between July 1, 1983 and June 30, 1985 were utilized. Cows were housed in a freestall confinement facility exposed to the environment. The diet consisted of a concentrate mix (16% protein) containing 50% corn and 50% sorghum grains, soybean meal, 1.5% sodium bicarbonate, and minerals in a self-feeder and alfalfa hay fed ad libitum. Cows were milked twice daily at 0930 and 2130 h. Two daily 30-min observations (0700 to 0900 h and 1600 to 1800 h) were conducted for estrous detection. A t calving, cows were assigned to four treatment groups. Control cows (n=88) were inseminated artificially (AI) at first observed estrus after 40 d postpartum. Time of first services was scheduled in the remaining cows using two 25-mg i.m. injections of PGF (PGF-1 and PGF-2; Lutalyse®; The Upjohn Company, Kalamazoo, MI) 11 d apart. Small groups of cows were treated together on Thursdays of each week (0800 to 0900 h) when cows were 40 to 46 d postpartum. Eleven days later (Mondays), cows were given a second injection (PGF-2) between 0800 and 0900 h. Following PGF-2 (0 h), cows were treated according to the following procedures: 1) AI at 80 h (n=86); 2) 100 /ag (i.m.) GnRH (Cystorelin ®, CEVA Laboratories, Overland Park, KS) at 72 h and A1 at 80 h (n=86); or 3) AI at 72 and 96 h (n=88). Injections of GnRH occurred on Thursdays (0800 to 0900 h), 80-h inseminations on Thursday afternoons (1600 to 1700

2187

h), and 72 and 96-h inseminations on Thursday and Friday mornings (0800 to 0900 h). Inseminations at 72 and 96 h were counted as one service. Cows were reinseminated according to observed estrus until determined pregnant by uterine palpation per rectum at 40 to 46 d after AI or removed from the herd for various reasons including infertility. Blood Collection

Blood was collected by coccygeal venipuncture from a random group of PGF-treated cows (n=176) at 0 and 48 h after PGF-1 and PGF-2. Samples were stored at 5°C for 24 h until serum was harvested b y centrifugation. Samples of serum were held at - 2 0 ° C until assayed for progesterone (P) by radioimmunoassay (21). lntraassay a n d interassay coefficients of variation were 10.3 and 15.1%, respectively. A luteolytic response after PGF-2 was indicated when P in serum exceeded 1 ng/ml at 0 h and was less than 1 ng/ml at 48 h. No luteolysis was indicated when P was greater than 1 ng/ml at 0 and 48 h after PGF. Serum P of less than 1 ng/ml at 0 and 48 h suggested that cows did not have a functional corpus luteum. Statistical Analyses

A different inseminator was used during the first and second year of the study. Because no effect of inseminator was found, data were combined. Intervals to first service, first service to conception, conception rate at the first three services, calving interval (estimated by adding 280 days to the interval from calving to conception), and services per conception and per cow were analyzed by the least squares procedures of the Statistical Analysis System (17). Treatment (n=4), lactation (primiparous or muhiparous), season of calving (October 1 to January 31, February 1 to May 30, or June 1 to September 30), and all two-way interactions were included as sources of variation. A measure of the variation associated with the mean intervals to first service, to first service conception, and to subsequent calvings was determined by Levene's test (12). This was accomplished by calculating a residual for each observation (absolute value of the difference between the observed value and treatment mean), which was subjected to analysis of variance. When the resulting F-test was significant, Journal of Dairy Science Vol. 69, No. 8, 1986

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variances were heterogeneous, and synchrony of fit for individual observations about the mean were compared by the relative magnitudes of the residual means for each treatment. Smaller mean residuals indicated less variability within treatment. Orthogonal contrasts were made between treatment means (control vs. timed AI groups, timed AI with and without GnRH, and one vs. two timed inseminations). Selected nonorthogonal contrasts were made by Scheffe's interval (17). Enumeration data (percentages) were tested for independence by chi-square. R E SU L TS Intervals from Parturition to First Service and Conception

Analyses of variance for various reproductive traits are in Table 1. Interval to first service was affected only by treatment as average intervals to first insemination were 6 d shorter (P<.001) for PGF-treated cows than controls (Table 2). By design, timed AI after PGF resulted in less variable intervals to first service, since the mean residuals for treated cows were 12% of those for controls. The range of first services was reduced to 7 d (54 to 60 d postpartum) compared with 83 d (40 to 122 d) for controls. Interval from first service to conception was altered by treatment (P<.05) and season of calving (P<.05). Treatment (P<.01) and season (P<.001) accounted for some of the variation in the mean residuals for this interval (Table 1). Cows inseminated at 72 and 96 h and those inseminated at 80 h that also received GnRH had longer (P<.01) and more variable (P<.01) intervals to conception after first service than cows inseminated at 80 h without GnRH treatment (Table 2). However, AI at 80 h without GnRH treatment resulted in less variable intervals from first service to conception than those of controls (P<.05) and the remaining AI groups (P<.01). Cows calving in the spring had longer (50 -+ 6 d; P<.O1) and more variable (P<.O01) intervals from first service to conception than cows calving in fall and winter (31 + 4 d), whereas cows calving in summer had intermediate intervals (40 + 6 d). Interval between first and second service averaged 36 +- 4 d for control cows, whereas that interval for all PGF-treated cows averaged Journal of Dairy Science Vol. 69, No. 8, 1986

34 + 3 d and was similar to controls. Intervals from second to third service also were similar for controls (36 +- 4 d) and PGF-treated cows (40 + 3 d). However, the proportion of PGFtreated cows with intervals from treatment AI (first service) to second service of less than 18 d (33/181; 18%) was greater (P<.01) than that for all control (intervals from first to second service for control cows and those from second to third services for all cows) cycles (3/155; 2%). Calving intervals were affected by treatment (P<.10) and season (P<.05) as were the residuals of those factors (Table 1). Although calving intervals were not lengthened significantly by timed insemination treatments compared with controls, GnRH-treated cows had longer (P<.01) calving intervals than their counterparts inseminated at 80 h (Table 2). As with the intervals to conception after first service, variation in calving interval was not reduced for treated cows, but variation was less (P<.05) for cows inseminated at 80 h (without GnRH) than for controls and for the remaining treated groups that were first inseminated by appointment. Cows calving during fall and winter seasons had shorter (370 +- 4 d; P<.05) and less variable (P<.001) calving intervals than those in spring (388 +- 6 d), whereas summer calvings were intermediate (378 -+ 6 d). Conception Rates and Fertility

Conception rate at first service (treatment) was altered (P<.01) only by treatment (Table 1). Control cows had higher (P<.01) conception rates at first service (51%) than all timed AI groups (Table 3). Conception rate at first service was similar among all timed A1 groups. Detection rate of estrus after PGF-2 was similar among treatments (Table 3). When only cows that were observed in estrus after PGF-2 were considered for determining conception rate at first service, conception rates of estrual cows were higher than those of all treated cows [33% (19/58), 41% (23/56), and 37% (23/63) for cows inseminated at 80 h, at 80 h plus GnRH, and at 72 and 96 h, respectively], but were still less (P<.05) than those for controls. Rates of conception at second and third services were similar for PGF-treated cows and controls, except that cows previously treated with GnRH and inseminated at 80 h had a lower (P<.05) conception rate at second ser-

T A B L E 1. L e a s t s q u a r e s a n a l y s e s o f v a r i a n c e o f i n d i v i d u a l o b s e r v a t i o n s a n d t h e i r r e s i d u a l s f o r r e p r o d u c t i v e treated with prostagiandin F2a (PGF) and gonadotropin-releasing hormone (GnRH).

traits of dairy cows after timed inseminations

in cows

Services

Source of variation Treatment C o n t r o l vs. t i m e d i n s e m i n a t i o n G n R H vs. n o G n R H O n e vs. t w o i n s e m i n a t i o n s

Days to first

First service to

Calving

service

conception

interval

df 3

**,2

,, ,3

1 1 1

***

***

Season Fall-winter vs. spring F a l l - w i n t e r vs. s u m m e r

2 1 1

Lactation number Treatment × season Treatment × lactation Lactation × season

1 6 3 2

,2

, ,3

,2

Conception

, ,3

1st

2nd

, ,2

rate

Con3rd

(3

0 Z

per ception

Cow

0

,2

,2

**2

Z

* *

** f

* * *

* * t"

*** **

***

**

***

**

* **

*** ***

* **

*** ***

~" *

< t~

e~

Z c3 > t~ Z

i The GnRH

(100/zg) was given 72 h after PGF.

2 Significance for individual observations. 3 Significance for synchrony e~

of fit of individual observations

about their mean (Levene's test). See Materials and Methods

for further details.

*P<.05. * *P<.01.

<

* * *P<.O01.

tP<.lO. Z o o0

ox

ro

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LUCY ET AL.

TABLE 2. Average (X,) and mean residual (Xr) days from parturition to first service, first service to conception, and to subsequent calving after timed inseminations at first service using prostaglandin F2a (PGF) and gonadotropin-releasing h o r m o n e (GnRH). Timed inseminations 1 Item

Control N

Days to first service

Xr N

First service to conception

Xr Calvinginterval

N Xr

80 h + GnRH

80 h

72 +

96 h

88 63 17

86 57 a 2a

86 57 a 2a

88 57a 2a

77 33 38

68 31 26 a

70 54a, b 45 b

72 44 40b

77 375 39

68 368 26 a

70 391 b 45 b

72 381 41b

aDifferent from control (P<.05). bDifferent from AI at 80 h (P<.01). i GnRH (100 #g) was given 72 h after PGF.

TABLE 3. Measures of fertility and reasons for culling after scheduled first services using prostaglandin F2a (PGF) and gonadotropin-releasing hormone (GnRH). Timed inseminations 1 Item

Control

80 h

80 h + GnRH

72 + 96 h

Conception rate First service Second service Third service

45/88 (51) 2 15/36 (42) 10/21 (48)

20/86 (23) a 32/64 (50) 12/30 (40)

26/86 (30) a 16/59 (27) b 17/38 (45)

27/88 (31) a 26/58 (45) 7/32 (22)

Observed estrus after PGF Services/conception Services/cow Pregnancy rate

1.9 -+ .1 2.0 -+ .2 77/88 (87)

58/86 (67) 2.0 -+ .1 a 2.2 +- .2 a 68/86 (79)

56/86 (65) 2.5 +- .2a, b 2.7 -+ .2 a 70/86 (81)

63/88 (72) 2.1 -+ .2 a 2.6 -+ .2 a 72/88 (82)

Reasons for culling Reproduction Milk production Mastiffs Ketosis Injury Death a

5 (6) 3 4 1 0 0 1

.

Different from control (P<.05).

bDifferent from timed AI at 80 h (P<.05). 1GnRH (100 ~g) was given 72 h after PGF. 2 Percent. Percentage of cows treated. Journal of Dairy Science Vol. 69, No. 8, 1986

8 (15) 2 1 1 3 3

9 (10) 2 2 0 2 1

11 (11) 2 1 0 1 1

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CONTROLLING FIRST SERVICE AND CALVING INTERVAL

vice (27%) than those without GnRH treatment (50%) (Table 3). Cows that calved during spring months had lower (P<.05) conception rates at second service (31%) than those calving during fall and winter months (46%) with summer being intermediate (43%). There were no seasonal effects on conception rate at first and third services. Conception rates did not appear to be related to milk yield of cows because daily milk yield of control cows (29.4 + .7 kg) nearest the day of first service was similar to that for PGFtreated cows (29.6 + .8 kg). Lactation yields (mature equivalent, 305-d, 2X) for controls (7665 + 144 kg) and PGF-treated cows (7673 +162 kg) also were similar. As expected, daily milk yield was greater (P<.001) for multiparous (34.1 + .7 kg) than for primiparous cows (24.9 + .8 kg), and both daily and full lactation yields were greater (P<.05) in spring than fall and winter or summer months. Services per conception (only cows that conceived) and services per cow (all cows regardless of their eventual conception status) were affected (P<.05) by treatment and season (Table 1). Control cows required fewer (P<.05) services per conception and per cow than PGFtreated cows (Table 3). Furthermore, cows previously treated with GnRH required more (P<.05) services per conception than those inseminated at 80 h and not receiving GnRH. Cows calving in fall and winter had fewer (P<.05) services per conception (1.9 -+ .1) and per cow (2.1 + .1) than those calving during spring (2.3 + .1 and 2.6 -+ .1) and summer (2.2 + .1 and 2.4 + .2). Despite lower conception rates at first service and more services per conception, pregnancy rates for control and treated cows were similar (Table 3). Reasons for cow disposal were similar; however, culling for reproductive failure was greater for cows treated with PGF than for control cows (Table 3). Efficacy of Prostaglandin F2e for Timed Insemination

Based on concentrations of P in serum from 176 cows at 0 and 48 h after each of two injections of PGF (PGF-1 and PGF-2), three different responses were observed. These are illustrated for treated cows after PGF-2 in Figure 1. Progesterone decreased (P<.05) 48 h

after PGF-2 for 127 cows, indicating luteolysis, but in another 23 cows, P in serum was unchanged (no luteolysis). Surprisingly, 26 cows had low concentrations of P in serum at 0 and 48 h after PGF-2 and could not have responded to PGF, despite a previous injection of PGF 11 d earlier (between 40 and 46 d postpartum). Comparison of the responses after the first and second injections of PGF is in Table 4. Before PGF-1, 123 of 176 cows (70%) had a functional corpus luteum (serum P greater than 1 ng/ml). This percentage increased (P<.01) to 85% 11 d later before PGF-2. Luteolysis occurred in 60% of all treated cows after PGF-1 and increased (P<.01) to 72% after PGF-2. However, the percentage of cows with luteolysis was identical (85%) after both PGF-1 and PGF-2. No luteolysis occurred in 10 and 13% of all treated cows after PGF-1 and PGF-2, respectively. In other words, 15% of cows with a functional corpus luteum failed to have luteolysis after either injection of PGF. Of 23 cows that failed to have luteolysis when serum P was greater than 1 ng/ml at 0 and 48 h after PGF-2 (Figure 1), 16 (70%) responded to PGF-1, 4 (17%) had low serum P at PGF-1, and 3 (13%) failed to respond to PGF-1. 5

5

T 4

[ ] Luteolysis at PGF-2 (n= 127) 72% [ ] No luteolysis at PGF-2 (n=23) I 3% [ ] Low progesteroneof PGF-2 (n=26) 15%

[ 1

v o x_

o

3

2

£1-

0h

48

Time A f t e r PGF Figure 1. Mean (-+ SD) serum progesterone concent-rations for dairy cows following the second (PGF-2) of two injections of prostaglandin F ~ (PGF) given 11 d apart. Cows are grouped according to their progesterone response: luteolysis, no luteolysis, or low serum progesterone at 0 and 48 h after PGF.

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LUCY ET AL.

TABLE 4. Responses to prostaglandin F2a (PGF) at 11-d intervals preceding timed inseminations. Injection of PGF 1 Item

PGF-1

PGF-2

No, cows treated

176

176

No. cows Functional corpus luteum Luteolysls No luteolysis Low serum progesterone

123 105 18 53

(70) 2 (60) (10) (30)

150 127 23 26

(85) a (72) a (13) (15) a

aDifferent from proportion of cows at first injection (P<.01). 1 First injection of PGF (25 mg) was given at 40 to 46 d postpartum. 2Percentage of cows treated.

F e w e r ( P < . 0 1 ) cows had low c o n c e n t r a t i o n s o f P in serum at 0 and 48 h after PGF-2 than after PGF-1 (Table 4). Of 26 cows with low P at PGF-2 (Figure 1), 9 (35%) r e s p o n d e d to PGF-1, 9 (35%) had low P at PGF-1, and 8 (30%) failed to respond to PGF-1 when P in serum e x c e e d e d 1 ng/ml. The p r o p o r t i o n of cows observed in estrus was related to the response after PGF. F e w e r (P<.05) cows (27/47 or 57%) were observed in estrus after PGF-2 w h e n cows had low serum P (n=7) or failed to respond to PGF-1 (n=40) than when cows r e s p o n d e d (luteolysis) t o b o t h injections of PGF (60/80 or 75%). C o n c e p t i o n rates also t e n d e d to be higher (P<.10) for cows that r e s p o n d e d to both PGF injections ( 3 2 / 8 0 or 40%) than for those only responding to PGF-2 (12/47 or 25%). Of 49 cows that either failed to respond to PGF-2 or had low concentrations o f P at 0 and 48 h after PGF-2, one cow conceived. T h a t cow had low P, was n o t observed in estrus, and was inseminated at 72 and 96 h after PGF-2. If only first-service c o n c e p t i o n rates of cows with luteolysis ( 1 2 7 / 1 7 6 ) are considered by t r e a t m e n t group, 10 of 43 (23%) cows inseminated at 80 h, 18 of 43 (42%) cows inseminated at 80 h and receiving GnRH, and 16 of 41 (39%) inseminated at 72 and 96 h conceived at first service. Gonadotropin-releasing h o r m o n e t e n d e d to increase (P=.07) first-service conception rates for cows inseminated at 80 h. HowJournal of Dairy Science Vol. 69, No. 8, 1986

ever, these percentages were still less (P<.05) than that of controls (51%). However, when cows with luteolysis and observed estrus were considered after PGF-2, c o n c e p t i o n rate app r o a c h e d that of controls ( 9 / 3 0 or 30%, 14/30 or 47%, and 15/27 or 55%). Resulting calving intervals of these cows were shorter (P<.05) than those for cows that failed to have luteolysis or could n o t have responded to PGF because o f l o w P in serum. These differences were 13, 27, and 43 d, respectively, for the three groups of PGF-treated cows.

DISCUSSION

Timing first services to occur at approximately 57 d p o s t p a r t u m using PGF allowed efficient scheduling of inseminations. A l t h o u g h average interval to first service was within 6 d of that for control cows, variability was reduced to 12% of control intervals. U n f o r t u nately, variability of calving intervals was n o t reduced. However, by delaying any first insemination of PGF-treated cows to 54 to 57 d postp a r t u m (14 to 17 d longer than controls), we did not lengthen calving intervals nor delay interval to conception. We should p o i n t o u t that the value of estrous s y n c h r o n i z a t i o n as a tool to test our hypothesis was not fully realized because the m e t h o d s o f t i m e d insemination resulted in l o w c o n c e p t i o n rates. A m o r e effective approach to improve c o n c e p t i o n rate at first service and possibly reduce variation in calving intervals w o u l d involve synchronizing estrus and only inseminating cows observed in estrus. A single dose of PGF f o l l o w e d by insemination at estrus might produce acceptable results, because percentage of total cows with luteolysis after PGF-1 was 60 and only increased to 72 w h e n two injections o f PGF were utilized (Table 4). A proven alternative to reduce interval to c o n c e p t i o n is to inseminate all cows at observed estrus and then treat remaining cows (not observed in estrus) with PGF, based on detection of a palpable corpus l u t e u m (16). The impact of these systems on reducing variability of subsequent calving intervals has n o t been determined. Success of our t r e a t m e n t s d e p e n d e d on presence of a viable corpus luteum, efficacy of P G F to regress that corpus luteum, and the presence of a new corpus l u t e u m responsive to PGF within 11 d after PGF-1. Ability of PGF treat-

CONTROLLING FIRST SERVICE AND CALVING INTERVAL

ments to control estrus by inducing luteolysis after two PGF injections was poor compared with previous work in heifers (6, 21). Even though 85% of treated cows having a corpus luteum responded with luteolysis after each PGF injection, this was less than the 91% in a previous report (16). However, these results are similar to recent reports for observed estrus in lactating cows after PGF (11). Another limitation encountered was absence of a functional corpus luteum (low concentrations of P in serum) in 26 (15%) of 176 cows at PGF-2. This percentage seems rather high in view of the fact that all treated cows received PGF 11 d earlier and the average postpartum interval to first ovulation for our herd was 19 + 1 d in a previous study (20). These represented, in part, cows that responded to PGF-1 but failed to have serum P that exceeded 1 ng/ml at PGF-2, indicating a viable corpus luteum. Based on daily blood samples in selected cows after PGF-1, we have determined that these cows had a delayed rise in serum P resulting in low P 11 d later at PGF-2. A second reason for low P at the time of PGF-2 was failure of cows to respond to PGF-1. The third reason for low P at PGF-2 was anestrus or anovulation, because 5% (9/176) of the cows were apparently not cycling by 51 to 57 d postpartum. It is not surprising that insemination of 28% (49/176) of the cows (those that failed to respond to PGF or had low serum P) only resulted in one conception. Insemination of those cows not in estrus reduced overall conception rates and probably contributed to our inability to reduce variability of resulting intervals to conception and calving. Despite low conception rates at first service, PGF treatments did not prolong intervals from calving to conception. Part of the reason that PGF treatments did not prolong conception intervals might be due to a greater proportion of PGF cows that were re-inseminated within 18 d after the timed AI. However, those cows with luteolysis and observed estrus tended to have improved rates of conception and decreased intervals to conception than control cows or cows treated with PGF without luteolysis or estrus. Compared with their own second service conception rates and those of controls at first service, PGF-treated cows had lower first

2193

service conception rates. Cows previously treated with GnRH also showed poor conception at second service. This may be related to reduced P in serum during the luteal phase after GnRH treatment (13; M. C. Lucy and J. S. Stevenson, 1986, unpublished data). Several studies have suggested a positive relationship between P in serum during the luteal phase preceding AI and the success of that insemination (7, 8). Because P was reduced in our GnRHtreated cows after first service as in another experiment (M. C. Lucy and J. S. Stevenson, 1986, unpublished data), this might account for the low second service conception rates. The resulting low first and second service conception rates for GnRH-treated cows also explained the longer intervals to conception for this treatment group. In contrast, for cows with luteolysis (verification that cows were estrual), GnRH (at 72 h) improved first service conception rates when insemination occurred at 80 h. This increase in conception rate has been reported by others (9). Therefore, low conception rate at second service may result in cows treated with GnRH during a nonestrous period (e.g., luteal phase), because reduced P in serum preceded second service. Our study demonstrated that average duration and variability of calving intervals were unaffected by our treatments, even though first services did not begin until 54 to 57 d postpartum for PGF-treated cows and variability of those intervals to first service was reduced to 12% of that for controls. Lower conception rate at first service and less than optimal estrous synchrony probably prevented reduction of the mean and variance for calving intervals. Our results also demonstrated that an acceptable calving interval for the control cows (375 d) was achieved with a reasonable twice daily estrous detection program similar to that shown by Allalout (1). Other treatments using estrous synchronization that optimize first service conception and also reduce variability of intervals to first service may prove useful in reducing variability and length of calving intervals for all cows in a herd. ACKNOWLEDGMENTS

The authors express gratitude to M. K. Schmidt and K. D. Frantz for their assistance in conducting this study at the Kansas State Journal of Dairy Science Vol. 69, No. 8, 1986

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University Dairy Center, t o S. D u r h a m f o r her assistance in t h e l a b o r a t o r y , and t o B. Beckley f o r t y p i n g this m a n u s c r i p t . We a c k n o w l e d g e t h e d o n a t i o n o f p r o s t a g l a n d i n F2a (Lutalyse ®) by J. F. McAllister o f t h e U p j o h n C o m p a n y , K a l a m a z o o , MI, and the gift o f g o n a d o t r o p i n releasing h o r m o n e (Cystorelin ®) b y M. D. B r o w n of CEVA L a b o r a t o r i e s , Overland Park, KS. REFERENCES

I Allalout, S. 1979. The effect of calving interval, days open, previous days dry, and percentage cow days in milk on annual yield per cow. M.S. Thesis, Washington State Univ., Pullman. 2 Barr, H. L. 1975. Influence of estrous detection on days open in dairy herds. J. Dairy Sci. 58:246. 3 Bozworth, R. W., G. Ward, E. P. Call, and E. R. Bonewitz. 1972. Analysis of factors affecting calving intervals of dairy cows. J. Dairy Sci. 55: 334. 4 Britt, J. H. 1975. Early postpartum breeding in dairy cows: a review. J. Dairy Sci. 58:266. 5 Call, E. P. 1978. Economics associated with calving intervals. Page 190 in Large dairy herd management. C. J. Wilcox and H. H. Van Horn, ed. Univ. Presses Florida, Gainesville. 6 Cooper, M. J. 1974. Control of estrous cycles of heifers with a synthetic prostaglandin analogue. Vet. Rec. 95:200. 7 Folman, T., M. Rosenberg, Z. Herz, and M. Davidson. 1973. The relationship between plasma progesterone concentration and conception in postpartum dairy cows maintained on two levels of nutrition. J. Reprod. Fertil. 34:267. 8 Fonseca, F. A., J. H. Britt, B. T. McDaniel, J. C. Wilk, and A. H. Rakes. 1982. Reproductive traits of Holsteins and Jerseys. Effects of age, milk yield, and clinical abnormalities on involution of cervix and uterus, ovulation, estrous cycles, detection of estrus, conception rate, and days open. J. Dairy Sci. 66:1128. 9 Hansel, W., and J. Fortune. 1978. The application

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of ovulation control. D. B. Crighton, N. B. Haynes, G. R. Foxeroft, and G. E. Lamming, ed. Pages 237-263 in Control of ovulation. Butterworth, London. 10 Harrison, D. S., C. E. Meadows, L. J. Boyd, and J. H. Britt. 1974. Effect of interval to first service on reproduction, lactation and culling in dairy cows. J. Dairy Sci. 57(Suppl.):628. (Abstr.) 11 MacMillan, K. L. 1983. Prostaglandin responses in dairy herd breeding programmes. N.Z. Vet. J. 31: 110. 12 Milliken, G. A., and D. E. Johnson. 1984. Levene's test. Pages 19--25 in Analysis of messy data. Vol. I. Designed experiments. Lifetime Learning Publ., Belmont, CA. 13 Milvae, R. A., B. D. Murphy, andW. Hansel. 1984. Prolongation of the bovine estrous cycle with a gonadotropin-releasing hormone analog. Biol. Reprod. 31:664. 14 Olds, D.~ T. Cooper, and F. A. Thrift. 1979. Effect of days open on economic aspects of current lactation. J. Dairy Sci. 62:1167. 15 Pelisser, C. L. 1972. Herd breeding problems and their consequences. J. Dairy Sci. 55:385. 16 Plunkett, S. S., J. S. Stevenson, and E. P. Call. 1984. Prostaglandin Fza for lactating dairy cows with palpable corpus luteum but unobserved estrus. J. Dairy Sci. 67:380. 17 Statistical Analysis System. 1979. SAS User's guide. SAS Inst., Inc., Cary, NC. 18 Slama, H., M. E. Wells, G. D. Adams, and R. D. Morrison. 1976. Factors affecting calving interval in dairy herds. J. Dairy Sci. 59:1334. 19 Speicher, J. A., and C. E. Meadows. 1967. Milk production costs associated with length of calving interval on Holstein cows. J. Dairy Sci. 50:975. (Abstr.) 20 Stevenson, J. S., and E. P. Call. 1983. Influence of early estrus, ovulation, and insemination on fertility in postpartum Holstein cows. Therlogenology 19:367. 21 Stevenson, J. S., M. K. Schmidt, and E. P. Call. 1984. Stage of estrous cycle, time of insemination, and seasonal effects on estrus and fertility of Holstein heifers after prostaglandin F2a. J. Dairy Sci. 67:1798.