ing dairy cows without the need for estrus detection. ( Key words: reproduction, gonadotropin-releasing hormone, prostaglandin F2a, synchronized ovulation).
Reproductive Management of Lactating Dairy Cows Using Synchronization of Ovulation J. R. PURSLEY, MICHAEL R. KOSOROK,1 and MILO C. WILTBANK2 Department of Dairy Science, University of Wisconsin, Madison 53706
ABSTRACT Lactating dairy cows have poor reproductive efficiency because of low fertility and low rates of estrus detection. To eliminate the dependence on detection of estrus, we have recently developed a timed artificial insemination ( A I ) protocol that synchronizes the time of ovulation using GnRH and PGF2a. The effectiveness of this protocol as a management tool was compared with standard reproductive management. Lactating dairy cows ( n = 333) from three herds were randomly assigned at parturition to two groups. Control cows were managed according to the typical reproductive strategy of the farm that relied on detection of estrus, the a.m.–p.m. breeding rule, and periodic use of PGF2a. Treated cows had timed AI after synchronization of ovulation with GnRH and PGF2a. For both groups, the voluntary waiting period was 50 d postpartum. Pregnancy diagnosis was performed by ultrasound between 32 and 38 d post-AI. Nonpregnant cows were inseminated again using the original treatment until diagnosed as pregnant or until culled from the herd. Median days to first AI (54 vs. 83) and days open (99 vs. 118) were lower for treated cows than for control cows, respectively. Pregnancy rates for the first AI were similar (37% vs. 39%) for the two groups even though treated cows were bred at an earlier time postpartum. More treated cows than control cows were pregnant at 60 d (37% vs. 5%) and at 100 d (53% vs. 35%) after calving. Thus, this protocol allowed effective management of AI in lactating dairy cows without the need for estrus detection. ( Key words: reproduction, gonadotropin-releasing hormone, prostaglandin F2a, synchronized ovulation) INTRODUCTION Increased milk production and increased herd size have dramatically influenced the manner in which dairy producers manage reproduction. Approximately
Received March 13, 1996. Accepted July 29, 1996. 1Departments of Statistics and Biostatistics. 2Corresponding author. 1997 J Dairy Sci 80:301–306
70 to 80% of the dairy farms in the US currently use AI, and most use daily estrus detection and the a.m.– p.m. breeding rule to manage AI. However, because the rate of estrus detection is 15 d after AI were assumed nonpregnant and were reinseminated. Nevertheless, all control cows, including those that had been reinseminated, were evaluated for pregnancy at 32 to 38 d after the initial AI. If Journal of Dairy Science Vol. 80, No. 2, 1997
cows in the control group were diagnosed open and had not been inseminated since the AI in question, the herd manager was informed of the pregnancy diagnoses to allow a prompt management decision concerning reinsemination. Cows in the treated group that were observed in estrus between the time of AI and pregnancy diagnosis were not reinseminated, and any reproductive management decisions were delayed until after pregnancy evaluation at 32 d post-AI. If cows were not pregnant, they were treated again with the synchronization protocol beginning at 32 d post-AI. If the ultrasound technician was unsure of the diagnosis, cows received the initial injection of GnRH and were rechecked 1 wk later, prior to the injection of PGF2a. Pregnancy rates for each AI were defined as the number of cows pregnant at 32 to 38 d post-AI divided by the total number of cows that received that AI. Statistical Analyses Covariables that were considered in the analysis of pregnancy rates per AI included season, farm, parity, treatment by parity interaction, and farm by parity interaction. Only the effect of season was significant; therefore, pregnancy rates per AI were stratified by season and analyzed using the Mantel-Haenszel chisquare test. Time to AI outcomes were censored when cows became pregnant, were culled from the herd, or when cows died. Time to pregnancy outcomes were censored at the time of pregnancy or at 32 d prior to either being culled from the herd or death, because this time represented the last possible time that they could have had a detectable pregnancy. Time data were analyzed by Kaplan-Meier survival curve estimation and the log rank statistics. The P values for the median were obtained by bootstrap methods ( 5 ) . Survival curves for time to second AI were obtained from the convolution of the time to first AI and the time between the first and second AI; survival curves for time to third AI were obtained from the convolution of time to second AI and time between second and third AI ( 9 ) . This approach was necessary to avoid bias that resulted from changes in the number of cows that were eligible for each successive AI. For example, cows that became pregnant after the first AI were excluded from the group that were evaluated at the second AI. RESULTS Time to First, Second, and Third AI Figure 1 illustrates the difference between the times to first AI for the treated and control groups
303
SYNCHRONIZATION OF OVULATION
nonpregnant control cows were reinseminated at all detected estruses (Table 1). Pregnancy Rates per AI Synchronization of ovulation and timed AI resulted in similar pregnancy rates at first AI compared with standard reproductive management (Table 1). In addition, pregnancy rates per AI were similar for second (Table 1 ) and third AI between treated and control cows. Days to Conception
Figure 1. Survival curves for days to first AI in lactating Holstein cows managed with standard reproductive strategies ( ◊) versus timed AI after synchronization of ovulation ( ♦) .
using Kaplan-Meier curve for survival estimates. Because the synchronization protocol allowed the scheduling of AI with respect to the voluntary waiting period, the median days to first AI were less ( P < 0.001) for treated cows than for control cows (Table 1). In addition, the range of days to first AI was less variable ( P < 0.001) for cows in the treated group, 50 to 58 d, than for cows in the control group, 50 to 162 d (Figure 1). Also, times to second and to third AI were less ( P < 0.001 and P < 0.01, respectively) for cows in the treated group than for cows in the control group (Table 1). The intervals among first, second, and third AI were similar for control and treated cows. The mean interval between breedings was about 42 d, although the nonpregnant cows in the treated group could not be reinseminated until 42 d after AI, and
Similar pregnancy rates per AI combined with less variation and fewer days to first and subsequent AI in the treated group led to a decrease ( P < 0.05) in median days open (Figure 2). The survival curves indicated a greater likelihood for treated cows to be pregnant than for control cows to be pregnant at all times postpartum ( P < 0.001 by log rank statistic). The median days to conception were less ( P < 0.05) for treated cows than for control cows (Table 1). In addition, more treated than control cows were pregnant at 60 and 100 d postpartum ( P < 0.01). Culling Rates Of the cows in this study, more treated cows were culled than control cows that received AI (Table 2). Also, of the cows that received at least one AI, mean days postpartum on the day of culling were greater for control cows than for treated cows. No difference ( P > 0.05) was detected in the number of cows culled from the control group ( n = 35) compared with those culled from the treated group ( n = 25). DISCUSSION The present study demonstrated that the use of a protocol that allows effective timed AI can improve
TABLE 1. Median days postpartum ( P P ) at AI, pregnancy rate ( P R ) per AI, and median days to conception in lactating Holstein cows1 inseminated after a detected estrus (control) or time inseminated after synchronization of ovulation (treated). AI First
Second
Third
Pregnant
Treatment
PP
PR
PP
PR
PP
PR
60 d PP
Control Treated P
(d) 83 54 0.25
(d) 128 96 0.15
(d) 170 140 0.15
5 37
