مقایسه چهار برنامه هم‌زمان‌سازی فحلی بر عملکرد تولیدمثلی گاوهای شیری نژاد هلشتاین

Introduction: Reproductive efficiency is one of the most important determinants of profitability and sustainability in modern dairy production systems. Delayed conception after calving prolongs the open period, increases insemination and veterinary costs, reduces milk production efficiency over the lifetime of the cow, and ultimately contributes to involuntary culling. In high-producing Holstein dairy cows, reproductive performance is often compromised by metabolic stress, negative energy balance, postpartum uterine disorders, and suboptimal expression of estrus, all of which make estrus detection and timely insemination difficult under commercial farm conditions. For this reason, estrus synchronization and ovulation control protocols have become central tools in herd reproductive management. Synchronization programs are designed to regulate ovarian activity, synchronize follicular wave emergence, induce ovulation at a predictable time, and allow artificial insemination to be performed with reduced dependence on visual estrus detection. Several protocols have been developed for this purpose, among which Ovsynch, Heat Synch, Short Synch, and Double Ovsynch are commonly applied. Although all of these programs aim to improve fertility, their field performance may differ because of variations in hormonal sequence, degree of presynchronization, follicular responsiveness, and the physiological condition of cows at the start of treatment. Therefore, comparative evaluation of these protocols under practical herd conditions is necessary to identify the most efficient strategy for improving reproductive indices in dairy cattle. The objective of the present study was to compare the effects of four estrus synchronization protocols contain; Ovsynch, Short Synch, Heat Synch, and Double Ovsynch on reproductive performance in Holstein dairy cows. Particular emphasis was placed on comparing first-service pregnancy rate and the interval from calving to first postpartum insemination, as these indicators directly affect herd fertility, calving interval, and economic return.
Materials and methods: The study was conducted to determine which protocol offers the greatest reproductive advantage under routine dairy farm management. This study was conducted on 400 Holstein dairy cows, which were registered after calving and subjected to careful reproductive health evaluation before initiation of synchronization treatment. All cows underwent a standardized postpartum examination that included both clinical assessment and ultrasonographic examination of the reproductive tract. The purpose of this preliminary screening was to identify cows suitable for inclusion in the reproductive program and to reduce the influence of postpartum pathological conditions on fertility outcomes. Clinical examinations focused on overall postpartum reproductive health, with particular attention to the presence or absence of uterine infection, abnormal vaginal discharge, delayed uterine involution, or other clinically evident disorders. In addition, ultrasonography was used to assess uterine condition and ovarian activity. Cows were included in the study only when uterine involution was considered complete and no signs of metritis, endometritis, or significant ovarian abnormality were observed. This screening step was important to ensure that differences in reproductive performance could be attributed more reliably to the synchronization protocols rather than to confounding postpartum disease. Following confirmation of suitability, cows were randomly assigned to one of four treatment groups, with 100 cows in each group. Each group received one synchronization protocol: Ovsynch, Short Synch, Heat Synch, or Double Ovsynch. The hormonal schedules were applied according to the standard principles of each protocol. These protocols differed in timing, sequence of hormonal administration, and degree of presynchronization before timed insemination. Artificial insemination was then performed at the appropriate scheduled time according to the design of each program. The main reproductive outcome measured in this study was the first-service pregnancy rate, defined as the proportion of cows that became pregnant following the first insemination after completion of the assigned synchronization treatment. Another important parameter evaluated was the interval from calving to first postpartum insemination, which reflects the efficiency of returning cows to breeding activity after calving. These variables were recorded and compared across the four experimental groups to determine the relative effectiveness of each protocol in improving reproductive performance.
Results and discussion: The results demonstrated clear differences among the four estrus synchronization protocols in terms of reproductive success. Among all treatment groups, the Double Ovsynch protocol produced the best overall performance and was associated with the highest first-service pregnancy rate, reaching 51.31%. This result indicates that more than half of the cows in this group conceived after the first insemination, suggesting that this protocol provided more effective synchronization of ovulation and improved the likelihood of insemination at an optimal physiological stage. The Short Synch protocol ranked second, with a first-service pregnancy rate of 43.65%. Although its performance was lower than that of Double Ovsynch, it still showed an improved reproductive outcome compared with the traditional Ovsynch protocol. The Heat Synch treatment yielded a pregnancy rate of 42.98%, which was very close to that observed for Short Synch, indicating a moderate level of effectiveness in promoting conception after first service. In contrast, the Ovsynch protocol had the lowest first-service pregnancy rate, recorded at 38.52%, suggesting that under the conditions of this study, it was less effective than the other synchronization programs. In addition to conception rate, protocol choice also influenced the timing of first postpartum breeding. Cows treated with Double Ovsynch had the shortest calving-to-first insemination interval, indicating that this protocol facilitated earlier and more organized return to insemination after calving. A shorter interval to first service is highly desirable because it contributes to reduction of days open and supports a shorter calving interval at the herd level. Therefore, the superiority of Double Ovsynch was evident not only in pregnancy rate but also in improving the overall efficiency of postpartum reproductive management.
Conclusion: The findings of this study indicate that the reproductive performance of Holstein dairy cows can be significantly influenced by the estrus synchronization strategy used after calving. Among the four protocols evaluated, Double Ovsynch proved to be the most effective, achieving the highest first-service pregnancy rate (51.31%) and the most favorable calving-to-first insemination interval. These results suggest that the added presynchronization and improved control of ovarian status provided by Double Ovsynch may better prepare cows for timed insemination and increase the probability of successful conception. From a practical perspective, implementation of Double Ovsynch may offer important economic and managerial advantages in commercial dairy herds by reducing open days, improving reproductive efficiency, and enhancing herd productivity. Although this protocol may require greater planning, stricter compliance, and potentially higher hormonal and labor inputs, its superior fertility outcomes may justify its use, particularly in herds facing challenges with estrus detection or low conception rates. Overall, the study supports the recommendation that Double Ovsynch be considered a preferred synchronization protocol for improving reproductive efficiency in Holstein dairy cows under managed farm conditions.
Data availability statement: The data that support the findings of this study are available on request from the corresponding author.
Conflicts of interest: The authors declare no conflicts of interest.
Acknowledgment: The authors thank the Mihan Company in Zarandeh. Central Province, Arak.