Resynchronization improves reproductive efficiency of suckled Bos taurus beef cows subjected to spring-summer or autumn-winter breeding season in South Brazil

Abstract

The objective of the present study was to evaluate the effect of estrus resynchronization (RE) after the first Timed Artificial Insemination (TAI) protocol on reproductive efficiency of suckled Bos taurus beef cows (n = 1052; Angus and Brangus) subjected to two different breeding season (BS) in Rio Grande do Sul, Brazil. In experiment 1, cows were maintained in native pasture on conventional BS (spring-summer), and in experiment 2, cows were maintained in a grazing area cultivated in an integrated crop-livestock system on modified BS (autumn -winter). Experiment 1 cows (n = 92) were randomly distributed in two reproductive managements: 1) TAI at the first day of the BS (Day 0), followed by natural service (NS) on day 10 until the end of the BS (TAI + NS; n = 45) or, 2) TAI on day 0, followed by RE 22 days after the first TAI and then followed by NS on day 42 until the end of the BS (TAI + RE22 + NS; n = 47). Experiment 2 cows (n = 960) were subjected to one of four reproductive managements: 1) only NS for 90 days of the BS (NS; n = 266); 2) TAI on day 0, followed by NS on day 10 until the end of the BS (TAI + NS; n = 200); 3) TAI on day 0, followed by RE 22 days after the first TAI and then followed by NS on day 42 until the end of the BS (TAI + RE22 + NS; n = 249) or, 4) TAI on day 0, followed by RE 30 days after the first TAI and then followed by NS on day 50 until the end of the BS (TAI + RE30 + NS; n = 245). In experiment 1, conception rate after first TAI was similar for treatments TAI + NS (42%, 19/45) and TAI + RE22 + NS (48%, 23/47; P = 0.4107). Total pregnancy rates at day 60 of the BS were 55.6% and 66.6% for TAI + NS and TAI + RE22 + NS, respectively (P = 0.006). In experiment 2, only 3% of the females submitted to NS were pregnant 30 days after the onset of BS, a lower rate compared to other groups (TAI + NS = 40%; TAI + RE22 + NS = 39.8%; TAI + RE30 + NS = 40%; P < 0.001). At 70 days of the BS, the groups TAI + RE22 + NS and TAI + RE30 + NS obtained greater pregnancy rate (66.3% and 69.4%, respectively) than other groups (NS = 16.9%; TAI + NS = 48%; P < 0.001). Pregnancy rate at the end of BS was greater in RE groups (TAI + RE22 + NS = 81.5%; TAI + RE30 + NS = 83.7%) than the NS (45.1%) or TAI + NS (71%). In conclusion, resynchronization increases pregnancy rate per AI (P/AI) and reproductive efficiency of suckled Bos taurus beef cows during spring-summer or autumn-winter breeding season in natural pasture or cultivated pasture, respectively.

Introduction

High reproductive performance in beef cattle herds is an essential requirement to ensure maximum production and satisfactory economic return. In this context, the incorporation of reproductive programs in the farm routine is an organized approach to optimize beef cattle reproductive results and profitability [1]. Increased reproductive efficiency at the start of a beef cattle breeding season (BS) is desirable and its implementation provides substantial improvement in the economic and biological efficiency of the process due to the positive impact on birth rate [2].

In tropical countries like Brazil, BS occurs during the spring and summer (October to March) when there is a greater supply of natural pastures and, consequently, calving occurs in the winter-spring transition (August to December) [3]. It is important to adjust the calving period when there is a greater supply of fodder or to standardize calf lots. Additionally, cows conceiving early in BS present longer time for uterine postpartum recovery and can successfully conceive the next season.

A very peculiar scenario has changed livestock production in southern Brazil, especially in Rio Grande do Sul, where is traditional with extensive creations in native fields [4]. According to Lopes et al. (2009), this modification occurred with the introduction of agriculture in native field areas that thus increased supply of fodder in June to September by the presence of cultivated pastures of oat (Avena strigosa Schreb.) and ryegrass (Lolium multiflorum Lam.) after soybean crop (Glycine max) [5]. Therefore, breeding season on farms with crop-livestock integration occurs in the autumn-winter months with an increasingly necessary concentration of calving season lasting less than 90 days.

Large-scale use of AI promotes genetic and economic gains through the use of genetically superior bulls [6]. Postpartum anestrus [7] makes implementation of AI programs based on estrus detection in beef herds difficult, so estrus detection can be faulty when used in large herds with many lots of animals, in addition to increasing the cost of the activity.

However, the use of Fixed Time Artificial Insemination (FTAI) promotes the withdrawal of suckled beef cows from the anestrous condition [8] and avoids delays in resuming cyclicity. These cows conceive at the beginning of BS, which shortens the interval from calving to conception and leads to reduced economic losses. Therefore, breeding programs for beef herds under pasture conditions should focus on increasing pregnancy rates at the onset of BS. The increase of this pregnancy rate at the beginning of the BS is possible with the use of FTAI when performing AI management of the females with less time spent, which allows inseminating more females in a single day without the need for estrus observation [9].

In addition, estrus resynchronization is a technology to perform a second TAI in a previously inseminated female in a short period. The objective of the technique is to increase the number of calves from AI, eliminate estrus observation, reduce the number of bulls to be transferred and increase herd reproductive and productive efficiency. Resynchronization of estrus in beef cows without previous knowledge of gestation was demonstrated as a safe method using 1 mg of estradiol benzoate (EB), which facilitates higher pregnancy rates compared to the use of GnRH [10].

Traditional resynchronization program was initiated at pregnancy diagnosis about 28–32 days after TAI [11,12]. Cows diagnosed as nonpregnant immediately initiated a second TAI protocol. Additionally, this resynchronization was only performed in nonpregnant cows. The evolution of Resynch programs allowed the reduction of the interval between AI to compact the period of BS and the interval between two subsequent parturitions. Thereby, the Resynch 22 was performed 22 days after the previous AI, eight days before pregnancy diagnosis [10]. At that time, cows diagnosed as pregnant were excluded from the following treatments and nonpregnant cows continued the synchronization treatment. The advantage of Resynch 22 is to reduce the interval between AI to 32 days. However, the P4 device and estradiol treatment should be done in all cows and pregnancy diagnosis must be performed in a fixed schedule, different from traditional Resynch [13].

This is the first research performed in suckled beef cows, predominantly Bos taurus, to evaluate the impact on pregnancy rate during BS using different forms of estrus synchronization followed or not by estrus resynchronization. In this way, the present study hypothesized that the use of estrus and ovulation resynchronization after the first TAI has a greater ability to concentrate more pregnancies at the beginning of BS.

The objective of this study was to compare the efficiency of resynchronization protocols against the use of just one TAI and natural service on the pregnancy rates of suckled beef cows in the conventional (spring-summer) or modified (autumn-winter) breeding season.

Section snippets

Experiment 1

Use of TAI + NS or TAI + RE22 + NS during the conventional BS (spring-summer).

Experiment 1

The conception rate after first TAI was similar between treatments [TAI + NS = 42% (19/45), TAI + RE22 + NS = 48% (23/47), P = 0.4107]. In the TAI + RE22 + NS group, the conception rate after resynchronization was 66.6% (16/24), while conception in the group TAI + NS was 26.9% (7/26, P = 0.006). The cumulative pregnancy rate in the first 60 days of BS was greater in the TAI + RE22 + NS group (80.8%, 38/47, P = 0.008) in comparison to TAI + NS group (55.6%; 25/45). However, the final pregnancy

Discussion

The results of this study confirmed the hypothesis that it is possible to obtain a higher number of pregnancies and with satisfactory indices using early or conventional resynchronization. In addition, when pregnant cows are placed in natural service after two AI, pregnancy rates above 80% are obtained in the autumn-winter breeding season. These strategies improve herd reproductive efficiency compared with natural service without estrus induction or synchronization [15]. The crop-livestock

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

Hertape Calier Saúde Animal, Brazil; MSD Animal Health, Brazil; Zoetis Animal Heatlh, Brazil ; FATEC UFSM (041030).

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