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Introduction
Reproductive efficiency is an important strategy for maximizing the productivity of cattle herds. Among the current strategies used to improve reproductive performance, ovulation synchronization has become the best technique for optimizing herd management and providing genetic improvements with greater efficiency.
Among beef cattle, approximately 40 to 60% of inseminated females become pregnant after the first cycle of fixed-time artificial insemination (FTAI), with slight variations in rates depending on several factors including the animals, farm management, body condition score, postpartum time and hormonal treatments [2311]. Therefore, females that do not conceive after the first service need to be re-inseminated as soon as possible to increase the reproductive performance of the herd [46719]. If these females are not re-inseminated in a short period of time, the interval between calving and conception increases, decreasing the reproductive efficiency of the herd.
The resynchronization of females who failed the first FTAI is an effective strategy to increase the number of pregnancies achieved by AI during a short breeding season [19]. Ovulation resynchronization can increase the percentage of non-pregnant cows that are subjected to a second insemination [13] and be an advantageous strategy when the number of bulls is insufficient to mate with all of the cows that did not become pregnant after the first FTAI [8]. During resynchronization, all cows inseminated in the first round of FTAI should be evaluated by ultrasonography, and those that did not become pregnant receive the same hormonal protocol for a second round of FTAI [718].
One of the most common hormonal protocols for FTAI or resynchronization in cattle involves the use of a progesterone source, such as an intravaginal device or norgestomet ear implant, in combination with 1 mg of estradiol benzoate (EB), 0.5 mg of estradiol cypionate (EC) [1019] or 100 µg of GnRH [16]. Temporary calf removal (TCR) for 48 to 54 h or 300 IU of equine chorionic gonadotropin (eCG) have also been successfully applied at the time of device withdrawal and in conjunction with a luteolytic agent and inducer of ovulation to improve the reproductive performance of resynchronized Bos indicus cows [4].
Resynchronization protocols have been evaluated in different categories of female beef and dairy cattle (Bos taurus) [19], non-lactating cows and beef heifers (Bos taurus) [78], multiparous lactating beef cows (Bos indicus) [4], non-lactating cows and cycling beef heifers (Bos indicus) and crossbred cows from Bos indicus and Bos taurus parents [16]. Nevertheless, no studies have examined different cohorts in a single resynchronization study, particularly with heifers or newly calved cows. Therefore, the present study was conducted to investigate the conception rate in heifers, primiparous and multiparous lactating cows after a large-scale FTAI program, followed by resynchronization using an estradiol/P4-based protocol.
Materials and Methods
Location, animals and management
This study was conducted during the 2011/2012 breeding season in South America at latitude 21°15'23'' and longitude 52°2'29''. The climate in this region is tropical, with an average temperature of 24℃ and a rainy season from November to January. Data for this study were collected from Bos indicus (Nelore) herds including heifers, as well as primiparous and multiparas lactating cows (n = 2,464) that were selected to receive a conventional protocol of FTAI in two commercial beef farms in Southern Brazil under the same management.
Heifers (24 to 27 months old) and cows (36 to 180 months old) were selected based on an adequate body condition score (BCS) and postpartum period, normal estrous cycles and health status. The primiparous and multiparous lactating cows were 30 to 50 days postpartum (average 45 days), and only females with a BCS between 2.0 and 4.0 (average 3) on a scale of 1 to 5 [1] were used for the FTAI program. The herds were kept in an extensive system that allowed for continuous grazing of Brachiaria spp. and were given ad libitum access to mineralized salt and water.
Hormonal treatment
On a random day of the estrous cycle (Day 0), heifers (n = 903), primiparous lactating cows (n = 338) and multiparous lactating cows (n = 1,223) underwent a conventional estradiol/P4-based FTAI protocol. The protocol consisted of the insertion of an intravaginal device containing 1 g of progesterone (P4) (DIB; MSD Animal Health, Brazil; cows) or an ear implant containing 3 mg of norgestomet (Crestar; MSD Animal Health; heifers) and intramuscular (i.m.) administration of 2 mg of EB (Gonadiol; MSD Animal Health). On Day 8, the devices were removed and the animals received i.m. injections containing 250 µg of cloprostenol (Ciosin; MSD Animal Health), 300 IU of eCG (Novormon; MSD Animal Health) and EC (1.0 mg for lactating cows and 0.5 mg for heifers, ECP; Zoetis, Brazil). All females were inseminated by two experienced inseminators using frozen-thawed semen of bulls 48 h after device removal (Fig. 1).
Thirty days after the first FTAI (Day 28 to 32), all animals were subjected to a pregnancy diagnosis by transrectal ultrasonography (Aloka SSD 500; Aloka, Japan) and non-pregnant females received the same hormonal treatment described above.
Heifers received norgestomet due to poor estrus synchronization results when a new P4-releasing intravaginal device (first use) was used [591417]. To resynchronize ovulation, all females were implanted with a previously used P4 source (device or implant) for 8 days. After their initial use, the devices/implants were individually washed with water, then soaked in a solution of ascorbic acid, citric acid and lactic acid (Kilol-L; Quinabra, Brazil) for approximately 10 minutes. Thereafter, the devices were dried and stored at room temperature until use.
Pregnancy diagnosis
Pregnancy diagnosis was performed by ultrasonography 30 days after each FTAI cycle. The first evaluation occurred on Day 30 (28 to 32 days), while the second was conducted 30 days later. In both evaluations, females were scanned by transrectal ultrasonography using an Aloka SSD-500 ultrasound equipped with a 5 MHz linear transducer (Aloka). The conception rate was calculated by dividing the number of pregnant cows by the total number of females inseminated on Day 30 and 30 days after the second FTAI.
Statistical analysis
The data are presented as proportions. The conception rates corresponding to the first FTAI, second FTAI and overall rate within 30 days of the second FTAI (first FTAI + resynchronization) were compared across categories (heifers, primiparous cows and multiparous cows).
Statistical analysis was performed using Minitab. The significance level for rejecting the null hypothesis was p ≤ 0.05. A chi-square test was used to determine differences in conception rates among the categories of females.
Results
Of the 2,464 female bovines subjected to an estradiol/P4-based FTAI protocol followed by ovulation resynchronization, 1,985 animals became pregnant, resulting in an average conception rate of 81% 30 days after the second FTAI (Table 1). Heifers exhibited a higher conception rate (85%; 770/903) than the primiparous cows (76%; 257/338) or multiparous cows (78%; 958/1,223; p = 0.0001).
The average conception rate after the first FTAI was 55% (1,367/2,464), and there were no differences between the conception rates of the heifers (57%; 514/903), primiparous cows (51%; 173/338) or multiparous cows (56%; 680/1,223; p = 0.193). The average conception rate after the second FTAI (resynchronization) was 56% (618/1,097), and the conception rate was higher in heifers (66%; 256/389) than in the primiparous cows (51%; 84/165) and multiparous cows (51%; 278/543; p = 0.0001).
Discussion
Data from the current study demonstrated that it is possible to achieve high pregnancy rates in a short period of the breeding season using only FTAI after resynchronization of heifers, primiparous and multiparous lactating cows. The higher pregnancy rate after the second FTAI in heifers should be considered strategic information for large scale AI programs in beef herds.
Thirty days after the first FTAI procedure, we used 1 mg of EB in combination with a P4 source (intravaginal device or norgestomet ear implant) to induce a new follicular wave. A recent study evaluated the use of resynchronization employing a progestin-based FTAI protocol in beef cattle (Bos indicus) with two different inducers of new follicular waves (EB vs. GnRH). This study showed that both inducers resulted in similar pregnancy rates (70.5% and 63.2%, respectively). Additionally, 1 mg of EB 22 days after FTAI did not affect pre-established pregnancy [16]. Another study of previously inseminated lactating beef cows (Bos taurus) used resynchronization with P4 + 1 mg of EB or P4 + 0.5 mg of EC. The P4 sources in combination with estrogen treatment did not decrease the conception rates, resulting in pregnancy rates of 86% and 65%, respectively [19].
In the present study, the conception rate following the first FTAI protocol did not differ among the cohorts evaluated (heifers (57%), primiparous cows (51%) and multiparous cows (56%)). Additionally, these results were very similar to those observed by Campos et al. [4], who achieved conception rates ranging from 47 to 54% in Nelore multiparous cows after evaluating data from the first FTAI in an estrus resynchronization program.
The second FTAI resulted in higher conception rates in heifers than cows (66% vs. 51%, respectively), demonstrating that the category influences the conception rate of re-inseminated females. Some field studies have shown that Bos indicus cows exhibit different pregnancy rates following the first and second FTAI depending on the category of the resynchronized females [416]. In our field experience, there is a decrease in pregnancy rate following the second FTAI relative to the rate after the first FTAI of primiparous cows. In the present study the pregnancy rate was similar when a second synchronization of ovulation was performed.
The resynchronized females from the present study achieved an average pregnancy rate of approximately 80% (after both the first and second FTAI) in only 80 days of the breeding season. Despite the better efficiency of resynchronization in heifers (85%), this protocol also provided promising results in primiparous (76%) and multiparous (78%) cows. By resynchronizing cycling heifers and non-lactating cows (Bos indicus and crossbred ½ Bos indicus ½ Bos taurus), Sá Filho et al. [16] achieved a 75% pregnancy rate, which was similar to the data described here. Similarly, Campos et al. [4] reported pregnancy rates of 76.6% and 74% following the resynchronization of lactating cows treated with 300 UI of eCG or temporary calf removal, respectively.
Lactating cows in the postpartum period represent a category of females with higher energy requirements [3]. During this period, the cow's nutritional status and presence of the calf exert strong influences on the female's energetic balance that could negatively influence breeding potential and the success of FTAI [1215]. This is particularly true in primiparous cows, which have the highest energy demands [20]. Remarkably, in the present study, primiparous cows achieved a conception rate similar to multiparous cows that was similar to the average found in other studies of multiparous cows (74-76%) [4].
In conclusion, ovulation resynchronization of females who failed the first FTAI is a feasible and effective technique, providing an average conception rate of 80% in beef cattle. The best result was found in heifers, with a 66% pregnancy rate being observed after the second FTAI and an 85% rate after the two FTAI procedures.
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