| Livestock Research for Rural Development 31 (9) 2019 | Guide for preparation of papers | LRRD Newsletter | Citation of this paper |
The effect of the season of the year on the reproductive activity of Dorper and Dorper x Pelibuey ewes under tropical conditions was evaluated. Thirty-nine ewes per each breed group were evaluated in two seasons: Reproductive (September-January) and non-reproductive seasons (March-June). In each season, estrous was detected for 45 days and ovulation was confirmed by laparoscopy 7 to 9 days after estrus. The ewes received natural service at the second estrus, pregnancy diagnosed at 35-40 days post-service, and prolificacy was registered at lambing. The rate of ewes in estrus, duration of estrus, estrous cycle length, average corpus luteum (CL) and fertility were also evaluated. The data were analyzed using analyses of variance and Chi-square tests. The rate of ewes in estrus (97.4 vs 82.1%; P <0.05) and estrus duration (36.2 vs 16.9 h; P <0.05) was higher in the reproductive season, but no breed group differences were found (p> 0.05). The duration of the estrous cycle (17.35 days), average of CL (1.39), fertility (84.85 %) and prolificacy (1.36 lambs) were similar between seasons and breed groups (p> 0.05). In conclusion, Dorper ewes and Dorper x Pelibuey ewes do not show reproductive seasonality under the tropical conditions of this study.
Key words: reproductive seasonality, ovulatory rate, prolificacy, tropic
The reproductive activity of sheep raised under temperate conditions presents seasonality; however, in the tropics, where the photoperiod variation is minimal, reproductive seasonality in hair sheep is usually not observed. Several studies report that the hair sheep show reproductive activity throughout the year, with a slight reduction in the reproductive activity of the sheep during the months of February-May (González et al 1992; Cruz et al 1994; Arroyo et al 2007; de la Isla-Herrera et al 2010; Arroyo 2011). This allows the establishing of breeding seasons throughout the year, as a strategy to increase flock productivity.
On the other hand, in the Mexican tropics, interest in the Dorper breed has been increasing in recent years, knowing little about the seasonal reproductive behavior of this breed and its crosses with other hair breeds, such as Pelibuey. The Dorper breed was introduced with the purpose of improving the productive characteristics of tropical sheep. It has been reported to have non-seasonal reproductive behavior, high fecundity and good prolificacy, as well as a short anestrus period after weaning; aspects that makes it a breed of interest for meat production in arid (Elias et al 1985) and tropical areas. However, under tropical conditions, some sheep-producers mention that the reproductive activity of Dorper sheep decreases, during the spring and early summer. In addition, although it is reported that the Dorper breed was created to produce efficiently under adverse conditions (Elias et al., 1985, Cloete et al 2000), results of its reproductive behavior is scarce under humid and sub-humid tropical conditions. . Therefore, it is important to generate information that allows the establishing of more efficient reproductive management strategies in this breed and its crosses in different seasons of the year. The objective of the present work was to evaluate the effect of the season of the year on the reproductive activity of Dorper and Dorper x Pelibuey sheep under tropical conditions.
The study was carried out in the "El Chaparral" Ranch, located in Sucila, Yucatan, Mexico (21 ° 07 'and 21 ° 14' N and 88 ° 16 'and 88 ° 25' W). The predominant weather of the region is subhumid warm with summer rain (Aw0), average annual temperature of 26.4 ºC and average annual rainfall of 900 to 1000 mm (Orellana et al 2009). Two periods were established, depending on the reproductive activity of the sheep in the tropics: The period of lower reproductive activity (March to June) and the period of greatest reproductive activity (September to January) (Cruz et al 1994; Heredia 1994; de la Isla-Herrera et al 2010).
Seventy-eight ewes, 39 Dorper and 39 F1 (Dorper x Pelibuey) were evaluated. All of them were multiparous (2 to 4 births), 2 to 4 years old, with body condition between 2.5 and 4 points (scale 1-5, Russel, 1984), clinically healthy, empty and without raising a lamb. Ewes of each breed group were randomly distributed in each season of the year. Both breed groups were managed under diurnal grazing (8 h/day) in meadows of Star grass (Cynodon nemfluensis) and Pasto Guinea (Panicum maximum) with irrigation. In the corral the ewes received 350 g / day of commercial feed (16% PC and 10.1 MJ / kg DM), as well as Taiwan grass (Pennisetum purpureum) chopped ad libitum. The animals had access at all times to fresh water, shading and mineral salts.
Estral activity of the sheep was evaluated by direct observation, twice a day (06:00 and 18:00 h) for one hour, 45 days in both seasons (Reproductive and non-reproductive). The detection of the estrous was carried out with the help of rams with aprons, which were introduced to the groups, only during the observation hours. The females detected in estrus were temporarily moved away from the group to allow the males to continue detecting other females (Aké-López et al 2013).
Ovulation and the number of corpora lutea were evaluated in the ewes that presented estrous and was performed by laparoscopy on day 7 to 9 post-estro. The mean was calculated by dividing the number of corpora lutea in both ovaries between the total number of ewes.
Ewes were served by natural mating until the second estrous was observed, to obtain the duration of the estrous cycle. The mating services were carried out at 0 and 24 hours after estrous was observed, using males with good seminal quality. Pregnancy diagnosis was carried out by ultrasound, 35-40 days after service and followed until the ewe lambed. All ewes diagnosed pregnant lambed. Fertility was obtained dividing the number of lambs born between the total number of ewes served x 100, and number of lamb born was recorded as prolificacy.
The effect of season of the year, breed group and the interaction on estrus duration, estrous cycle, number of corpora lutea and prolificacy was determined by analysis of variance, using the GLM procedure (SAS 2003). Data of presence of estrous (yes, no), ewes ovulated (yes, no) and lambing ewes (yes, no) were analyzed by Chi square tests (SAS 2003). Differences between breed groups and seasons were established at 5% significance. No interaction was found, therefore only the main effects are reported.
The season of the year had effect on the ewes that showed estrus (p <0.05), being the highest in the reproductive season (97.44%, Table 1). In addition, estrous duration was longer in the reproductive season (36.2 h) than in the non-reproductive season (16.9 h, p <0.05), but the duration of the estrous cycle was similar (17.4 and 17.3 days). Breed group had no effect on estrous related traits (p> 0.05).
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Table 1. Results by season and breed group for estrous traits of Dorper and Dorper x Pelibuey (F1) ewes under tropical conditions |
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|
Traits |
Season |
Breed group |
||
|
Reproductive
|
Non-reproductive |
Dorper
|
F1
|
|
|
Ewes in estrous (%) |
97.4a |
82.1b |
89.7a |
89.7a |
|
Estrous duration (h) |
36.2±2.1a |
16.9±0.9b |
28.9±2.6a |
25.7±2.1a |
|
Estrous cycle duration (d) |
17.4±0.3a |
17.3±0.2a |
17.4±0.2a |
17.3±0.4a |
|
Ewes that ovulated (%) |
100 a |
100 a |
100 a |
100 a |
|
Corpus luteum (Number) |
1.39±0.08a |
1.38±0.11a |
1.49±0.1a |
1.29±0.09a |
|
a,b. Different literals in the same row indicate differences at p<0.0). CL= corpus luteum |
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All ewes showing estrous ovulated (100%), so no differences were found between seasons and breed groups (p> 0.05). In addition, averages were similar for season and breed group for the number of corpora lutea (Table 1).
There was no effect (p> 0.05) of season on fertility (84.8%) and prolificacy (Table 2).
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Table 2. Results by season and breed group for fertility and prolificacy of Dorper and Dorper x Pelibuey (F1) ewes |
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|
Trait # |
Season |
Breed group |
||
|
Reproductive |
Not reproductive |
Dorper |
F1 |
|
|
Fertility (%) |
85.3 (29/34) |
84.4 (27/32) |
81.8 (27/33) |
87.9 (29/33) |
|
Prolificacy |
1.48 ± 0.11 |
1.22 ± 0.08 |
1.44 ± 0.11 |
1.28 ± 0.08 |
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Fertility= lambing ewes/served ewes x 100; Prolificacy=
number of lambs/lambing ewe;
|
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Several studies report that the reproductive activity of hair sheep under tropical conditions occurs throughout the whole year. However, the frequency of reproductive activity decreases in certain months of the year, which generally ranges from mid-March to early June (González et al 1992; Heredia 1994; Martínez et al 1995; de la Isla-Herrera et al 2010). de la Isla-Herrera et al (2010) in Pelibuey ewes, found season differences on estrous rate, reporting that, from February to May, the estrus activity decreased (45%) in comparison with the period from August to November (100% of the sheep that showed estrous activity). This agree with what was found in this study, although, the gap between seasons was lower (82.05% vs 97.4%). On the other hand, Cruz et al (1994) in Pelibuey ewes, under grazing conditions in the humid tropics of Veracruz, Mexico, found similar values to those reported in this study. These researchers found seasonal variations in the presentation of estrous, and reported that estrous activity was greater in the month of August (100%) than in April (81.2%); however, the differences were not significant (p> 0.05).
In the present study, it was observed that the season also influenced estrous duration, being shorter in the March-June period (16.9 h) compared to the months of September-January. These results disagree from what was found by de la Isla-Herrera et al (2010) in Pelibuey ewes, who did not find season effect on estrous duration, with a general average of 24.9 h. On the other hand, the value found here in the reproductive season (36.2 h) is similar to that mentioned by Elias et al (1985), who in Dorper reported a mean of 36 h. These results show that Dorper and Dorper x Pelibuey ewes modified their estrus behavior according to the season of year, in principle reducing the estrous duration in the non-breeding season.
The estrous rate and the estrus duration of the Dorper and F1 sheep showed an influence of season, probably due to environmental effects, mainly to high temperatures, as previously reported (González et al 1992; Heredia 1994; Isla-Herrera et al 2010). These authors mention that, independently of differences in age, weight or body condition, ewes reduce their reproductive activity in the spring-summer period. In this study, the decrease in the estrous rate was not as marked (15.3% difference); however, the duration of estrus did show an important difference (19.3 h), which may result in a decrease in herd fertility.
Regarding the duration of the estrous cycle, no difference was found (p> 0.05) between the breed groups and season of the year (17.4 days). These results are consistent with the length of estrus cycle reported for hair sheep in the tropics (Heredia 1994; de la Isla-Herrera et al 2010; Arroyo 2011), and for Dorper sheep (17.6 days), in arid regions (Elias et al 1985). This indicates that the estrus cycle of the Dorper and Dorper x Pelibuey sheep, under tropical conditions, is not affected, as reported in Pelibuey sheep under the same environmental conditions (de la Isla-Herrera et al 2010).
The absence of effect of season and breed group (p> 0.05) on the ovulation rate of the sheep (100%), and on the number of corpora lutea (1.39 CL), differs from other studies (de la Isla-Herrera et al 2010). These authors, in Pelibuey sheep, regardless of their body condition, observed a reduction in the ovulation rate (60%) and in the mean of corpora lutea (1.3 ± 0.16 CL) in the months of February to May, compared to the months of August to November (100% and 1.8 ± 0.13 CL respectively). In addition, under tropical conditions, it has been reported that from February to June (non-reproductive season) sheep show a significant decrease in their ovarian activity (Rosa and Bryant 2003; de Isla-Herrera et al 2010). However, this did not was observed in the Dorper and Dorper x Pelibuey sheep studied in the present study. The corpora lutea mean was not affected (p <0.05), being higher than that reported by Greeff et al (1993), who obtained a mean of 1.0 for Dorper and 1.2 for the F1 sheep (Dorper x Romanov); similar result to that reported by Erasmus et al (1994), who found in the Dorper breed a mean of 1.43 CL.
No season or breed group effect (p> 0.05) was found on fertility (84.85%) and prolificacy (1.36 lambs). The fertility rate is slightly higher than that reported by van Niekerk (1998), Schoeman et al (2000) and Zishiri et al (2013), who obtained fertility rates between 80 and 82% in Dorper and their crosses. In addition, it is slightly below the 90% reported by Cloete et al (2000).
In relation to prolificacy, the mean here obtained is higher than that reported by van Niekerk (1998), Schoeman et al (2000) and Zishiri et al (2013) who found means of 1.23 to 1.28 lambs at lambing. However, it is slightly below the mean reported by Elias et al (1985), Schoeman and Burger (1992), Erasmus et al (1994) and Gavojdian et al (2013), who reported 1.40 and 1.49 lambs per ewe lambing. The fertility rate and the prolificacy found here indicate that the Dorper and Dorper x Pelibuey sheep are adapted to the tropical conditions of the region. However, it must be considered that the management conditions of the farm can influence animal behavior.
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Received 21 August 2019; Accepted 21 August 2019; Published 1 September 2019