Season and parity number influence the conception rate of zebu breed cows in South-eastern Mexico

J C Segura Correa, J G Magaña Monforte, J R Ake Lopez and V M Segura Correa¹

Campus de ciencias Biológicas y Agropecuarias, Universidad Autónoma de Yucatán, Mérida, Yucatán, México
jose.segura52@hotmail.com
¹ Instituto Nacional de Investigaciones Forestales Agrícolas y Pecuarias, Tizimín Yucatán. México

Abstract

The aim of this study was to determine the effect of season of the year and parity number on conception rate of beef cows from three zebu breeds under the tropical conditions of the southeastern of Mexico. Information of 2064 calving to conception intervals (CCI) from 423 cows calving from 2006 to 2013 was used. The statistical model that described the reproductive status of the cow (conceived=1 or not conceived=0) at 90, 120, 150 and 180 days postpartum, included the effects of cow breed (Brahman, Guzerat and Nellore); year of calving (2006-2013), season of calving (dry, rainy and windy) and parity number (1 to 7). Data were analyzed to get the conception rate and odd ratios using binomial logistic regression.

Overall conception rates at 90, 120, 150 and 180 days postpartum were 23.8, 37.0, 44.6 and 56.5%, respectively. Breed, parity number, season and year of calving had significant effects on conception rate. The best conception rate was for the Nellore cows followed by the Brahman and Guzerat. At 90 days postpartum, the conception rates were 48.09, 22.11 and 10.64%, whereas at 180 days the rates were 75.32, 58.21 and 37.67%, for those breeds in the same order. The worst conception rates belong to the rainy season and first parity cows. In conclusion, the overall conception rate, here obtained, was low (44.6% at 120 days postpartum) which suggest a possibility of improvement of this trait through better management that allows mating of cows in the season of better pasture availability. Breed, parity number and season were important factors that affected the conception rate. Under the conditions of this study, the Nellore cows had the highest conception rate.

Key words: Bos indicus, cattle, reproductive performance, tropics

Introduction

Cattle production in Mexico is practiced in ranches under a wide range of environmental conditions and different managements, but with a common task, to overcome the postpartum anestrus (period of time after calving when the cow is not in estrus). Cows should present normal and fertile estrous cycle after calving within a limited time to conceive each year and to contribute to the profitability of the ranch. To maintain a calving each year, cows must have a calving to conception interval equal to or less than 85 days.

The resumption of the post-partum ovarian cycle in beef cow is influenced by several factors, among them breed, parity number, nutritional status, weight changes during the postpartum period and suckling (Diskin and Kenny 2014). The immediate resumption of postpartum ovarian activity facilitates the possibilities that a cow presents a short calving to conception interval (CCI), a feature of economic importance, which may affect the rate of calvings and calf crop per cow per year. In general, it is mentioned that zebu cows restart cyclical activity between 40 and 60 days after calving (Abeygunawardena and Dematawewa 2004; Venkantanaidu et al 2007).

There are some studies, with variable results, on the conception rate of zebu cattle to a given time (120 days postpartum), which have determined the effect of cow breed and season of calving (Delgado et al 2004; Aban et al 2008). Some authors report effect of breed and/or season and others do not. Then it is important the study breed and environmental factors that influence reproductive traits to take decision according to the specific conditions of each ranch or region. The effect of the parity number has been more consistent, where first parity cows have commonly longer CCI and lowest gestation rates than multiparous cows (Segura et al 1989; Aban et al 2008).

Taking into consideration that each day a cow remains non-pregnant, it has an extra cost, and it affects the profitability of the ranch; the objective of this study was to determine the effect of season of the year and parity number on conception rate of three zebu breeds, under the tropical conditions of southeastern Mexico.

Materials and methods

The study was conducted in a commercial ranch located in the cattle region of Yucatan, Mexico. The region had a warm and rainy tropical climate with rains in summer, with average annual temperature of 26.5 C, relative humidity of 78% and annual rainfall of 1,100 mm (INEGI, 2004). Management of cattle was the common in the region. Cows were kept under extensive conditions with free mating throughout the year, although few cows were artificial inseminated and data were not included in the statistical analysis. The cows belong to the Brahman, Nellore, Guzerat, Brown Swiss breeds and crossbred. The feeding of livestock was based on the consumption of Panicum maximum and Brachiaria brizantha grass using a rotational system, in evening hours (17:6 to 00:00 h).

During the first month postpartum, calves and cows remained together in the corral (7:00 to 16:00 h); and from the second month to weaning, at 8 months approximately, calves went out to pasture with their dams. During daylight, the animals were kept in the corral where they were provided water and mineral salts. In the dry season (scarce of pasture) animals were provided with bales of forage. The calves were vaccinated against clostridia, bovine paralytic rabies and leptospira. External parasites were controlled with immersion baths when animals were seen with ticks. Internal parasites were controlled by deworming every six months. The herd was free of brucellosis.

Data of calving to conception interval (CCI; n= 2064; 423 cows) were obtained from 2006 to 2013 by subtracting from the calving interval, the average length of gestation (283 days). The CCI data, thus obtained were classified into cows that conceived at 90, 120, 150 or 180 days postpartum. Only the information of the Zebu cows (Brahman, Nellore and Guzerat) were used, deleting those for Swiss and crossbred cows, due to the limited number of observations. To determine the effect of season of calving on conception rate, the data by month of calving were grouped in three seasons, according to the climatic conditions: dry season (February to May), the rainy season (June to September) and windy season, characterized by fresh winds with sporadic rains (October to January). The data of the cows with 7 or more parities were grouped in a single category.

The statistical model that described the reproductive status of the cow (conceived = 1 or not conceived = 0 at 90, 120, 150 or 180 days postpartum) included the fixed effects of cow breed (Brahman, Guzerat and Nellore); year of calving (2006-2013), season of calving (dry, rainy and windy) and parity number (1, 2, …7). Reproductive status of cow with a binomial distribution, were analyzed by logistic regression procedures (SAS 2010).

Results

The results of the conception rates by breed, number of calving and season are shown in Table 1 and Figures 1, 2 and 3, respectively. The overall conception rates were 23.8, 37.0, 44.6 and 56.5% at 90, 120, 150 and 180 days postpartum. Differences were found (P<0.05) in conception rate by breed, parity number, season and year of calving. With regard to the breed, the best conception rate was for the Nellore cows, followed by the Brahman and Guzerat. At 90 days postpartum, the rates were 48.09, 22.11 and 10.64%, while at 180 days the rates were 75.32, 58.21 and 37.67% for the breeds mentioned above, in that same order.

Figure 1. Effect of breed on the conception rate (%) at different intervals post-partum in Zebu cows

Figure 2 shows how the conception rate increased with parity number until the fifth calving; to fall in the sixth and increase again in cows with 7 or more calvings. Likewise, the best season corresponded to the dry season (Figure 3).

Figure 2. Effect of parity number on conception rate at different intervals post-partum in Zebu cows.

Figure 3. Effect of season of calving on conception rate at different intervals post-partum in Zebu cows.

In terms of odd ratios, to 120 days postpartum, the results indicated that the chances of conception of a Brahman or Nellore cow were 2.52 and 7.02 times more, compared to Guzerat cows (Table 2). Also, the chances of conception were lower in first parity cows and calving in the rainy season.

Discussion

The discussion of the results focuses on conception rate at 120 days postpartum, because most of the literature on this topic and zebu cattle is evaluated at that time; thus is it, based on one calf per cow every 13 months. However, the discussion applies to the results obtained at 90, 150 and 180 days postpartum.

The overall conception rate (44.6%), here obtained, at 120 days postpartum, confirms the low reproductive level that characterized the production of beef cattle in the tropical regions, managed under extensive conditions (Sullivan and O'Rourke 1997; Lôbo 1998; Magaña and Segura 2001; Osorio and Segura 2002; Aban et al 2008; Celebrate and Nanda 2008; Ayres et al 2014). In addition, this result is poor when compared to the values of 60 and 79%, reported in Zebu and B. taurus x B. indicus crosses in herds managed under programed breeding season in Latin America (Plasse et al 1997; 2002). In Nigeria, Campbell et al (1996) obtained a conception rate of 36.9% in Zebu cattle; while, in double purpose cows, in the same region of this study, Osorio and Segura (2002), and Magaña-Monforte et al (2013) reported conception rates at 120 days postpartum of 20% and 30%, respectively. However, when conditions improve through the supplementation with grains, fertility could be improved up to 47% at 90 days postpartum, as a result of a positive energy balance of the cow (Aguilar-Perez et al 2009). Entwistle (1983) mentioned that it is feasible to achieve 70 to 75% of calves weaned per year in zebu herds in the tropics. In this regard, in the south of the United States, Vargas et al (1999) reported in Brahman, gestation rates of 63 and 90% in cows with one, and two or more parities, respectively. Both, the results of Entwistle (1983) and Vargas et al (1999), were based on programed breeding seasons of two and three months, prior to a strategy that consisted in the definition of the best time for breeding, to harmonize food resources with the requirements of the cattle, and to allow a better reproductive performance. However, this strategy for cattle, is not practiced in most regions of the tropics of Mexico. Breeding cattle, throughout the year, loses the advantage of harmonizing and adjusting food resources with the increased requirements of the cows, resulting in the decline in the annual calving rate, and productivity of the herd.

The overall conception rate to 120 days postpartum, in this work, is similar to that reported (40%) by Aban et al (2008) but lower in comparison to what has been reported (60 to 79%) by other authors (Rios et al 1996; Plasse et al 1997; 2002). Under poor nutritional conditions, the body condition of the cows is poor, because they have to feed their calf and sacrifice their energy requirements for reproduction, having as a result, low conception rate. It is expected a better reproductive performance when cows are kept in paddocks with sufficient quantity and quality of pastures to meet their requirements. It has been reported that nutrient intake restriction in early postpartum, causes a negative energy balance (Butler and Smith 1989; Aguilar-Perez et al 2009); the duration and magnitude of this depending on the amount of fat body reserves available at calving. The poor reproductive performance of cows suffering a severe negative energy balance, cause long period of postpartum anoestrus (Jolly et al 1994; Aguilar-Perez et al 2009). In addition, the poor reproductive performance of zebu cows could be explained due to the suckling effect of calves. Suckling activates a complex system of neural responses and hormonal feedback loops that reduces the frequency and amplitude of LH pulses that alter the release of GnRH; which in turn affects the ovarian function (Williams 1990; Matiko et al 2008; Diskin and Kenny 2014). The effect of time of suckling on conception rate has been reported in the literature (Moore 1984). Bastidas et al (1984) found that suckling twice a day, from 30 days postpartum on, improved the gestation rate of Zebu cattle. Similarly, Randel (1981) detected a difference of 72 days in CCI, in favor of cows with controlled suckling. Given that the three zebu breeds, in this study, were under the same management conditions, indicates that Guzerat cows might be more sensitive to the effects of suckling compared with Brahman and Nellore cows. This type of stress experienced during lactation, may be due to a more temperamental maternal behavior and more protective instinct to calves during suckling (Williams 1990). The odds of a cow to conceive at 120 days were 2.5 and 7 times higher for Brahman and Nellore compared with Guzerat. Differences in conception rates among breed groups have been notified by Aban et al (2008) in the same region of this study.

The effect of season, here found, coincides with the results of Aban et al (2008), who reported that the season of calving influenced (P<0.05) the pregnancy rate at 120 days postpartum in Zebu and crossbred cows. Also, agree with the results of Magaña and Segura (2001) and Magaña et al (2002), in the same region of this study, who observed a better reproductive performance in cows that calved in the windy season compared to the other two seasons. Difference between seasons of the year can be explained due to the lack of pasture that occurs in the dry season, affecting the reproductive performance of cattle. The highest conception rate of the cows calving in the dry season, indicates that they conceived in the rainy season, when their body condition was poor, caused by the seasonal variation in grass availability, due to the rain distribution (Aban et al 2008). The odds of a cow calving during the dry season were higher in comparison with those ones that calved in the rainy season (Table 1).

The lowest conception rate (38.18%) of the first parity cows could be attributed to the fact that these increased their nutritional requirements to sustain their vital functions of maintenance, growth and lactation in comparison to the older cows, especially under extensive management conditions of the tropics (Plasse et al 1997; Vargas et al 1999). In addition, the highest conception rate of cows with 7 or more parities could be explained by the fact that they have survived to culling in previous parities. The odds of conception for a cow with 4 or 5 parities was 3.6 and 4.2 times that for first parity females. A practice to implement to improve fertility, chiefly of first parity cows, is energy supplementation, accompanied with temporal weaning (Jolly et al 1996; Huanca et al 2005).

The effect of year on conception rate (results not shown) could be attributed to year differences that, indirectly affect the quantity and quality of pasture in the farm, and to changes in management. The reason of highest conception in last years, could be attributed to the fact that latter years only included cows that have calved, when data collection was ended. It is common to find out, in most studies, that last years have better calving to conception and calving intervals (Segura et al 1989; Magaña et al 2002).

Conclusions

• The low conception rate, found in this study, suggests the possibility of improvement of this trait, using appropriate management to program parities when pasture is available. Breed, parity number and season were important factors that influenced conception rate. Under the conditions of this study, Nellore cows had the best performance.

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Received 4 July 2017; Accepted 23 October 2017; Published 2 November 2017

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