Effect of mature size and body condition of Nelore females on the reproductive performance
Introduction
The reproductive traits in beef cattle are mostly important for economic purposes, since they determine the number of bred animals; affect the generation intervals and, consequently, the selection intensity. The scrotal circumference has been used to improve sexual precocity and reproduction performance of males [1]. In addition, this trait is a reasonable indirect indicator of female puberty as long as it is measured at a young age (400–440 days of age) [2]. Terakado et al. [3] showed that the scrotal circumference influence reproductive performance of cows and indicated that selection based on the largest scrotal circumference at 12 months of age should be result in higher heifer pregnancy rates and younger age at first calving. On the other hand, Grossi et al. [4] have reported that the selection for scrotal circumference should be inefficient to improve the reproductive performance of female cattle.
The female reproductive traits can also be used to improve the beef cattle reproductive efficiency. The age at first calving is the most common trait used to assess sexual precocity and it is an easily obtained. On the other hand, shorter gestation periods allow females to have longer recovery time and, consequently, increase their possibility of getting into heat in the subsequent mating season, thus generating better conception rates [5]. For calving interval, the reproductive efficiency could be reached with value of 365 days, i.e., one calf per year [6]. The number of days to calving allows identifying the most fertile animals in the herd, as well as earlier female breeding in the mating season and bulls producing sexually precocious progenies [7].
The weight, height and body condition of females are traits related to maintenance and production efficiency commonly evaluated in Zebu breeding programs. The body size and weight of cows are relevant because approximately half of the dietary energy expended in the process of beef production is used on the maintenance of cows [8]. In addition, the size of cows may affect the herd reproductive efficiency. Overall, calves born from larger females are heavier at weaning due, in part, to the dams' larger milk production, fact that makes these cows more nutritionally demanding [9,10]. Thus, limited food sources may lead the animal's body to use its reserves in order to meet the nutritional requirements for maintenance and lactation, mainly in primiparous cows. The phenomenon is stronger among these cows because of their higher nutritional needs since they are still growing; moreover, it prolongs the anestrus period after calving [11,12]. Therefore, the use of large-size females may be detrimental to extensive and food-restricted production systems.
The female body condition score, which is used to infer body composition and energy balance, may affect the weaning weight of progenies, as well as cows' ability to breed in the following mating seasons [13,14]. The breeding process may be compromised by the negative energy balance; thus, the body condition at calving is associated with the rate of cows getting into early heat in the following mating season [15]. According to Grecellé et al. [16], prolonged postpartum anestrus were observed in Nelore x Hereford cows showing low body condition scores at late pregnancy and early lactation, fact that led to late heat, increased calving interval and reduced pregnancy rate. Similarly, Santos et al. [13] found that low body condition scores may negatively affect the conception rate in cows. In contrast, Mulliniks et al. [17] reported that body condition score had no effect on the pregnancy rate of cows.
Few studies have genetically related the size and body condition score of cows to traits indicating precocity and reproductive efficiency in male and female beef cattle. Thus, the present study has estimated the parameters and correlations among weight, height and body condition score of females and reproductive traits (scrotal circumference, age at first calving, gestation length, days to calving and calving interval) in Nelore animals. In addition, it has also obtained direct and correlated responses aiming at determining whether changes in scrotal circumference and other reproductive traits may affect the size and body condition of cows.
Section snippets
Data and traits
Data from productive and reproductive records of 119,944 Nelore females and 139,283 Nelore males born between 1984 and 2016 on 234 farms located in the southeast and middle-west of Brazil and provided by Conexão Delta G breeding program were used. The following traits measured in females were analyzed: mature weight (MW), mature height (MH), body condition score (BCS), age at first calving (AFC), gestation length (GL), days to calving (DC) and calving interval (CI). In addition, scrotal
Results
The posterior distributions of heritability for MW, MH and BCS are shown in Fig. 2. The mean of heritability estimated were 0.46 ± 0.02 (MW), 0.35 ± 0.01 (MH) and 0.17 ± 0.02 (BCS), indicating that these traits should present different selection responses. The environmental variability estimated for BCS may be due to the extensive production system in which the cows are kept, which is influenced by climatic variations that affect the pasture growth. Thus, though the intermediate BCS was
Discussion
The heritability estimated for MW and MH (Fig. 2) indicate that these females' traits can be used as selection criteria, with a higher expected response for adult size indicator traits due to the importance of the direct additive genetic effect in its expression, when compared to the body condition. For MW and MH, the heritability estimates were in agreement with those obtained by Pedrosa et al. [21], Regatieri et al. [22], Boligon et al. [23] and Silveira et al. [24] for Nelore breed. In
Conclusions
Mature weight and height can be used as selection criterion to improve the size of cows. Selection based on breeding values for body condition score of cows should present slower responses. Negative impact of increase the size of Nelore females on their reproductive performance is expected to be small in tropical regions. The use of sires with high genetic breeding values for SC should result in slight reduction of BCS of cows, but no effective genetic response in female size is expected.
Acknowledgment
The authors would like to thank the Rio Grande do Sul Research Foundation (FAPERGS - Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul) for their financial support and Conexão Delta G for use of the dataset. A.A. Boligon was supported by a fellowship from CNPq (‘Conselho Nacional de Desenvolvimento Científico e Tecnológico’).
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