Effect of body condition score on ovary score and pregnancy rate in timed artificial insemination programs in Nellore femalesAbstract
The objective was to evaluate the effect of body condition score (BCS) at the beginning of a timed artificial insemination (TAI) protocol on ovary score (OS) and pregnancy per TAI (P/TAI) in Nellore females. A total of 1,790 non-pregnant females classified according to parity (586 heifers, 421 primiparous cows, and 783 multiparous cows) were assigned to a TAI protocol. Evaluations of BCS [1 = very thin to 9 = very fat] and OS [1 = ovary with small follicles (<5 mm), 2 = ovary with large follicles (>7 mm) and 3 = ovary with corpus luteum] were performed. The TAI protocol was based on estrogen and progesterone, and pregnancy was diagnosed 30 days after TAI. BCS, OS and P/ TAI were affected by animal category. No effect of BCS on P/TAI was detected in multiparous cows, whereas a tendency was observed in primiparous cows a tendency and a significant effect was detected in heifers. Primiparous cows and heifers with better BCS had greater P/TAI. An effect of BCS was detected in all categories evaluated, and females with higher BCS also had higher OS. In conclusion, BCS at the beginning of the TAI program influences the OS in all categories analyzed and the P/TAI in primiparous cows and heifers.
Keywords:
beef cattle; heifers; multiparous; primiparous; reproductive efficiency
Resumo
Objetivou-se com este estudo avaliar o efeito do escore de condição corporal (ECC), no início do protocolo de inseminação artificial em tempo fixo (IATF), no escore de ovário (EO) e na taxa de prenhez por IATF (P/IATF) em fêmeas Nelore. Um total de 1.790 fêmeas, classificadas de acordo com a paridade (n= 586 novilhas, 421 primíparas e 783 multíparas), foram submetidas ao protocolo de IATF. Foram realizadas avaliações de ECC [1- muito magra a 9- muito gorda] e EO [1- ovário com folículos pequenos (<5 mm); 2- ovário com folículos grandes (>7 mm); 3- ovário com corpo lúteo]. O protocolo de IATF foi a base de estrógeno e progesterona e o diagnóstico de prenhez foi realizado 30 dias após a IATF. A categoria animal influenciou o ECC, EO e P/IATF. Não houve efeito do ECC na P/IATF das vacas multíparas, mas nas vacas primíparas houve uma tendência e um efeito nas novilhas foi detectado. As vacas primíparas e novilhas com melhores ECC apresentaram melhor P/IATF. Foi detectado efeito do ECC no EO em todas as categorias avaliadas e as fêmeas com melhores ECC também tiveram melhores EO. Em conclusão, o ECC no início do protocolo de IATF influencia o EO em todas as categorias avaliadas e a P/IATF nas primíparas e novilhas.
Palavras-chave:
eficiência reprodutiva; gado de corte; multíparas; novilhas; primíparas
1. Introduction
In cow–calf beef systems, production efficiency is related to reproductive performance. This, in turn, is associated with the profitability of the system. Regardless of animal category, the main objective is to produce one calf per cow each year (1). Nutritional management is the major factor that interferes with reproductive function (2), and body condition score (BCS) evaluation (1–9 scale) is the easiest and most efficient tool for assessing cattle nutritional status (3). The BCS at calving is the main factor influencing the success of post-partum re-conception (2).
Furthermore, nutrition is responsible for maintaining metabolic homeostasis (4), and insulin-like growth factor 1 (IGF-1) and other hormones synthezised by hepatic, adipose, and gut tissues are used as nutritional mediators of reproductive function (2). Ginther (5) reported that the ovarian follicles growth and development before ovulation are completely dependent on blood-derived IGF-1. Leptin secreted by adipose tissue is primarily responsible for the onset of puberty in heifers (6), and a recent study suggested that leptin can act at both the brain and pituitary levels, regulating gonadotropin-releasing hormone (GnRH) receptors on gonadotrope cells in the anterior pituitary gland (7). Insulin and glucose are also metabolic hormones that can affect reproduction in female beef cattle and are modulated by nutrition (2).
Studies have shown that beef cows with a BCS of ≥5.0 at the beginning of the breeding season have increased pregnancy rates (8–10). In addition, the BCS also affects ovarian function, and as it is known cows with a greater BCS have larger dominant follicle size, greater corpus luteum (CL) area, higher estrus rate and ovulation rate (11, 12).
This experiment was conducted to evaluate the impact of the BCS at the beginning of the timed artificial insemination (TAI) protocol on pregnancy rate and ovary score. It was hypothezised that cows with a BCS of ≥5.0 would present better ovary scores and higher pregnancy rates.
2. Material and methods
The experiment was conducted on 10 commercial farms, located in the states of Minas Gerais, Mato Grosso do Sul e Rio de Janeiro, Brazil, and cows were managed on extensive pastures systems, in tropical conditions with ad libitum access to water and a complete mineral-vitamin mix. The study was approved by the Ethics Committee on the Use of Animals of the Federal University of Uberlandia as a research project related to zootechnical management (No. A006/21).
2.1 Animals, sampling, and management
A total of 1,790 Nellore females (Bos indicus), born between 2005 and 2017 and classified according to parity (586 heifers, 421 primiparous lactating cows, and 783 multiparous lactating cows), were included in this experiment.
The breeding season was carried out during the rainy season, between December and March. Data collected from December 2016 to February 2021 were evaluated. All cows were assigned to a TAI protocol. The cows were inseminated with semen from Angus, Nellore, Charolais, and Brangus bulls. The TAI program was followed by natural service with Nellore bulls.
The ovulation synchronization protocol for TAI started 30 days after parturition of the last calf in each batch of cows. On day 0 (d 0), the cows received a 2.0mg intramuscular (i.m.) injection of estradiol benzoate and received an intravaginal progesterone-releasing device containing 1.2 g of progesterone. On d 8, the intravaginal progesterone-releasing device was removed and each cow received i.m. injections of 1.5 mg of equine chorionic gonadotropin, 2.0 mg of estradiol cypionate and 1.0 mg of prostaglandin F2α. Forty-eight hours later, the cows were inseminated on d 10.
Cow BCS was evaluated by a single technician at the beginning of the protocol (d 0) using a 1-to-9 scale (1 = very thin, 9 = very fat). The ovary score was assessed on d 0 by transrectal ultrasonography using a 4.0 MHz - 7.0-MHz transducer (DP-10 Vet; Mindray, Shenzhen, China), and females were classified according to an ovary score ranging from 1 to 3 (1 = ovary with small follicles (<5 mm), 2 = ovary with large follicles (>7 mm), and 3 = ovary with CL; adapted from Wiltbank et al. (13)).
Pregnancy status was evaluated 30 days after TAI by transrectal ultrasonography. Pregnancy per TAI (P/TAI) was calculated by dividing the number of cows that became pregnant by the number of cows that received TAI and multiplying the result by 100.
2.2 Statistical analysis
Binomial data were analyzed by logistic regression and continuous data were analyzed by analysis of variance both in the Minitab software. The model included the effects of BCS, animal category, ovary score as well as their possible interactions . Statistical significance was established at P ≤ 0.05 and a statistical tendency was defined as 0.05 < P ≤ 0.10.
3. Results
Animal category significantly influenced BCS, ovary score and P/TAI (P ≤ 0.05; Table 1). The results obtained in the present study indicate that heifers and primiparous cows had lower P/ TAI than multiparous cows.
An effect of BCS (P < 0.05) on ovary score was detected in all evaluated categories. Females with a BCS of ≥6.50 had higher ovary scores (Table 2). This finding is consistent with the results reported by Vedovatto et al. (15), who found that cows with a greater BCS had better development of ovarian structures.
Effect of BCS within each animal category at the beginning of the TAI protocol on ovary score.
No effect of BCS on P/TAI was detected in multiparous cows (P > 0.05; Table 3). Primiparous cows and heifers with a better BCS had higher P/TAI, confirming the results reported by Vedovatto et al. (15). A similar pattern was also observed in other studies, which reported a 90% correlation between the BCS at the beginning of the TAI protocol (d 0) and the conception rate in cows subjected to hormonal protocols, as well as higher conception rates in TAI programs among females with a better BCS (16-18).
4. Discussion
A possible explanation for the low P/TAI observed in the heifers in the present study is due to the lower BCS and ovary score reported in this category, as also observed by Nishimura et al. (12). Heifers with low ovary scores have small follicles and, consequently, small preovulatory follicles (12). As it known the POF vascularization and size are correlated with CL vascularization and size (19). In addition, preovulatory follicle size is closely related to the amount of progesterone secreted by the CL during early pregnancy (20, 21).
Uterine function and embryonic development are dependent on progesterone concentration and can be impaired when progesterone concentrations are inadequate (22). Consequently, embryonic mortality may occur and may be partly attributable to low circulating progesterone concentrations after ovulation (19). These embryonic losses impair pregnancy rate and may explain the low P/TAI observed in this study.
In agreement with the results obtained by Grillo et al. (14) primiparous females are expected to perform worse than multiparous females because of the increased nutritional requirements associated with the stress of calving and the combined effects of growth and first lactation (14). According to Ciccioli et al. (23) , insufficient nutrient intake during the pre- and post-partum periods has significant effects on post-partum reproductive performance in primiparous cows. Moreover, inhibition of luteinizing hormone (LH) pulses can also be caused by the presence of a suckling calf in post-partum cows (12), and the negative effect of suckling on the estrous cycle of beef cows accentuates post-partum anestrus, especially in females with nutritional deficiencies, negatively affecting pregnancy rates in TAI programs (13).
Cows with a greater BCS have larger dominant follicle size, greater CL area, and higher ovulation rates (12) because they may have higher circulating concentrations of IGF-1, leptin, insulin, thyroxine, glucose, and non-esterified fatty acids (2, 23). These physiological changes may affect follicular development (12).
Low serum glucose concentrations can also impair GnRH secretion because this condition is detected by the hypothalamus (24). Insulin is the main regulator of glucose homeostasis (25) and serves as a metabolic signal that influences LH secretion by the pituitary gland. It also increases follicular responsiveness to gonadotropins (26) and, in the ovary, can stimulate cell proliferation and steroidogenesis (27). Therefore, poor nutritional status can impair reproduction in female beef cattle through several mechanisms, as confirmed by the results of this study.
The absence of a BCS effect on P/TAI in multiparous cows may be related to the homogeneity of the BCS within this category. In the present study, 73.95% (579/783) of multiparous cows had a BCS between 4.50 and 6.00.
5. Conclusion
These results demonstrate the importance of appropriate nutritional management to maintain females in adequate body condition at the beginning of the TAI program. In conclusion, BCS at the beginning of the TAI program influenced the ovary score in all categories analyzed and P/TAI in primiparous cows and heifers.
Data availability statement
The complete dataset supporting the results of this study is available to reviewers upon request.
Generative AI use statement
Acknowledgements
The authors thank the participating beef farms and their employees for allowing the use of their animals in this study. This study was financed in part by the Coordination for the Improvement of Higher Education Personnel (CAPES), Brazil—Finance Code 001. Author R. M. Santos received a scholarship from the National Council for Scientific and Technological Development (CNPq) (Research Productivity Grant, PQ; Process No. 306873/2022-2).
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