Conception rate of Holstein and Japanese Black cattle following embryo transfer in southwestern Japan

1 INTRODUCTION

Heat stress in summer is a major contributing factor in lowering reproductive performance among lactating dairy cattle. Heat stress inflicts heavy economic losses worldwide (Wolfenson, Roth, & Meidan, 2000). It has been reported that the conception rate to artificial insemination (AI) of lactating dairy cattle decreased during summer in Japan (Nabenishi et al., 2011). Several causes of reduced reproductive performance during heat stress have been reported, including lowered bovine oocyte competence (Al-Katanani, Paula-Lopes, & Hansen, 2002), reduced dry matter intake (De Rensis & Scaramuzzi, 2003), and early embryonic death (Ealy, Drost, & Hansen, 1993).

Putney, Drost, and Thatcher (1988) have indicated that bovine embryos are sensitive to the harmful effects of a high ambient temperature during the initial stages of embryonic development and blastocyst formation. This suggests that during heat stress, the conception rate can be increased by embryo transfer (ET). It is assumed that the embryos transferred into recipients at 7–8 days after estrus have already passed through the most thermosensitive periods of development (Hansen, 2007). However, this hypothesis has not been proven in Japan under field conditions.

On the other hand, Nabenishi and Yamazaki (2017) have reported a cyclical change in conception rate to AI of Japanese Black cattle, with high levels in summer and low levels in winter. This pattern contrasts with that of Holstein dairy cattle. Since the reproductive performances of both breeds have different potentials, the conception rate to ET of Japanese Black cattle is expected to be different from that of Holstein dairy cattle.

The ambient temperature and relative humidity are important environmental factors. The temperature–humidity index (THI) is used to assess the effects of environmental factors on conception rate to AI of dairy (Al-Katanani, Webb, & Hansen, 1999; García-Ispierto et al., 2007; Huang et al., 2008; Nabenishi et al., 2011) and beef cattle (Nabenishi & Yamazaki, 2017). Therefore, THI may be an effective index of the reproductive performance of cattle. Therefore, this study aimed to quantify and compare conception rate to ET of Holstein and Japanese Black cattle in southwestern Japan. We also investigated whether THI and conception rate to ET of both cattle are correlated.

2 MATERIAL AND METHODS

A 10-year retrospective epidemiological survey was conducted. The trial was conducted from May 2000 to April 2009. Embryos were obtained from the Miyazaki Livestock Research Institute. All the embryos were collected non-surgically via the uterine cervical canal with a balloon catheter at 7 days after estrus using superovulation treatment in Japanese Black donor cattle. Compact morula- to expanded blastocyst-stage transferable embryos (IETS standards; Codes 1–2) were preserved using a modified one-step dilution method using 1.4 mol/L glycerol at a cooling rate of 0.5°C/min to −32°C (Dochi et al., 1998). Embryos were thawed by placing the straws in a 30°C water bath and kept for 5 min. Thawing and transfer of embryos were performed at the client's farm in southwestern Japan (located between latitude 31°21′ and 32°50′ North and between longitude 130°42′ and 131°53′ East) by an ET technician. ET was conducted by 57 technicians during the experimental period. Embryos were transferred 7–8 days after natural estrus. The ET recipients comprised healthy multiparous Holstein and Japanese Black cattle. In total, 1,148 ET procedures were performed. The recipient numbers for Holstein and Japanese Black cattle were 621 and 527, respectively. Pregnancy was diagnosed by rectal palpation at 50–60 days following ET.

The daily mean ambient temperature and relative humidity data during the corresponding period were obtained from the local meteorological observatory, which were made available by the Japan Meteorological Agency (Miyazaki Local Meteorological Observatory). THI was calculated using the formula reported by Nabenishi et al. (2011) and is described below.

T: temperature (°C); RH: relative humidity (%)

The daily mean THI values on the day of ET were classified into five groups as follows: ≤45, 46–55, 56–65, 66–75, and >75 to assess conception rate to ET of Holstein and Japanese Black cattle. The daily mean THI throughout the study period was 62.2 (range, 39.0–80.7), all THI were evenly classified into five groups in this study. All animal treatments and procedures were approved by the Animal Care and Use Committee at Miyazaki Livestock Research Institute.

3 RESULTS

Monthly mean temperature, humidity, and THI throughout the study period are summarized in Table 1. Monthly mean THI was high from July to August, rapidly dropped from October to December, was the lowest in January, and then gradually elevated toward July. Daily mean (± SD) temperature and relative humidity for the 10-year study period were 17.0 ± 7.6°C (range, 1.0°C−29.8°C) and 73.0% ± 10.6% (range, 41.0%–96.0%), respectively.

During the study period, the conception rates of Holstein and Japanese Black cattle did not show significant differences (45.4% and 42.3%, respectively). Conception rates to ET of Holstein and Japanese Black cattle for each month are summarized in Table 2. In Holstein cattle, conception rates in August, September, and October were lower than in the other months. Conception rates in August–October were significantly lower (p < .05) than in April. In particular, th econception rate in October was the lowest. In Japanese Black cattle, conception rates in December and January were lower than in the other months. The conception rate in December was significantly lower (p < .05) than in February, March, May, June, August, and September. The conception rate of Japanese Black cattle in December was significantly lower (p < .05) than of Holsten cattle.

Figure 1 shows the relationship between the conception rates of both cattle and the daily mean THI on the day of ET. The conception rate of Japanese Black cattle declined as THI decreased, exhibiting significantly lower levels in the ≤45 THI class than in any other THI class (p < .05). By contrast, in Holstein cattle, there was no correlation observed between conception rate and THI on the day of ET.

4 DISCUSSION

In this study, no significant difference was observed in conception rate to ET between Holstein and Japanese Black cattle under field conditions in Japan. However, a different trend of conception rates to ET in Holstein and Japanese Black cattle was reported. In Holstein cattle, conception rates in August, September, and October were lower than in the other months. A previous study has reported that conception rate to AI of lactating dairy cattle decreased during summer in Japan (Nabenishi et al., 2011). Although conception rate to ET was slightly higher than after AI (42.3% vs. 29.5%), it was probably still compromised by heat stress. ET can be used to bypass the harmful effects of heat stress on oocyte quality that limit embryonic development (Al-Katanani et al., 2002; Zeron et al., 2001). However, these results indicate that timed ET improved conception rate under heat stress conditions when fresh embryos were transferred (Al-Katanani et al., 2002). In this study, frozen-thawed embryos collected from superovulated donors were used for ET. It is possible that the type of embryo played an important role in reduced conception rates during summer. Furthermore, heat stress can also affect endometrial prostaglandin secretion, leading to embryo loss (Putney, Mullins, Thatcher, Drost, & Gross, 1989). On the other hand, reduction in conception rates in Japanese Black cattle in summer was not observed. Because beef cattle are considered to be thermotolerant (Sakatani, Takahashi, & Takenouchi, 2016), it is suggested that the conception rate of Japanese Black cattle in summer is not affected by heat stress.

During autumn, when ambient temperatures decrease, conception rate to AI in dairy cattle remains lower than in winter (Hansen, 1997). This may be explained by the susceptibility of the ovarian follicles to heat stress (Badinga, Thatche, Diaz, Drost, & Wolfenson, 1993; Wolfenson et al., 1995) and a period of 40–50 days is required for the small antral follicles to develop into large dominant follicles (Lussier, Matton, & Dufour, 1987). Thus, exposure to summer heat stress during the early stages of follicular development may impair the later follicular function in autumn (Roth, Meidan, Shaham-Albalancy, Braw-Tal, & Wolfenson, 2001). Interestingly, in this study, conception rate to ET of Holstein cattle in October was the lowest although THI was decreased and the cattle were no longer exposed to heat stress. In addition, no relationship was observed between conception rate and THI on the day of ET. These findings indicate that there may be a delayed effect of heat stress on the transferred embryos. The mechanism underlying the delayed effect of heat stress, which induces a reduction in conception rate to ET of Holstein cattle, remains unclear. This may be different in the case of AI. Low conception rate to ET in autumn could be associated with a delayed effect of heat stress on corpus luteum function in cattle that were previously exposed to heat stress during summer. Some studies (Wilson et al., 1998) have reported that luteolysis was effected under heat stress. In the conception cycle, the effect of progesterone is probably associated with the need for a synchronous development of the embryo, and a delayed or advanced development of the corpus luteum will lead to higher rates of implantation failure (Lamming & Royal, 2001). In fact, during the conception cycle, low progesterone concentrations can lead to the failure of implantation (Lamming & Royal, 2001; Mann & Lamming, 1999). It has been suggested that the delayed effect of heat stress on follicular steroidogenesis (Roth et al., 2001) is associated with the development of the corpus luteum.

In Japanese Black cattle, conception rates in December and January were lower than in the other months. In addition, the conception rates of Japanese Black cattle declined as THI on the day of ET decreased. This pattern contrasts with that of Holstein dairy cattle; however, this is consistent with the findings of Nabenishi and Yamazaki (2017), who also reported a reduced conception rate to AI during winter compared with summer in Japanese Black cattle. They suggested that cold environment has a chronic effect on conception rate to AI. At the beef farms included in this study, no particular measures have been implemented to increase the feeding amount during the cold season. Accordingly, there might have been a chronic energy insufficiency during the cold season. It is possible that negative energy balance had a carryover effect on the quality of the developing oocytes during the cold season before AI (Nabenishi & Yamazaki, 2017). However, the results of this study suggest that even after an oocyte is successfully ovulated and fertilized, pregnancy is still not guaranteed. A certain aspect of protein metabolism may further compromise the successful embryo development (Rhoads, Rhoads, Gilbert, Toole, & Butler, 2006). A relative shortage of energy to synthesize the bacterial proteins will result in an accumulation of excessive ammonia in the rumen (Sinclair, Sinclair, & Robinson, 2000). This is absorbed through the ruminal wall and is converted into urea in the liver, resulting in high plasma urea nitrogen. In addition, negative energy balance may also exacerbate the detrimental effects of urea in the reproductive tract (Butler, 2005). Therefore, early embryonic mortality can result from the potential toxicity of the direct by-products of protein catabolism (ammonia and urea) for the embryo (Leroy, Opsomer, Van Soom, Goovaerts, & Bols, 2008). The effects of a negative energy status can also result in an inadequate corpus luteum function leading to suboptimal progesterone concentrations (Leroy, Van Soom, Opsomer, Goovaerts, & Bols, 2008). If the negative energy status does indeed result in low plasma progesterone, pregnancy would not be maintained regardless of the competency of the embryo. Accordingly, in this study, it is suggested that these factors in the cold season may have reduced the potential success of ET compared with that in the other period. In contrast, the decrease in conception rate of Holstein cattle in winter was not observed. In this study, the cattle in the dairy farms were fed uninterruptedly; thus, there was no energy insufficiency situation even in winter. In addition, the Holstein cattle have originated from Europe, and it is a cold-resistant breed. These factors might explain why conception rate to ET of Holstein cattle is not affected by the cold season.

In conclusion, no significant difference was observed in conception rate to ET between Holstein and Japanese Black cattle in Japan. Nevertheless, we observed different trends of conception rate to ET in Holstein and Japanese Black cattle. In Holstein cattle, conception rates in August–October were lower than in the other months, in particular, the conception rate in October was the lowest. By contrast, the conception rate of Japanese Black cattle in December–January was lower than in the other months. The conception rate of Japanese Black cattle declined as THI decreased. In Holstein cattle, there was no correlation observed between conception rate and THI on the day of ET. These observations suggest the importance of an appropriate management that considers seasonal reactivity in each breed. Good management includes measures against hot conditions during summer in Holstein cattle, and measures against cold conditions during winter in Japanese Black cattle.