Repeat-breeder (RB) cows are a major source of economic waste due to their
decreased fertility. Embryo transfer (ET) is an alternative tool to improve
the fertility of RB cows. The aims of the present study were to evaluate the
effects of recipient parity and the season on pregnancy rates following ET
in RB Japanese Black beef cattle. Embryos were transferred nonsurgically to
recipients, consisting of 155 heifers (< 2 years old) and 172 cows
(< 8 years old), which were defined as RB cattle. Of the recipients
that were presented for ET, 57 recipients received a fresh embryo and 270
recipients received a frozen embryo. There were no differences in the
pregnancy rates between cattle that received fresh embryos or frozen
embryos. The rates of recipients with pregnancy, abortion, stillbirth, and
normal calving were similar between heifers and cows. In cows, the pregnancy
rates were lower (
Repeat breeding is considered to be one of the most important reproductive disorders in cattle. Repeat-breeder (RB) cows are defined as cows with three or more consecutive artificial inseminations (AIs) without conception and no clinical signs of disease (Gustafsson and Emanuelson, 2002). Although several causes of repeat breeding have been described, e.g. oestrus detection errors, endocrine dysfunction, and infections (Kendall et al., 2009; Moss et al., 2002; Perez-Marin and Espana, 2007), the particular reason for repeat breeding often remains speculative. Hormonal treatment, such as gonadotropin-releasing hormone (GnRH), has been used to increase the rate of pregnancy for RB cows (Kharche and Srivastava, 2007), but success has been limited. It has been demonstrated that embryo transfer (ET) can increase the probability of pregnancy in dairy cows by minimizing the negative effects of high milk production and heat stress on both the quality of the oocyte and early development of the embryo (Vasconcelos et al., 2006). Moreover, the ET technique has been introduced as an efficient tool to improve fertility for RB dairy cows (Dochi et al., 2008; Son et al., 2007). These studies indicate that ET may be an effective technique to achieve satisfactory conception rates in RB beef cattle. However, limited information concerning the effects of ET on the pregnancy rates of RB animals is currently available for beef cattle because ET studies have been conducted mainly with dairy cows.
The aims of the present study were to evaluate the efficacy of ET in RB Japanese Black beef cattle and to investigate the effects of recipient parity and the season on pregnancy rates following ET.
The animals receiving embryos were Japanese Black beef cattle, consisting of 155 heifers (< 2 years old) and 172 cows with one or more deliveries (< 8 years old), which were defined as RB animals. The RB animals were defined as animals that failed to conceive after three or more AIs. All RB recipients were raised in the Cattle Breeding Development Institute and on farms (Kagoshima, Japan), housed in a stanchion barn within an adequately wide area, and fed according to Japanese beef cattle feeding standards for beef cattle. The RB heifers with AIs (mean frequency: 7; range: 3–16) and RB cows with AIs (mean frequency: 6; range: 3–14) were selected from the farmers' herd records during regular herd visits. Only cattle with no clinical signs of uterine disorders, i.e. endometritis, and no abnormalities of the ovaries, i.e. ovarian cysts, were enrolled.
All procedures were approved by the Animal Research Committee of the Yamaguchi University and Cattle Breeding Development Institute of Kagoshima Prefecture. The owners of RB females had given permission for ET.
Embryo collection was performed according to a method described by Isobe et
al. (2013). All embryos were recovered from Japanese Black donors
(2–15 years old), which were bred at the Cattle Breeding Development
Institute or farms (Kagoshima, Japan). Superstimulation was induced by
intramuscular administration of 18 AU follicle stimulating hormone (FSH;
Kyoritsu Seiyaku Corp., Tokyo, Japan) that was administered in a series of
decreasing doses over a 3-day period, beginning from day 10–14 of the
oestrous cycle. Each donor was given two doses (375
Embryo freezing and transfer were performed by the procedures used by Isobe
et al. (2013). The freezing medium consisted of 5 % (
Either a single fresh (57 embryos) or frozen–thawed embryo (270 embryos) was transferred nonsurgically into the uterine horn, ipsilateral to the ovary bearing the corpus luteum, on days 6–8 of the oestrous cycle without hormonal treatments. Three trained technicians performed all of the transfers. The assessment of pregnancy was performed on days 30–40 after ET by transrectal ultrasonography (5.0 MHz transducer, Honda HS-101V; Honda Electronics Co., Aichi, Japan).
Effects of parity on the rates of pregnancy, abortion, stillbirth, and normal calving after the transfer of embryos to repeat-breeder Japanese Black beef cattle.
Effects of the season on pregnancy rates after the transfer of
embryos to repeat-breeder Japanese Black beef cattle
The percentages of cows that experienced pregnancy, abortion, stillbirth, and
normal calving were analysed using a chi-square analysis with Yates'
correction. To investigate seasonal effects on the pregnancy rates after the
transfer of embryos, the results were compared among spring (March to May),
summer (June to August), autumn (September to November), and winter (December
to February). The mean minimal and maximal average air temperatures between
2009 and 2014 in the experimental location were 12.5 and 20.8
When the data of heifer and cow recipients that experienced pregnancy were
combined to assess any effect of embryo freezing, there were no differences
in the pregnancy rates between cattle receiving fresh embryos and
frozen–thawed embryos (31.6 %,
The rates of recipients with pregnancy, abortion, stillbirth, and normal
calving were similar between heifers and cows (Table 1). When evaluating the
effects of season on the pregnancy rates of recipients, the pregnancy rates
in cows were lower (
The maternal environment of most RB cattle has been suggested to be satisfactory for establishing and maintaining pregnancy following ET (Tanabe et al., 1985). Our data showed that approximately 30 % of RB beef cattle can become pregnant by ET, even if they did not conceive following AI. These results are in agreement with previous studies that ET is an alternative tool for the treatment of repeat breeding (Dochi et al., 2008; Son et al., 2007). Moreover, we found that parity did not affect the rates of pregnancy, abortion, stillbirth, and normal calving. In general, heifers are more appropriate recipients because they are less likely to be under nutritional stress or to have a history of compromised uterine or systemic health in the post-partum period (Broadbent et al., 1991). However, the pregnancy rates following the transfer of fresh and frozen embryos have been reported to be not affected by the parity of recipients (Hasler, 2001). Our findings also indicate that, in RB beef cattle, heifers are not expected to be more likely than cows to become pregnant after ET.
Oocytes and embryos at the early stages are extremely sensitive to heat stress (Ealy et al., 1993). The effect of heat stress has been suggested to be due in part to the intrauterine environment, in which there is a decrease in blood flow to the uterus and an increase in uterine temperature (Roman-Ponce et al., 1978). These changes according to heat stress may inhibit embryonic development and increase early embryonic loss (Rivera and Hansen, 2001). However, embryos at 3 days and older are thought to be more resistant to heat stress than oocytes and early stage embryos. Several studies observed that ET can be applied to dairy herds to improve fertility compared with AI, particularly during the hot season of the year (Ambrose et al., 1999; Baruselli et al., 2011; Chebel et al., 2008). Therefore, ET is a strategy that bypasses the negative effects of heat stress during the periods of follicular development and early embryonic development. In the present study, we observed that the seasonal effect on the pregnancy rates following ET was different between heifers and cows. In heifers, the season did not affect the pregnancy rates in the recipients. In contrast, a significant difference was observed in the pregnancy rates of cows by season, resulting in a decreased pregnancy rate of recipients in the hot season. The reason for these discrepancies in the seasonal effect on the heifers and cows is unclear. Sartori et al. (2002) reported that cows had greater increases in body temperature in response to increases in environmental temperature than heifers. It has been suggested that an increase in environmental temperature that causes pronounced heat stress may augment the detrimental effects of negative energy balance in cows (Rensis and Scaramuzzi, 2003). Impairment of early embryonic development has been shown to be associated with a negative energy balance and poor body condition (Kendrick et al., 1999). Although nutrient requirements vary throughout the year, even in beef cattle (Laurenz et al., 1991), the variation in nutrient requirements for cows seems to be more than for heifers. Therefore, it is likely that heat stress affects the reproductive performance of cows directly and indirectly; the indirect actions may be mediated through alterations in energy balance. However, to identify the seasonal effects on the RB recipients, the relationship between energy balance and parity in beef cattle must be compared in future studies.
In conclusion, the results of this study indicate that the parity of recipients does not affect pregnancy rates following the transfer of fresh and frozen embryos in RB beef cattle. The hot season affected the pregnancy rates of cows. However, we confirmed that ET is an alternate treatment for Japanese Black beef cattle that are repeat breeders.
All authors contributed to the work described in the paper and all take responsibility for it. T. Ono, T. Isobe, Y. Morita, L. T. K. Do, F. Tanihara, M. Taniguchi, and M. Takagi as co-authors made a significant contribution to the conception and design of experiments, as well as the analysis and interpretation of data. Moreover, T. Otoi as a corresponding author participated in drafting the article as well as reviewing/revising it for contents.
The authors thank the technical staff of the Cattle Breeding Development Institute, Kagoshima Prefecture, for ET. Edited by: S. Maak Reviewed by: two anonymous referees