Abstract
Abstract. 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 (P < 0.05) in summer (June to August) than in spring (March to May) and winter (December to February). In heifers, however, there were no differences in the pregnancy rates among the seasons. Our findings indicate that in RB Japanese Black beef cattle, the parity of the recipients does not have an effect on the pregnancy rates following the transfer of fresh and frozen embryos. However, heat stress may affect reproductive performance in RB Japanese Black cows.
Highlights
Repeat breeding is considered to be one of the most important reproductive disorders in cattle
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 %, 18/57 vs. 31.9 %, 86/270)
Our data showed that approximately 30 % of RB beef cattle can become pregnant by embryo transfer (ET), even if they did not conceive following artificial inseminations (AIs)
Summary
Repeat breeding is considered to be one of the most important reproductive disorders in cattle. 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. 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. 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
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