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Abstract. Although the number of dairy farms is decreasing, that of large farms is increasing in Japan. Milk production in Japanese dairy cows has increased from 62 kg/ year to 88 kg/ year over the last 2 decades. However, Japanese dairy cows are experiencing a sustained decline in reproductive performance, calving intervals, and days open; further, the number of inseminations required for conception have increased, and the conception rate has decreased. In order to improve fertility in high milk-producing dairy cows, it is necessary to evaluate their reproductive characteristics. In this study, the postpartum body condition score (BCS) was remarkably low, and the functional recovery of reproduction was consequently delayed. Moreover, the results indicate that the estrus duration varies among individual cows. However, it is possible to improve the conception rate by inseminating cows 8-12 h after the onset of estrus. Reproductive management systems suitable for the current dairy farming system with large herd sizes are required. Key words: Artificial insemination, Body condition score, Conception rate, Dairy cows, Reproduction (J. Reprod. Dev. 56: S61-S65, 2010) lthough milk production per dairy cow has dramatically increased over the last 2 decades, the reproductive performance of dairy cows has declined worldwide [1][2][3][4][5][6]. The cause of this low reproductive performance may vary considerably across countries and is probably multifactorial. The factors that contribute to the decrease in the reproductive performance of dairy cows are increase in milk production per cow, concrete flooring, confined housing, and increase in herd size.According to the latest livestock statistics reported by the Ministry of Agriculture, Forestry and Fisheries, in July 7, 2009 (http:// www.maff.go.jp/toukei/sokuhou/data/siyou_doukou09/ siyou_doukou09.pdf), the total number of dairy farms in Japan is 22,300. This number is decreasing; however, the number of farms with more than 100 cows (aged above 24 months) is increasing, and this number is 1,860 (8.3%). The number of dairy farms with more than 300 cows is 173. This trend in the increase in herd size in Japan is the same as that in other countries such as the USA [2].According to data presented by the Livestock Improvement Association of Japan (http://liaj.lin.gr.jp/japanese/kentei/ kentei.html), over the last 2 decades, milk production (305 d) in Japanese dairy cows has increased by approximately 62 kg/year in Hokkaido and 88 kg/year all over Japan, except in Hokkaido (Fig. 1). This trend in the increase in milk production in Japan is the same as that in other countries [2,4]. However, a sustained decline in reproductive performance has been observed in Japanese dairy herds (Table 1) The Hokkaido Artificial Insemination Technician Association Inc. conducts annual surveys and reports the conception rates at the first artificial insemination (AI) in dairy cattle in the Hokkaido district. The conception rate in dairy cows was found to decrease from 53.4% in 1989 to 41.2% ...

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ABSTRACT. The present study was conducted to examine the effects of culture systems and culture media on developmental competence and freezability of bovine embryos obtained by in vitro culture of in vitro matured and fertilized (IVM-IVF) oocytes. No significant difference was observed in the proportions of oocytes developed to blastocysts, the speed at which the oocytes reached the blastocyst stage and the number of cells, when the IVM-IVF oocytes were cultured in CR1aa with or without cumulus cells. Nevertheless, more of the IVM-IVF oocytes cultured either with or without cumulus cells in CR1aa were seen to reach the blastocyst stage much sooner than those cultured with cumulus cells in TCM199 (P<0.05). The proportion of embryos developed to the blastocyst stage by day 7 in CR1aa culture was significantly higher than embryos cultured in TCM199. Viability after frozen-thawed blastocysts were obtained in vitro, was seen in a significantly higher percentage of embryos cultured in TCM199 and developed to the hatched blastocysts than in those cultured in CR1aa (P<0.05). These results indicate that CR1aa was superior to TCM199 for the potential developmental of IVM-IVF oocytes to blastocysts during in vitro culture regardless of co-culture with or without cumulus cells. But the freezability of blastocysts developed in CR1aa was inferior to those developed in TCM199. KEY WORDS: culture medium, developmental competence, freezability, in vitro culture, in vitro produced bovine embryo.J. Vet. Med. Sci. 64(10): 887-891, 2002 In vitro culture of bovine embryos derived from IVM-IVF have succeeded in producing calves [10,21]. An adequate in vitro culture system for bovine zygotes is required for large scale embryo production by IVM-IVF and genetic improvement by means of ovum pick-up and IVM-IVF. This culture system is used for the development and efficient utilization of embryonic techniques in cattle, for example for embryonic or somatic cell nuclear transfer and in transgenic animals. Bovine IVM-IVF zygotes were first cultured with cumulus cells [8,10], oviduct epithelial cells [9,21], granulosa cells [13], amnion cells [2] and buffalo rat liver cells [36] in TCM199 supplemented with bovine serum, but there are many differences between in vitro produced bovine embryos and those produced in vivo. It was reported that in vitro produced embryos have darker cytoplasm and lower density [24,25]. CR1aa medium was developed for in vitro culture of bovine IVM-IVF zygotes [15,29,30], but the optimum culture system for IVM-IVF oocytes and freezability of blastocysts cultured in CR1aa still remain to be clarified. The chilling sensitivity should be considered as a factor affecting the quality of in vitro produced bovine blastocysts.The aim of the present study was to examine the effects of culture systems and culture media on the development competence and the quality of bovine embryos obtained by in vitro culture of IVM-IVF oocytes. MATERIALS AND METHODS Collection of oocytes and in vitro maturation:Bovine ovaries were obtained ...