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Abstract. The aim of the present study was to show the distribution of cows by days in milk (DIM) at first AI, effect of DIM at first AI on reproductive performance and some factors influencing DIM at first AI. A total of 767 Holstein Frisian cows that calved from January 2004 to December 2007 in 14 dairy herds were used. The first AI conception rate (FAICR) was 34.0%. Seventy-five percent of the cows were first inseminated within 100 days after calving. FAICR increased linearly up to 100 DIM. A one unit (20 days) longer DIM at first AI within the first 100 days postpartum increased the likelihood of a 2.4% FAICR. However, cows first inseminated at an earlier stage of lactation showed better reproductive performance in terms of pregnancy rate and calving to conception interval than cows first inseminated at a later stage. A one day increase in the interval from calving to first AI reduced the likelihood of 0.85 days to become pregnant. Herd or region located in southwestern Japan, tie-stall herd, first AI in spring, higher parity, uterine infection, metabolic diseases and/or mastitis and abnormal resumption of postpartum ovarian cycles contributed to delaying first AI. Key words: Dairy cows, DIM at first AI, Reproductive performance (J. Reprod. Dev. 57: [643][644][645][646][647][648][649] 2011) ecreased conception rate at first AI in dairy cows has been reported worldwide [1][2][3][4], resulting in an increased number of services per conception, delayed conception and increase of days open. Our recent study also described that approximately 14% of 613 dairy cows in nine dairy herds in Japan had repeat breeding and that the repeat breeders showed extremely poor reproductive performance [5]. Nutritional interactions with reproductive performance in the early postpartum period through high feed consumption required to meet energy requirements have been recognized as reducing the first AI conception rate [6]. Changes in reproductive physiology due to elevated steroid metabolism [7] and metabolic changes in follicular fluid of the dominant follicle in early lactation [8] have been reported to be potential causes of reduction of the first AI conception rate in lactating dairy cows.Esslemont and Kossaibati [9] have provided a fertility management assessment checklist. To be considered good fertility management, 95% of the cows in the herd after calving must be serviced by keeping the average calving to first service interval at less than 70 days, the overall heat detection must be more than 55% and the pregnancy rate must be 50% or more. The requirements to achieve this target or high reproductive efficiency in a dairy herd are a disease-free transition period, high rates of submission to AI and a high conception rate per service [2]. However, many current dairy herds have difficulty breeding and achieving a high first AI conception rate in an optimum time.Conception rate has been reported to be higher in the later stages of lactation than in the early stages of lactation [3,[10][11][12]. Likewise, following an Ovsy...