Days in milk at first AI in dairy cows; its effect on subsequent reproductive performance and some factors influencing it.

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.

Reproductive performance of repeat breeders in dairy herds.

The objectives were to characterize repeat breeding in dairy cows, including reproductive performance and risk factors. Data from 613 Holstein Friesian cows in nine dairy herds across Japan were enrolled. A repeat breeder was defined as a cow that did not become pregnant after three inseminations, despite no clinically detectable reproductive disorders. In contrast, cows that became pregnant within three inseminations were considered to have normal fertility. Of the 613 cows, 87.3% eventually became pregnant after repeated AI (maximum calving to conception interval was 435 d). Mean (+/-SEM) first AI conception rate, days in milk at first AI, calving to conception interval and service per conception were 38.3%, 82+/-2 d, 125+/-3 d, and 2.0+/-0.1 times, respectively. Normal fertility cows (n=479) required only 114+/-3 d to conceive and 1.7+/-0.1 inseminations per pregnancy, whereas repeat breeders (n=86) required significantly more days to conceive (211+/-10) and more inseminations per pregnancy (4.7+/-0.2). Based on survival analysis, it took 94 d after calving for 50% of normal fertility cows to become pregnant, compared to 155 d for repeat breeders. For repeat breeders, 31.4, 50.0, and 58.1% became pregnant within 210, 300, and 435 d after calving, respectively. The risk factors for repeat breeding were parity (relative risk [RR]=0.809; P=0.058), resumption of postpartum ovarian cycles (RR=1.928; P=0.009), and days in milk at first AI (RR=0.991; P=0.039). In conclusion, repeat breeder dairy cows had very poor reproductive performance. Lower parity, abnormal resumption of postpartum ovarian cycles, and shorter days in milk at first AI were risk factors for repeat breeding.