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Abstract: The objectives were to determine the effects of elevated blood urea concentrations on: (i) the response to superovulation, fertilisation rate, and early embryonic development in beef heifers, and (ii) embryo survival from days 7 to 35 of gestation. In Experiment 1, heifers (18–24 months) were allocated at random (n =20 per treatment) to one of the following diets: (i) ad libitum grass silage plus 5kg commercial beef concentrates per day (controls); (ii) ad libitum grass silage plus 5kg concentrates and 250g feed grade urea per day (HE/HU); or (iii) ad libitum wheaten straw plus 250g feed grade urea and 50g vitamin/mineral mix per day (LE/HU). Serum urea concentrations were monitored throughout the experiment. Oestrus in heifers was synchronised using an intravaginal releasing device (CIDR®, InterAg, New Zealand). Oestrus was detected and in vitro produced blastocysts (day 7, morphological grades 1 and 2) were transferred to the heifers 7 days later (19 days after start of treatment diets). The heifers were maintained on the dietary treatments for a further 28 days, when pregnancy status was determined by transrectal ultrasonography. Detected pregnancies were terminated using 15mg luprostiol and recycled for Experiment 2. In Experiment 2, following a 14-day dietary rest period, the heifers were re-allocated at random to the three dietary treatments above. Heifers were treated with a CIDR for 8 days and 15mg luprostiol was given 12h before pessary withdrawal. They received 144mg pFSH (Folltropin®-V, Vetrepharm, Canada) given as 8 injections over 4 days commencing on day 6 of CIDR/dietary treatment. Heifers were artificially inseminated 48h after progesterone pessary withdrawal using commercial semen of proven fertility by a competent inseminator. The heifers were maintained on their diets until slaughter, 3 days post insemination when corpora lutea numbers were determined and embryos were recovered and cell numbers determined visually. Serum urea concentrations were greater in heifers on LE/HU than in those on HE/HU diets, which in turn were greater than controls (7.1±0.5, 4.9±0.3 and 3.2±0.1mmol/L, respectively; P <0.05). There was no effect of diet type on pregnancy rate at day 35 (42%, 47% and 46%) and on the number of corpora lutea following superovulation (5.2±0.8, 5.8±1.5 and 6.8±1.1) for heifers on control, HE/HU and LE/HU diets, respectively. The total number of embryos recovered per heifer was not different between the three groups (2.7±0.6, 3.4±1.1 and 4.8±0.8 for heifers on control, HE/HU and LE/HU diets, respectively; P >0.05), but the number of embryos with 8 or more cells at recovery was greater in heifers on LE/HU than on control diets (3.4±0.8 compared with 1.0±0.3; P <0.05). However the percentage of embryos recovered with 8 or more cells was not different between groups (70.0±13.3, 86.9±7.2 and 76.5±7.9%, for heifers on control, HE/HU and LE/HU diets respectively). Fertilisation rate, expressed as the proportion of embryos with more than one cell at recovery relative to the total number of embryos recovered, was less in the heifers on the control diet than in the other two dietary treatments (61.3±11.8, 92.0±3.5 and 86.8±5.4% for heifers on control, HE/HU and LE/HU diets, respectively; P <0.05). Deleterious effects of urea on reproduction were not found, suggesting that adverse effects of urea are likely to take place at the early oocyte development stage prior to ovulation or fertilisation following an increase in protein intake. [Copyright &y& Elsevier]