ABSTRACT Superovulation has the potential to increase the rate of genetic improvement through the female line. In sheep, Corriedale, a dual-purpose (wool, meat) breed, is numerically important in Argentina, where it is well distributed throughout all the area and managed under extensive conditions. However, little is known about the superovulatory response in Corriedale ewes and no studies have reported developing protocols that could be easily applied in such open flocks. The aim of the study was to evaluate the viability of simplifying superovulatory treatments in Corriedale ewes, during the breeding season. Two experiments were designed, using Corriedale ewes in the autumn breeding season. The ewes were synchronized with progestogen intravaginal sponges and superovulated according to different treatments. In Experiment 1, simplified schedules based on ovine FSH (oFSH) in combination or not with eCG were tested: A1 (n=13): 176 units’ oFSH given as a single injection in saline along with 500 IU eCG 48 h before sponge removal (B-SR); B1 (n=12): 176 units’ oFSH dissolved in saline were divided into 4 equal doses and administered at 12-h intervals starting 24 h B-SR. C1 (n=13): 176 units’ oFSH given as a single injection in long acting vehicle (polyvinylpyrrolidone; PVP) 24 h B-SR. D1 (n=14): 176 units’ oFSH dissolved in saline were divided into 6 equal doses and administered at 12-h intervals starting 24 h B-SR. E1 (n=7): 132 units’ oFSH dissolved in saline, divided into 4 equal doses and administered at 12-h intervals starting 24 h B-SR, along with 500 IU eCG given at the time of the first oFSH injection. F1 (n=7): 132 units’ oFSH given as a single injection in PVP along with 500 IU eCG, 24 h B-SR. In Experiment 2, the simplified protocol that exhibited the best performance in Experiment 1 (A1) was selected, A2 (n=16) and compared with the same protocol but using less oFSH (132 units), B2 (n=14), and with the most conventional protocol of oFSH (176 units in 8 decreasing doses, 72 h B-SR), C2 (n=17). Ewes were exposed to rams fitted with crayons and signs of oestrus were checked at 6-h intervals from sponge withdrawal. The interval to preovulatory LH peak and the magnitude of the surge were determined from blood samples collected at 6-h intervals and analysed using a sandwich enzyme immunoassay (EIA). P4 concentration at oestrus was measured by RIA in blood samples extracted at the time of oestrus onset. Six days after the onset of oestrus, ewes were submitted to laparotomy. Numbers of corpora lutea (CL) and persistent follicles (PF) were recorded. Percentages of ovulated (at least one functional CL) and superovulated (at least three functional CL) ewes were computed. The uterine horns were flushed and collected ova (CO: unfertilized ova and total embryos) were registered. Embryos (TE) were morphologically evaluated and those classified as viable (excellent or good quality; VE) were recorded. The rates of recovery (CO/CL), fertilization (TE/CO) and viability (VE/TE) were calculated. Blood samples were collected at embryo recovery to determine P4 by RIA. Variables were analysed by Chi-square or Fisher Exact Test, or by Analysis of Variance as appropriate. In Experiment 1, when comparing A1, B1 and C1, results were as follows. The onset of oestrus and the LH peak occurred earlier in those ewes that were co-treated with eCG (A1) (P<0.05). P4 concentration at oestrus was higher (P<0.05) in A1. The rate of superovulation differed between A1 and C1 (100% and 58.3%, respectively; P<0.05). The ovulation rate was higher in A1 (13.8 ± 1.9) than in B1 (6.2 ± 1.1) or C1 (4.8 ± 1.0) (P<0.05). The number of persistent follicles was no dependent on treatment (P>0.10). Progesterone secreted per corpus luteum was not different among groups, so that total concentration was higher in A1 because of its higher ovulation rate (P<0.05). The number of collected ova was higher in A1 (8.4 ± 1.4) than in B1 (3.1 ± 1.1) or C1 (3.2 ± 1.1) (P<0.05). Group A1 tended to produce more embryos (5.4 ± 1.6) than did B1 (1.4 ± 0.4) (P=0.06). However, C1 had higher rates of fertilization and viability than the other groups (89.7% and 76.9% in C1 vs. 64.2% and 53.2% in A1 and 45.9% and 40.5% in B1; P<0.05). Finally, A1 tended (P=0.08) to produce more viable embryos (4.5 ± 1.3) specially when compared to B1 (1.2 ± 0.4). The alternative of dividing the oFSH into 6 (D1) instead of 4 (B1) equal doses failed to increase ovulation rate, but it produced higher (P<0.05) rates of recovery (65.4% vs. 49.3%), fertilization (82.3% vs. 45.9%) and viability (72.5% vs. 40.5%). So, D1 resulted in more viable embryos than B1 (3.1 ± 0.7 vs. 1.2 ± 0.4; P<0.05). The choice of combining eCG to the oFSH administered either into 4 equal doses (E1) or in one injection in PVP (F1) reduced the intervals to the oestrus and the LH peak (P<0.05). Group E1 had higher rates of fertilization and viability than did B1 (78.3% vs. 45.9% and 73.9% vs. 40.5%; P<0.05) but although its number of viable embryos was duplicated (2.8 ± 1.7 vs. 1.2 ± 0.4), differences were not significant. Group F1 overcame the low percentage of superovulation of C1 (100% vs. 58.3%) and increased the number of corpora lutea (F1: 9.4 ± 1.4 and C1: 4.8 ± 1.0). However, it did not raise the production of viable embryos because of its low recovery rate (F1: 53% vs. C1: 68.4%; P<0.05). In Experiment 2, the conventional protocol (C2) resulted in a later oestrus onset and LH peak (30.4 ± 1.7 and 33.3 ± 2.5 h in C2 vs. 22.0 ± 1.4 and 24.6 ± 2.4 h in A2, and 20.1 ± 1.0 and 19.0 ± 1.8 h in B2; P<0.05). Ovulation rates were high regardless of treatment. C2 had a low incidence of persistent follicles (0.3 ± 0.2), which was significantly different with A2 (1.3 ± 0.5) (P<0.10). P4 concentration per corpora lutea was lower (P<0.05) in C2 in comparison with A2 and B2. Recovery rate was greater in C2 (72.5% vs. 57.3% in A2 and 56.7% in B2; P<0.05), but the number of collected ova was similar among groups. Fertilization rate was dependent on treatment (A2: 54.2%; B2: 81.6% and C2: 93.9%; P<0.05) and C2 tended to produce more embryos than the others (7.3 ± 1.0 vs. 4.3 ± 1.1 in A2 and 4.8 ± 0.8 in B2; P=0.07). Viability rate was lower in A2 (42.4%) when compared to B2 (68.4%) and C2 (71.2%) (P<0.05). Finally, the number of viable embryos did not depend on treatment (3.3 ± 1.0, 4.0 ± 0.8 and 5.5 ± 0.8 for A2, B2 and C2; P>0.10). In conclusion, the study supports the use of simplified protocols to superovulate Corriedale ewes during the autumn. Among them, the reduced-dose oFSH given once along with eCG is the most appropriate superovulatory schedule because it combines simplicity and a lower dose of gonadotrophin (which implies a reduction in cost), without decreasing embryo production. Keywords: Superovulation; Sheep; Hormones; Embryos.