Abstract:To evaluate the reproductive effects of a short-term dietary protein supplementation (Days -10 to -3) before timed AI (TAI = Day 0), 471 Merino ewes grazing native pastures were estrous-synchronized when there were either long intervals between prostaglandin administrations (two prostaglandin injections 15 or 16 d apart; PG15 and PG16, respectively) or with a progesterone-eCG (P4-eCG) protocol, resulting in a 3 × 2 experimental design. Ovulation rate on Day 8 (OR), non-estrous-return to Day 21 (NRR21), and fertility, prolificacy and fecundity on Day 70 were evaluated. The interaction between estrous synchronization protocol and supplementation was not significant for any of these variables (P > 0.05). Supplementation increased OR, prolificacy and fecundity (+0.14, +0.15 and +0.14, respectively, P 0.05). Ewes treated using the PG15 and PG16 protocols had a lesser OR (-0.27), prolificacy (?0.22) and fecundity (?0.20) than ewes treated using P4-eCG protocol (P 0.05 for both variables), without significant differences between the PG15 and PG16 groups. In conclusion, a short-term dietary protein supplementationbefore TAI improved OR, prolificacy and fecundity of ewes which were estrous-synchronized by imposing long interval PG (15 or 16 d apart) or P4-eCG-based protocols. There was a greater OR, prolificacy and fecundity when there was use of the P4-eCG compared to longinterval PG-based protocols. Estrous-non-return rate after AI and fertility as a result TAI were not affected by either the supplementation or the estrous synchronization protocols used.