Summary Fertility can be improved prior to breeding by improving nutritional management, especially in range sheep operations. Feeding fatty acids (FA), including omega-3 (ω-3) FAs, have been shown to have positive effects on pregnancy parameters. The objective of this study was to determine the effects of feed- ing supplemental flaxseed via a Flaxlic® Sheep Tub during the flushing period on reproductive performance of spring lambing multiparous ewes. Multiparous Rambouillet ewes (n = 240; aged 2 to 7 years) weighing an average of 70.8 ± 10.4 kg were ran- domly assigned to 24 pens (10 ewes/pen) and fed a flushing diet for 35 d. Ewes were assigned to receive a Flaxlic® Sheep Tub (FLX; n = 12) or not (CON; n = 12). Tubs were weighed on d 0, 3, 7, 10, 14, 17, 24, 29, and 35. Tubs weighing less than 5 kg were replaced with new tubs. Weight data for all ewes was taken on d 0, 7, 14, 21, 28, and 35, with two-day weights on d -1 and 0 and d 34 and 35. Serum samples were taken for serum proges- terone concentration analysis on d 0, 7, 14, 21, 28, and 35 from 60 ewes per treatment (5 ewes/pen). On d 36, ewes and rams were comingled for breeding for 35 d. Birthweight, birth type, and sex of lambs were recorded at lambing for first cycle (149 to 166 days post ram turnout) and the overall lambing season. Ini- tial and final weights were not different between treatment groups (P = 0.47 and 0.23, respectively). No treatment x day interactions (P = 0.82) or treatment effects (P = 0.83) were observed for serum progesterone concentration. A day effect was observed for serum progesterone concentration, which was higher on d 28 and higher still on d 35 (P < 0.001). No differ- ences were observed between treatments for 1st cycle (P ≥ 0.26) or overall (P ≥ 0.61) pregnancy rate, prolificacy rate, or lambing rate. Our results indicate that ewes fed supplemental flax during the flushing period exhibited no economically beneficial responses in reproductive efficiency. Key Words: Ewes, Fatty Acids, Fertility, Flax, Flushing, Sheep ©2020, Sheep & Goat Research Journal Sheep & Goat Research Journal, Volume 35, 2030 - June 17 Volume 35, 2020 - June Impacts of Flax on Female Reproductive Traits When Supplemented Prior to Breeding in Sheep 1 Amanda M. Long 1,2 , Kimberly A. Vonnahme 2,5 James D. Kirsch 2 , Alison R. Crane 3 , Travis W. Hoffman 2 , and Christopher S. Schauer 1,6 1 Hettinger Research Extension Center, North Dakota State University, Hettinger, ND 58639 2 Department of Animal Sciences, North Dakota State University, Fargo, ND 58108 3 Department of Animal Sciences and Industry, Kansas State University, Manhattan, KS 66506 4 Support for this research was provided by New Generation Supplements and AmeriFlax. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of New Generation Supplements or AmeriFlax. The authors would like to thank David Pearson, Donald Drolc, Don Stecher (Hettinger Research Extension Center, North Dakota State University), and Ethan Schlegel (New Generation Supplements) for their assistance throughout the project. 5 Present address: Zoetis, Inc., 10 Sylvan Way, Parsippany, NJ 07054. 6 Corresponding author: christopher.schauer@ndsu.edu