- The objective of this study was to determine if supplementing linoleic or α-linolenic acids, and timing of supplementation around AI will alter supplemental intake, ADG, and serum progesterone (P4), cholesterol, and triglyceride concentrations in replacement Angus crossbred heifers. In yr 1, 54 heifers, stratified by age, were assigned to: barley and soybean meal (CON); CON with 2% estimated DMI as flaxseed oil (F2); CON with 4% estimated DMI as flaxseed oil (F4); and CON with 4% estimated DMI as safflower oil (S4). In yr 2, timing of supplementation was evaluated on 51 heifers fed CON, F4 prior to AI (d 0) then CON (PREFLAX), CON prior to AI then F4 (POSTFLAX), or F4 throughout (FLAX). Isonitrogenous supplements were individually fed once per day and orts quantified. Supplement DMI (DMI[subscript SUPP]) was equal to 25% of estimated daily DMI (2.5% x BW). Grass hay and water were provided ad libitum. Heifers were estrous synchronized using the 14-day CIDR-PG protocol. Data were analyzed as a completely randomized design using repeated measures with heifer as the experimental unit and heifer within diet as the error term. For yr 1, means were compared using the following contrasts: CON vs. OIL(F2, F4, S4), F2 vs. F4, and F4 vs. S4. Across mean comparisons were used in yr 2. No differences (P > 0.10) occurred in AI conception rates in either yr. Compared to CON, OIL had lower (P < 0.05) DMI[subscript SUPP], but greater (P < 0.05) P4 and cholesterol concentrations on d 22 and 29. Triglyceride concentrations were similar (P > 0.10) between CON and OIL during feeding. The F2 group had greater (P < 0.01) DMI[subscript SUPP] than F4, but there were no differences (P > 0.10) in P₄, cholesterol, and triglyceride concentrations. Supplemental DMI was lower (P < 0.01) for F4 versus S4, but no differences (P > 0.10) in P₄ concentration occurred. The S4 heifers had greater (P < 0.05) cholesterol concentrations on d 22 verses S4. In yr 2 PREFLAX and FLAX tended (P < 0.10) to have lower DMI[subscript SUPP] pre-AI, while FLAX tended to be lower (P < 0.10) post-AI. Progesterone concentrations were similar among supplements. The feeding of flaxseed oil resulted in greater cholesterol (P < 0.05) and triglyceride (P < 0.10) concentrations post-AI with POSTFLAX and FLAX heifers having greater concentrations than PREFLAX and CON on d 22 and 29. Feeding flaxseed oil around breeding has the potential to increase reproductive performance in beef heifers by increasing circulating progesterone and cholesterol.