Previous work by our group showed that supplementing diets with Ca salts containing fish oil fatty acids improved pregnancy per AI (P/AI) in dairy cows. Objectives were to determine the effects of supplementing DHA by feeding algae on lactation and reproductive performance of Holstein cows. The hypothesis was that feeding DHA improves P/AI in dairy cattle. Weekly cohorts of cows were blocked by parity and randomly assigned at 27 DIM to a control [CON; n = 372, 115 primiparous and 258 multiparous] or an algae supplemented (ALG; n = 366, 105 primiparous and 261 multiparous) diet. Cows were fed the same TMR; however, ALG received 100 g/cow/d of a 17% DHA supplement top-dressed and hand-mixed onto the TMR for 120 d. Milk yield was recorded daily and sampled every 3 wk for analysis. Cows were subjected to the Presynch-Ovsynch protocol starting at 44 ± 3 DIM and those in estrus after 58 ± 3 DIM were inseminated on the same day, otherwise, they received timed AI at 80 ± 3 DIM. Pregnancy was diagnosed on d 60 after AI. Data were analyzed by ANOVA, logistic regression or the Cox’s proportional hazard model using the GLIMMIX and PHREG procedures of SAS. Intake of DM did not differ between treatments (ALG = 24.8 vs. CON = 25.5 ± 0.6 kg/d). Milk yield was greater (P = 0.01) in ALG than CON (43.6 vs. 42.5 ± 0.3), but yield of energy-corrected milk was similar between treatments and averaged 39.2 ± 0.2 kg/d. Feeding ALG reduced (P = 0.03) content and yield of milk fat (3.08 vs. 3.23 ± 0.03% and 1.30 vs. 1.34 ± 0.01 kg/d); however, ALG increased (P < 0.01) yields of protein (1.23 vs. 1.20 ± 0.01 kg/d) and lactose (2.07 vs. 2.02 ± 0.01 kg/d). Feeding ALG increased (P = 0.04) the proportion of primiparous cows that resumed estrous cyclicity by 58 DIM (77.6 vs. 65.9%) and P/AI at first AI (47.6 vs. 32.8%). Feeding ALG increased (P < 0.01) P/AI at all AI in all cows (41.4 vs. 30.8%). Cows fed ALG became pregnant 21 d (103 vs. 124 d) sooner (P < 0.01) than those fed Control (adjusted HR = 1.38; 95% CI = 1.13 to 1.69). Supplementing DHA by feeding ALG did not affect intake of DM, increased yields of milk and protein, but reduced milk fat content and yield. Cows fed ALG had improved estrous cyclicity, P/AI, and reduced interval to pregnancy.