Reproductive programs that rely primarily on timed artificial insemination (TAI) may reduce days to pregnancy in dairy heifers. Thus, the objective was to compare the reproductive performance of heifers managed for 1st service with programs that relied primarily on insemination to estrus (AIE) or TAI. Holstein heifers from 2 farms were randomized to receive 1st service at 12 mo of age [end of the voluntary waiting period (VWP)] with sexed semen after (1) PG+AIE (n = 279): AIE after PGF injections every 14 d (up to 3) starting at the end of the VWP, (2) PG+TAI (n = 294): AIE after 2 PGF injections 14 d apart (end of VWP at 2nd PGF). Heifers in PG+AIE and PG+TAI not AIE 9 d after the last PGF received the 5d-Cosynch protocol (5dCP = CIDR+GnRH-5d-CIDR-out+PGF-3d-GnRH⁺TAI), 3) TAI (n = 277): ovulation synchronization with the 5dCP protocol. Heifers not AIE by 72 h after PGF (end of VWP) received GnRH⁺TAI. After the 1st AI, heifers in all groups received AIE and TAI after 5dCP (initiated 31 ± 3 d post-AI) with conventional semen. Hazard of AI and pregnancy (days to event) for up to 45 d after VWP were analyzed with Cox’s regression and binomial outcomes by logistic regression in SAS. Results are presented in Table. More heifers were AIE in the PG+AIE and PG+TAI than the TAI group. However, the hazard of AI was greater in the TAI than the other groups. Pregnancies per AI were similar for all AI combined, AIE, or TAI services. Time to pregnancy was reduced for the TAI than the PG+AIE group but it was similar for the TAI and PG+TAI and the PG+AIE and PG+TAI groups. We conclude that a program for first AI that relied primarily on TAI reduced time to AI when compared with programs that resulted in most AIE (>80%) and reduced time to pregnancy when compared with a program with almost 100% AIE. Supported by NYFVI AOR15–020. Table 1.

	PG+AIE	PG+TAI	TAI	P-value
AIE, % (no.)	98.9a (279)	83.0a (294)	18.8b (277)	<0.01
Hazard of AI1, HR (95% CI)	Referent	1.16 (1.00–1.39)	1.85 (1.54–2.22)	<0.01
All AI P/AI 1st AI, % (no.)	42.3	46.9	43.0	0.54
P/AI AIE 1st AI	42.4 (276)	50.4 (244)	50.0 (52)	0.17
P/AI TAI 1st AI	—2	30.0(50)	41.3 (225)	0.17
Hazard of pregnancy3 (95% CI)	Referent	1.02 (0.84–1.25)	1.23 (1.02–1.49)	0.07

¹HR TAI vs PG+TAI 1.58 (1.33–1.87). ²PG+AIE group: TAI P/AI not included because n = 3. ³HR TAI vs PG+TAI 1.20 (0.99–1.46).