Low progesterone concentration during the development of the first follicular wave reduces pregnancy per insemination of lactating dairy cows.

Objectives were to determine the effect of progesterone (P4) concentration on fertility of lactating dairy cows induced to ovulate follicles of the first follicular wave. Lactating dairy cows (n=989) at 38±3d postpartum were balanced by parity and body condition score and randomly assigned to 3 treatments: first follicular wave (FFW), first follicular wave with exogenous P4 (FFWP), or second follicular wave (SFW). All cows had their estrous cycle presynchronized with 2 injections of prostaglandin (PG) F(2α) given 14 d apart. Cows in the FFW and FFWP treatments started the ovulation synchronization protocol 3 d after the last PGF(2α) of the presynchronization protocol, whereas SFW cows received a GnRH injection (100 μg of gonadorelin diacetate; Cystorelin, Merial Ltd., Duluth, GA) 3 d after the last PGF(2α) of the presynchronization protocol and started the synchronization protocol 7 d later. The synchronization protocol consisted of GnRH on d -10, PGF(2α) on d -3, and GnRH concurrent with timed artificial insemination (AI) on d 0. Cows in the FFWP treatment received 2 controlled internal drug release inserts containing 1.38 g of P4 from d -8 to -3. Progesterone concentration was determined on d -10, -8, -6, -3, and 0 from all cows and at 7, 14, and 21 d after AI from a subsample of cows (n=170). Cows (n=715) had their ovaries scanned by ultrasound on d -10, -3, and 7 d. Pregnancy was diagnosed at 38 and 66 d after AI. Concentration of P4 from study d -8 to -3 was lowest for FFW cows (1.4±0.1 ng/mL) and similar between SFW (3.7±0.2 ng/mL) and FFWP (3.7±0.1 ng/mL) cows. Diameter of the dominant follicle on study d -3 was greater for FFW cows (16.5±0.3 mm) than for SFW cows (15.4±0.3 mm), but diameter of the dominant follicle of FFWP cows was not different (15.9±0.3 mm) compared with that of SFW and FFW cows. The incidence of multiple ovulation was largest for FFW cows (SFW=19.5, FFW=33.6, FFWP=19.0%), but pregnancy per AI (P/AI) at 66 d was smallest for FFW cows (SFW=38.9, FFW=22.3, FFWP=32.0%). Anovular cows in the SFW (19.4 vs. 42.8%) and FFWP (22.1 vs. 37.2%) treatments had reduced P/AI compared with cyclic cows, despite having similar or greater P4 concentration from study d -8 to -3, respectively. Estrus and ovulation synchronization protocols for lactating dairy cows must result in growth of ovulatory follicle under P4 concentration >2 ng/mL to ensure high P/AI.