Supplemental progesterone and timing of resynchronization on pregnancy outcomes in lactating dairy cows

Abstract The objective was to determine the effect of exogenous progesterone (P 4 ) in a timed artificial insemination (TAI) protocol initiated at 2 different times post-AI on pregnancies per AI (P/AI) in lactating dairy cows. Cows (n=1,982) in 5 dairy herds were assigned randomly at a nonpregnancy diagnosis 32±3d post-AI to 1 of 4 resynchronization (RES) treatments arranged in a 2×2 factorial design using the Ovsynch-56 (GnRH, 7d later PGF 2α , 56h later GnRH, 16h later TAI) protocol. Treatments were as follows: cows initiating RES 32±3d after AI with no supplemental P 4 (d 32 RES-CON; n=516); same as d 32 RES-CON plus a controlled internal drug release (CIDR) insert containing P 4 at the onset of Ovsynch-56 (d 32 RES-CIDR; n=503); cows initiating RES 39±3d after AI (d 39 RES-CON; n=494); and same as d 39 RES-CON plus a CIDR (d 39 RES-CIDR; n=491). Cows were inseminated if observed in estrus before TAI. The P/AI was determined 32 and 60d after TAI. In a subgroup of cows (n=1,152), blood samples were collected and ovarian structures examined by ultrasonography on the days of the first GnRH (G1) and PGF 2α of Ovsynch-56. Percentage of cows with a corpus luteum (CL) at G1 was unaffected by timing of treatments, but percentage of cows with a CL at PGF 2α was greater for d 32 than for d 39 cows (87.9 vs. 79.4%). In addition, percentage of cows with P 4 ≥1ng/mL at G1 was unaffected by timing of treatments, but was increased for d 32 compared with d 39 RES cows on the day of the PGF 2α of the RES protocols (86.5 vs. 74.3%). Treatment did not affect ovulation to G1 or P/AI 32d after RES TAI (d 32 RES-CON=30.1%, d 32 RES-CIDR=28.8%, d 39 RES-CON=27.5%, d 39 RES-CIDR=30.5%). A greater percentage of d 39 RES cows underwent premature luteolysis during the RES protocol compared with d 32 RES cows. An interaction was detected between day of RES initiation and CIDR treatment, in which the CIDR increased P/AI 60d after TAI for d 39 (CON=23.7% vs. CIDR=28.0%), but not for d 32 (CON=26.9% and CIDR=24.2%) cows. Pregnancy loss was unaffected by treatment. In addition, cows had improved P/AI 60d after TAI when they received a CIDR and did not have a CL (CON-CL=28.2%, CON-No CL=19.2%, CIDR-CL=27.0%, and CIDR-No CL=26.5%) or had P 4 4 =27.8%, CON-Low P 4 =15.0%, CIDR-High P 4 =25.0%, and CIDR-Low P 4 =29.4%) at G1, but not if a CL was present or P 4 was ≥1ng/mL at G1. In conclusion, addition of a CIDR insert to supplement P 4 during the RES protocol increased P/AI for cows initiating RES 39±3d after AI but not 32±3d after AI.