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

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 AT (P/AI) in lactating dairy cows. Cows (n = 1,982) in 5 dairy herds were assigned randomly at a nonpregnancy diagnosis 32 +/- 3 d post-AI to 1 of 4 resynchronization (RES) treatments arranged in a 2 x 2 factorial design using the Ovsynch-56 (GnRH,7 d later PGF(2 alpha), 56 h later GnRH, 16 h later TAI) protocol. Treatments were as follows: cows initiating RES 32 +/- 3 d after AT 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 +/- 3 d after AT (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 60 d 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(2 alpha) of Ovsynch-56. Percentage of cows with a corpus luteum (CL) at 01 was unaffected by timing of treatments, but percentage of cows with a CL at PGF(2 alpha) was greater for d 32 than for d 39 cows (87.9 vs. 79.4%). In addition, percentage of cows with P-4 >= 1 ng/mL at 01 was unaffected by timing of treatments, but was increased for d 32 compared with d 39 RES cows on the day of the PGF(2 alpha) of the RES protocols (86.5 vs. 74.3%). Treatment did not affect ovulation to 01 or P/AI 32 d 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 60 d 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 60 d 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 <1 ng/mL (CON-High P-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 >1 ng/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 3 d after AT but not 32 +/- 3 d after AI.