79 Effect of intramuscular gonadotrophin-releasing hormone prior to and after conceptus attachment on pregnancy survival in lactating dairy cows

Lactating dairy cows have significant chances of pregnancy loss following conceptus attachment. It is not clear if these losses are due to altered conceptus development, corpora lutea (CL) regression, or a combination. This study hypothesised that treating cows with gonadotrophin-releasing hormone (GnRH), as a resynchronization strategy at specific times before pregnancy diagnosis, would induce accessory CL that may play a role in the retention of pregnancy. Lactating dairy cows (n = 362) received Double Ovsynch for first AI or Ovsynch for second to fourth AI. On Day 16 post-AI cows were randomly divided into three groups. Treated cows received 100 µg of GnRH intramuscularly on either Day 18 and 25 (G18/25; n = 138), or 25 (G25; n = 116) post-AI. All treated cows and controls (n = 108) received GnRH on Day 32 post-AI at the start of Ovsynch. All cows were diagnosed for pregnancy with ultrasonography on Day 34 post-AI. All cows had blood samples collected daily from Day 16 to 28 post-AI for determination of pregnancy-specific protein B (PSPB) and progesterone (P4). Ovarian structures were measured and mapped using ultrasonography on Days 18, 21, 25, 28, 32, and 35 post-AI. Luteal function was individually analysed using both P4 and CL volume. Pregnancy was also assessed with ultrasonography on Day 62 and between 115 and 199 days post-AI for determination of pregnancy loss. Presumptive conceptus attachment (pCA) was defined as an initial increase in daily PSPB (≥12.5%) followed with two more days of at least the same increase. Sensitivity for pCA was 100% and specificity was 82% compared to the Day 34 pregnancy diagnosis. Data were analysed using a mixed (continuous) or logistic (binomial) procedure in SAS 9.4. Treatments did not affect percent of cows with pCA (57% vs 66% vs 58%; P ≥ 0.14), time to pCA (21.1 ± 0.1 vs 21.0 ± 0.1 vs 21.1 ± 0.1 days; P ≥ 0.5), or pregnancy loss between pCA and 34 days post-AI (15% vs 12% vs 14%; P = 0.8), respectively, for G18/25, G25 and controls. Luteal function was maintained from Days 16 to 35 in 14/18 cows that had no accessory CL and lost pregnancies between conceptus attachment and Day 34 post-AI. There was no effect of treatment on pregnancy loss between attachment and Day 62 (P = 0.8) or 62 and 199 (P = 0.2). Lesser PSPB concentrations on the third day following pCA were associated with a greater probability of pregnancy loss after pCA (P < 0.01). On the third day following pCA, cows with GnRH-induced accessory CL that maintained pregnancy had greater PSPB concentrations in comparison with cows that maintained without accessory CL or those that lost pregnancy with or without accessory CL (P ≤ 0.01). In summary, treatment did not enhance pregnancy survival. Conceptus death between pCA and Day 34 pregnancy diagnosis appeared to occur both before and after loss of luteal support.