48 Timing of luteolysis and conceptus expulsion after induced pregnancy loss at three different timepoints after maternal recognition of pregnancy in cattle
Authors: Duran B. J. , Lemos-Motta J. C. , Rojas-Canadas E. , Hayden C. , Rykaczewski C. , Nall A. , Mussard M. L. , Garcia-Guerra A. 
Published: January 2021

Results from previous research indicate that induced pregnancy loss (IPL) on Day 36 of gestation in cattle resulted in luteolysis by gestation Day ∼45. The aim of this study was to test the hypothesis that IPL at 25, 35, or 55 days of gestation will result in luteolysis and whether the interval between IPL and luteolysis is positively associated with days of gestation at time of treatment. Pregnant non-lactating beef cows were randomly assigned to undergo IPL, consisting of an intrauterine infusion of 120mL of hypertonic saline (7.2%), at Day 25 (IPL25, n=15), Day 35 (IPL35, n=14), or Day 55 (IPL55, n=14). Corpus luteum (CL) volume and embryo/fetal viability were evaluated by transrectal B-mode ultrasonography daily for 19 days following treatment and every other day subsequently until luteolysis was identified or gestation Day 91. Blood flow through the CL was evaluated using colour-Doppler ultrasonography, and luteolysis was defined as the day when &lt;25% of the CL was represented with colour pixels. Conceptus expulsion was defined as the absence of a conceptus and fluid accumulation in the uterus was &lt;25%. Data were analysed using Proc Mixed in SAS (v9.4; SAS Institute Inc.). Cessation of conceptus heartbeat occurred in all animals within 24h of administration of treatment. Day of luteolysis occurred earlier (P&lt;0.01) in IPL25 (5.3±0.4d) than IPL55 (8.9±1.4d) cows, and luteolysis tended (P=0.07) to occur earlier in IPL35 (6.6±0.6d) than IPL55 cows; however, values for this variable in IPL25 and IPL35 cows were similar (P&gt;0.2). The variance in timing of luteolysis was greater for IPL55 than for IPL35 cows, whereas variance for IPL35 was greater than that for IPL25 (P&lt;0.05) cows. Conceptus expulsion was detected earlier (P&lt;0.01) in IPL25 (Day 4.2±0.4) and IPL35 (Day 5.8±0.5) than IPL55 (Day 10.4±1.4) cows; however, IPL25 and IPL35 cows had similar (P&gt;0.1) values for this variable. Values for luteolysis and conceptus expulsion were correlated in IPL35 (r=0.84; P&lt;0.01) and IPL55 (r=0.88; P&lt;0.01) cows, whereas there was no correlation among values for these variables in IPL25 (r=0.31; P=0.3) cows. There were effects of treatment (P&lt;0.01), time (P&lt;0.01), and treatment by time interaction (P&lt;0.01) on CL volume. Volume of the CL was less (P&lt;0.04) in IPL25-treated cows by treatment day 5 compared with that in IPL55-treated cows. Additionally, CL volume was less (P&lt;0.03) in IPL25 by treatment day 6 than in IPL35 cows, and IPL35 cows had a lesser (P&lt;0.01) CL volume starting on treatment day 10 than IPL55 cows. Induction of pregnancy loss at different times of gestation resulted in luteolysis; however, the interval between treatment and luteolysis tended to increase as gestation days increased. Furthermore, the variability in the timing of luteolysis increased as gestation days increased. Results from the present study support the working hypothesis of presence of a continuous mechanism for CL maintenance during pregnancy beyond the classical maternal recognition period in cattle.