Effect of AI on the incidence of short luteal lifespan in cows supplemented with P4 at early diestrus

In cattle, progesterone supplementation (P4-suppl) during early diestrus stimulates conceptus growth and the proportion of pregnant cows (P/AI). Despite such beneficial effects, a proportion (~40%) of non-inseminated and P4-suppl animals undergo reduction on luteal lifespan (early luteolysis) (Reprod. Dom. Anim. 49, 85-91, 2014) which may reduces the potential of further improvements of this strategy in inseminated cows. Accordingly, the presence of an embryo or conceptus between days 13 and 16 of diestrus alters gene expression in the endometrium (Biolreprod. 85,144-156, 2011). We hypothesized that presence of the conceptus in P4-suppl cows reduces the incidence of early luteolysis. Non-lactating multiparous cows (n = 94) were synchronized using an estradiol/P4- based protocol followed by estrus detection twice a day for 4 days. Cows detected in estrus (n = 66), were split to receive 12h after estrus an AI with semen from a single high fertility bull (n = 23; AI subgroup) or semen extender only (n = 43; non-AI subgroup). At AI, the pre-ovulatory follicle diameter was measured and ovulation (d0) was monitored every 12h by transrectal ultrasonography. On d3, an IM injection with 150 mg of long acting P4 (Sincrogest; Ourofino Saude Animal, Cravinhos, SP) was given to non-inseminated (P4-suppl group; n = 21) and inseminated cows (AI + P4-suppl group; n = 23). Non-inseminated and non-treated cows (Control; n = 22) received at the same moment (d3) a sham injection (NaCL 0.9%). From d3 to d23 CL development and function was evaluated by B-mode and Color Doppler ultrassonography. Criteria for classifying cows that underwent luteolysis was CL area <2.0 cm2 and colored blood flow signals that covered ≤ 25% of total CL area. Additionally, blood samples were collected every 48 h from d9 to d21 for measurement of P4 concentrations by RIA. On d30, pregnancy was checked by ultrasonography. Data were analysed with SAS v.9.3. Frequency of luteolysis was analyzed using Chi-square test and continuous variables using Kruskal-Wallis test. Treatment with P4 (P = 0.03) reduced luteal lifespan (Control: 17.6a ± 0.37; P4-suppl: 16.4b ± 0.37 and AI + P4-suppl non-pregnant cows: 15.9b ± 0.71 days). The proportion of cows in luteolysis on d15 was not affected by AI (Control: 4.5%b [1/22]; P4- suppl: 33.3%a [7/21] and AI + P4-suppl: 26.1%a [6/23]; P ≤ 0.05) but on d16, the proportion was similar between AI + P4-suppl and the control group and tended to differ between AI + P4-suppl and the P4-suppl (Control: 31.8%b [7/22]; P4-suppl: 61.9%a,X [13/21] and AI + P4-suppl: 34.8%a,b,Y [8/23]; a,bP = 0.05; X,YP = 0.07). On d30 the P/AI was 30.4% (7/23). Removing the D30 confirmed pregnant cows, the proportion of D30 non-pregnant cows that showed luteolysis until d16 was 50.0% (8/16). In conclusion, AI affected incidence of short luteal lifespan, but this effect is limited by the conceptus developmental stage.