172 Treatment with recombinant bovine somatotropin increases serum IGF-1 after timed artificial insemination but does not improve ovulation rate or subsequent pregnancy outcomes in Nelore cows

Insulin-like growth factor 1 (IGF-1) exerts several beneficial effects on ovarian function. However, the use of recombinant bovine somatotropin (rbST) as a strategy to improve pregnancy rates after AI or embryo transfer (ET) remains a matter of controversy. The aims of this study were to evaluate the effects of rbST treatment at the beginning of a timed AI (TAI) protocol on serum progesterone (P4) and IGF-1 concentrations 8 days after TAI and subsequent pregnancy rates. Lactating Nelore (Bos indicus) cows were randomly assigned to receive either 325 mg rbST s.c. (Posilac, Agener União; n = 63) or no treatment (control group, n = 65) on the first day (D0) of a TAI protocol. All cows received the same TAI protocol, consisting of the insertion of an intravaginal 0.5 g P4 device and injection of 2.0 mg estradiol benzoate i.m. on D0, followed by device withdraw and intramuscular injection of 0.723 mg of sodium cloprostenol, 1.0 mg of estradiol cypionate, and 400 IU of equine chorionic gonadotrophin on D9. On D11, TAI was carried out with frozen-thawed semen from a single Nelore sire and by the same technician. Eight days later (D19) blood samples were collected and centrifuged, and the serum stored for further P4 and IGF-1 analysis by electrochemiluminescence. Pregnancy diagnosis was carried out by transrectal ultrasonography 30 days after TAI. Data were retrospectively analyzed using the Glimmix or Freq procedures of SAS. As expected, treatment with rbST resulted in greater circulation IGF-1 concentrations 8 days after TAI (125.5 ± 7.6 ng mL⁻¹ vs. 99.0 ± 5.3 ng mL⁻¹ in the rbST and control groups, respectively; P = 0.0422). We observed a greater percentage of cows with subluteal P4 concentrations (<1 ng mL⁻¹) in the rbST group compared with controls (17.6% vs. 1.4%; P = 0.0009), suggesting ovulation failure to the TAI protocol. When these animals were excluded, we observed a trend toward a greater serum P4 in the rbST group compared with controls (6.8 ± 0.6 vs. 5.5 ± 0.4 ng mL⁻¹, respectively; P = 0.0937). Pregnancy rate, however, was lesser in rbST group than in controls (55.4% vs. 73.5%, respectively; P = 0.0344). A low and nonsignificant correlation was observed between P4 and IGF1 serum concentrations (R = −0.0467; P = 0.6066). In summary, treatment with 325 mg of rbST at the beginning of a TAI protocol had a negative effect on ovulation rate. Moreover, a possible positive effect of rbST treatment on luteal P4 secretion did not lead to greater pregnancy rates.

This study was supported by CAPES, DPG UnB, and FAPDF 00193-00002307/2022-11.