Effect of a puberty induction protocol based on injectable long acting progesterone on pregnancy success of beef heifers serviced by TAI

Induction protocols based on progesterone (P4) are used prior to a synchronization program for timed-AI (TAI) to increase number of pubertal heifers and pregnancy per AI (P/AI). Injectable, long-lasting P4 (iP4) is a novel, practical method to supplement P4 in cattle. Here, we aimed to test the effect of an induction protocol based on a single injection of iP4 on P/AI of heifers. Bos indicus (Nellore) heifers were classified as pubertal (PUB; n = 224) or prepubertal (PRE; n = 414) based on two ovarian ultrasonographyc exams conducted 10 d apart. Heifers with a corpus luteum (CL) in any of the exams were considered PUB. Within each puberty status, heifers were assigned to receive nothing (NoiP4) or an induction protocol (iP4). Induction consisted of a single injection of 150 mg of iP4 on D-31, followed by injections of 1 mg of estradiol benzoate (EB) and 150 mg of prostaglandin analogue (PGF) on D-21. On D-9, all heifers received 2 mg of EB + 75 mg of PGF associated to intravaginal P4-device insertion. On D-3, P4-releasing devices were removed and 150 mg of PGF injected. Heifers were inseminated based on estrus on D-1 or were TAI on D0. On D0, all heifers received a dose of GnRH analogue. On D-21, iP4 treatment stimulated a 50% increase in the uterine score (UTS) and a 19% increase in the diameter of the largest follicle of PRE heifers (P < 0.01). On D-9, PRE vertical bar iP4 group had a greater proportion (P < 0.01) of CL (63.3%) than PRE vertical bar NoiP4 group (11.6%). On D-3, exposure to 6 d P4-releasing device stimulated UTS of PRE|NoiP4 group in a similar fashion than the induction protocol, but it did not have any additional positive effect for PRE vertical bar iP4 heifers. P/AI of PRE vertical bar iP4 group was similar to that of the PUB groups (44.7 vs 46.9%), but was more than that of PRE vertical bar NoiP4 (34.2%). There was an overall 7.7% increment (P = 0.07) on P/AI of iP4 treated heifers (iP4: 46.0% vs. NoiP4: 38.3%). In conclusion, implementation of an induction protocol based on iP4 was effi-cacious to hasten puberty. Induction stimulated uterine development and follicular growth of prepu-bertal heifers, ultimately leading to pregnancy success similar to that of pubertal heifers. (C) 2020 Elsevier Inc. All rights reserved.