Evaluation of different nutritional and physical development parameters on reproductive efficiency of Nellore heifers at 13 months of age submitted to FTAI

The present study aimed to evaluate the relationship between nutritional and physical development parameters and pregnancy rate results in Nellore beef heifers. The study was conducted in three commercial beef herds located in Camapua, Tres Lagoas and Agua Clara-MS. A total of 650 heifers with 13.0 ± 0.9 months of age was submitted to the same synchronization protocol for fixed time artificial insemination (FTAI). On a random day of the estrus cycle (Day -10) the females were evaluated for live weight, body condition score (BCS, 1-5 scale), subcutaneus fat thickness at the rump area (SFTR; ultrassonographic exams done with Aloka SSD500, Aloka, Japan) and the relation height/rib depth (RIBD). Ten days later (Day 0), all animals received an auricular ear implant containing 3.0 mg of Norgestomet (CRESTAR®, MSD Animal Health) and 2.0 mg of estradiol benzoate (EB; Gonadiol®, MSD Animal Health) intramuscularly (IM). At Day 8, the auricular implants were removed and they received 0.6 mg IM of estradiol cypionate (EC; ECP®, Zoetis), 0.265 mg IM of cloprostenol (Ciosin®, MSD Animal Health) and 300 IU of equine chorionic gonadotropin (eCG; Novormon®, MSD Animal Health). All females were then inseminated 48 h after implant removal. The pregnancy diagnosis was performed by transrectal ultrassonography (Aloka SSD500, Aloka, Japan) 30 days after FTAI. All variables were analyzed with the GLIMMIX procedure of SAS 9.3. The females were classified in categories according to the cutoff values provided by the Receiver Operating Characteristic curve (ROC): ≤ 13.0 months (≤ 13m; average = 12.8 months) and >13 months of age (>13m; average = 14.5 months); low BCS (LBCS, ≤ 3.0) and high BCS (HBCS, > 3.0); low SFTR (LSFTR, ≤ 2.47 mm) and high SFTR (HSFTR, > 2.47 mm) ; higher (HRIBD, >44.0%) and lower RIBD (LRIBD, ≤44.0%). Interestingly, pregnancy rate to the FTAI protocol had a positive relationship with age [ 13m = 40.9% (160/391); P=0.01], BCS [LBCS = 29.7% (119/400) vs. HBCS = 45.4% (113/249); P<0.001], SFTR [LSFTR = 23.4% (62/265) vs. HSFTR = 44.4% (168/378); P=0.0003] and RIBD [LRIBD = 27.0% (77/285) vs. HRIBD = 41.9% (150/358); P<0.001]. In conclusion, parameters related to nutritional and physical development had a positive relationship with fertility of Nelore heifers at 13 months of age that were submitted to FTAI protocol. This information could be used to increase efficiency of FTAI programs used in young animals and develop a reproductive program in order to reduce the age at first calving and the interval between generations in Nellore breed.