8 EFFECT OF CIRCULATING LEPTIN LEVELS AT SYNCHRONIZATION ON SUBSEQUENT FIXED-TIMED ARTIFICIAL INSEMINATION PREGNANCY RATES IN CROSSBRED BEEF HEIFERS

Abstract

It is generally accepted that reproductive efficiency is the limiting factor in the profitability of most livestock operations. The heritabilities of reproductive traits are typically low, so most advances in reproductive efficiency must be achieved through changes in management practices. Recent research indicates that leptin may play a role in the reproductive processes for many domesticated livestock species. Leptin has been shown to affect nutrient intake, metabolic efficiency, and animal adiposity, and may be associated with age at puberty onset and the length of the postpartum interval in cattle. Therefore, the objective of this study was to determine whether circulating leptin concentrations during estrous synchronization would affect subsequent pregnancy rates after fixed-timed AI (FTAI) in crossbred beef heifers. Before the initiation of the spring breeding season, 54 crossbred (Red Angus × Simmental) long yearling heifers, averaging 326 ± 6.3 kg with a mean body condition score (BCS) of 5.2 ± 0.33, a mean reproductive tract score of 2.1 ± 0.57, and a mean ovary score of 2.7 ± 0.76, were synchronized with a controlled internal drug-release (CIDR) vaginal implant and 2 mg of estradiol benzoate (EB) on Day 0, a 25-mg injection of prostaglandin and CIDR removal on Day 7, and 2 mg of EB on Day 8, with FTAI occurring 52 hours post-CIDR removal. Blood samples were collected via jugular venipuncture 4 times: twice during the synchronization protocol starting 2 days post-CIDR insertion, once at insemination, and once 4 days after insemination. All females were inseminated with frozen–thawed semen from the same fertile bull. Pregnancy was determined 45 days postinsemination via transrectal ultrasonography. Twenty females (37%) were confirmed pregnant to FTAI. There was no difference in average weights (324 ± 6.6 v. 323 ± 3.7 kg), BCS (5.1 ± 0.1 v. 5.2 ± 0.07), reproductive tract scores (2.2 ± 0.5 v. 2.1 ± 0.5) or ovary scores (2.7 ± 0.2 v. 2.7 ± 0.1) for the pregnant and nonpregnant females, respectively. However, those females that were pregnant from FTAI had higher (P = 0.02) mean plasma leptin concentrations (3.33 ± 0.22 ng mL^–1) compared with heifers that did not conceive to FTAI (2.66 ± 0.19 ng mL^–1) across all sampling days. In addition, as early as 2 days post-CIDR insertion, females that became pregnant from FTAI tended to have higher (P = 0.13) plasma leptin concentrations (2.76 ± 0.18 ng mL^–1) compared with those heifers that did not conceive to FTAI (2.19 ± 0.15 ng mL^–1). These results indicate that circulating plasma leptin levels in beef cattle during synchronization may be associated with subsequent fertility and might be a useful tool to select females to increase the efficacy of FTAI protocols in beef cattle.