PSI-37 Post-weaning feed efficiency did not influence female fertility traits of commercial or Lowline Angus heifers

Abstract Feed costs represent ~50% of beef production costs making selection for improved efficiency without affecting female reproduction critical to the cow/calf sector. Objectives were to determine the relationship between post-weaning feed efficiency and female fertility in commercial (n = 19) and half-blood Lowline-influenced Angus heifers (n = 20). Yearling heifers were delivered to the CSU, Chico Farm cattle feeding unit 45-d post weaning and randomly assigned to one of three 7 x 18 m pens, equipped with GrowSafe feeding units. Heifers were given ad libitum access to water and a forage-based diet targeting 0.68 kg gain/d (TDN: 54%, CP: 14%, and NDF: 39.7%, on a DM basis) for 72-d. Feed efficiency measures included dry matter intake (DMI), feed conversion ratio (FCR) and residual feed intake (RFI). Heifer RFI was calculated as the difference between predicted DMI and actual DMI during the trial. Feed efficiency groups were identified as: 1) low RFI (0.5 SD above the RFI mean), 2) marginal RFI (+/- 0.5 SD about the RFI mean), and 3) high RFI (0.5 SD above the RFI mean). Serial weights were taken every 14-d to calculate average daily gain (ADG) and metabolic midweight (MMWT). Heifers were synchronized using a 14-d CIDR-PG protocol. Heifers received PG injection 16-d post-CIDR removal and were equipped with an estrous detection patch. Estrous behavior was observed morning and evening for 90-h. Heifers were artificially inseminated (AI) 12-h post-standing estrus and injected with 2 mL GnRH. Pregnancy determination via rectal palpation occurred 6-mos after initial AI. Reproductive measures included: first service conception (FSC), pregnancy rate (PR), and response to estrous synchronization (RES). Reproductive traits were analyzed using the GLM procedures of SPSS with a binary logistic response. Breed type and feed efficiency group were independent variables. Relationships among reproductive measures and performance were analyzed using logistic regression of SPSS, fitting the independent variables of ADG, DMI, and RFI. Traits FSC, PR, and RES did not differ among breed types or feed efficiency groups (P > 0.05) and DMI, ADG and RFI were not predictors of FSC, PR, nor RES (P > 0.05). Cumulatively, results indicate there is no relationship between feed efficiency and reproductive performance. Producers can select for low RFI to decrease feed costs without compromising reproductive performance.