Associations of blood parameters with age, feed efficiency and sampling routine in young beef bulls

Utilization of blood parameters as proxies for feed efficiency is an avenue to maximize profitability of the beef industry. Among other factors, age and sampling routine may impact the reliability of potential proxies for residual feed intake (RFI). Thus, the objectives were to assess associations of blood parameters with age and RFI under two sampling routines. Thirty-two crossbred bulls with an average body weight (BW) of 633 +/- 93 kg and 369 +/- 29 days of age were studied. Residual feed intake was calculated using average daily gain, BW and ultrasound traits for body composition. Seven blood samples for each bull were collected during a 33-day on-station sampling period and an additional sample was collected at slaughter for analysis of blood metabolites and hormones. Bulls were classified as younger (342 +/- 17 days of age) and older (395 +/- 4 days of age) and into efficient (-0.55 +/- 0.70 kg DM/day) and inefficient (RFI=0.55 +/- 0.29 kg DM/day). Means of blood parameters were compared between age and feed efficiency groups using a mixed model for on-station sampling and a general linear model for slaughter sampling. During the on-station sampling, glucose (P=0.01), potassium (P=0.01) and insulin-like growth factor 1 (P=0.01) were lesser in older bulls while urea (P=0.05), acetate (P=0.01), osmolality (P=0.01), testosterone (P=0.01) and follicle stimulating hormone (FSH; P=0.04) were greater in older bulls. At slaughter, carbon dioxide (P=0.01), brain-derived neurotrophic factor (P=0.05) and FSH (P=0.01) were greater in older bulls. Over the on-station sampling, osmolality (P=0.05) was greater in inefficient bulls while leptin (P=0.01) was greater in efficient bulls. On the day of slaughter, cholesterol (P=0.04) and alkaline phosphatase (P=0.04) were lesser in efficient bulls. Age and RFI classes interaction was observed for T-3 (P=0.01) during the on-station sampling where lesser T-3 blood levels where observed in efficient bulls within the younger group (P=0.01) and in older bulls within the inefficient group (P=0.05). Overall, these results support the association of blood parameters with variation in age and RFI and illustrate the impact of sampling routine on components of intermediary metabolism in yearling bulls, providing information to the development of proxies for RFI.