Relationships between Residual Feed Intake and Other Biological Variables

Opportunity exists to improve the efficiency of feed utilization to reduce cost of production and environmental impact on a per head basis. Focus has been placed on utilizing residual feed intake (RFI) to measure animal efficiency utilizing feeding systems that determine individual feed intake of group housed animals. Our goal was to identify biological traits that might serve as proxies for RFI to allow selection of efficient animals for those without the technology to measure RFI. Heart rate has been found to be highly related to RFI. Understanding other biological measures that are related to RFI may help refine selection practices. We utilized the data from 93 Angus bulls during Summer 2016 that were started on an 85 day performance test between 215 and 315 days of age and RFI was determined for the test period utilizing the GROWSAFE system. Pre-test and mid-test physiological (cortisol), immunological (neutrophil/lymphocyte and vaccine antibody titer) and behavioral (temperament) measures were assessed. At mid-test, RFI was negatively associated with antibody titer for bovine respiratory syncytial virus (r = -0.22, p = 0.04) and the ratio of neutrophils to lymphocytes (r = -0.22, p = 0.04). At mid-test, RFI was positively associated with the extent of activity when the animal was isolated in a pen for 30 secs (r = 0.23, p = 0.04). ADG was negatively associated with cortisol at mid-test (r = -0.27, p = 0.01). Dry matter intake was negatively associated with antibody titer for bovine respiratory syncytial virus both pre- (r = -0.33, p = 0.002) and mid-test (r = -0.32, p = 0.003) as well as neutrophil to lymphocyte ratio mid-test (r = -0.28, p = 0.01). The weight achieved by the end of the test was positively associated with the ratio of neutrophils to lymphocytes (r = 0.22, p = 0.04) and negatively associated with cortisol pre-test (r = -0.30, p = 0.004), total activity when isolated with an observer pre- and mid-test (r = -0.29, p = 0.006 and r = -0.28, p = 0.01) and the distance the observer was able to approach the animal (r = -0.29, p = 0.007). Animal efficiency, as measured by RFI was related to measures of reduced stress (cortisol and neutrophil to lymphocyte ratio) and calmer temperament. Growth rate was inversely related to mid-test cortisol and the final weight achieved was inversely related to pre-test cortisol and measures of fearfulness.