Effect of Residual Feed Intake, Gender, and Breed Composition on Plasma Urea Nitrogen Concentration in an Angus-Brahman Multibreed Herd

Plasma urea nitrogen can be used as an indicator of N use and excretion by an animal. The objective of this research was to assess the effect of residual feed intake (RFI) on plasma concentration of urea N (PUN) in 188 bulls, heifers, and steers (mean body weight = 652 lb, SD = 82 lb) ranging from 100% Angus to 100% Brahman. Calves were assigned to pens in a GrowSafe feeding facility by sire group and sex, and self-fed a total mixed ration (corn, cottonseed hulls, chopped grass hay, cottonseed meal, molasses, and mineral-vitamin supplement; 90% dry matter (DM), 14% crude protein (CP), 0.7 Mcal/lb DM of NEm, and 0.4 Mcal/lb DM of NEg). The pre-trial adjustment period lasted 21 d. Individual daily feed intake was collected during the 70-d feeding trial; body weights were recorded every 2 wk. Blood (jugular) was drawn on d 56 for PUN. Residual feed intake was computed as the difference between actual and expected feed intakes. The RFI groups were high (RFI > mean + 0.5 standard deviation (SD)), medium (RFI between ± 0.5 SD), and low (RFI < mean – 0.5 SD; SD = 4.4 lb DM/d). Data (PUN) were analyzed using a mixed model. Fixed effects were sex of calf, RFI group, and Brahman fraction of calf; daily feed intake was a covariate. Random effects were sire and residual. Overall ADG was 2.74 ± 0.55 lb/d. Brahman PUN concentration was greater than Angus (P<0.01). Sex affected PUN (P<0.01) concentration with bulls having the lowest and heifers the highest. PUN concentration was related to RFI (P=0.02), indicating that more feed efficient animals also had lower PUN.