Relationship Of Blood-Borne Markers Of Subclinical Infection And Residual Feed Intake Status In Brangus Heifers.

Residual feed intake (RFI) is a heritable feed efficiency measure. Mechanisms underlying variation in feed efficiency are currently poorly understood but recent microarray studies have indicated high RFI cattle exhibit elevated innate immune pathway activation in the absence of overt clinical infection. To address whether this is a heritable genetic signature of feed efficiency or simply the result of a confounding presence of sub-clinical infection, dry matter intake (DMI) was measured on 103 Brangus replacement heifers during a 70 d feeding trial conducted using the Calan gate system. While on test, feed intake was recorded daily with BW and hip heights recorded at 14 day intervals. Ultrasound measurements of rib eye area (REA) and backfat (BF) were recorded initially and prior to harvest. Carcass and growth data were analyzed using a mixed model with RFI level (L, H) as the independent variable. Residual feed intake (RFI) was defined as actual feed intake minus expected feed intake based on maintenance and production requirements. The basic regression model for determining RFI was Yi = β0 + β1ADG + β2MMWT + ei, where Yi = expected DMI, β0 = regression intercept; β1 = partial regression coefficient of DMI on ADG, β2 = partial regression coefficient of DMI on metabolic mid-test weight (MMWT) and ei = RFI. Blood samples were collected via venipuncture of the tail vein during the last weigh day and analyzed for a full complete blood cell count (CBC), plasma protein total solids (PT/S) and fibrinogen protein total solids so that resulting values could be analyzed across RFI class. The lsmeans for RFI were -1.51 and 1.56 for the L and H cohorts (P < .0001). As expected, DMI was higher for the H individuals versus the L heifers (P < .0001) while on test BW gain was not different between the two groups (P < 0.79). No lsmeans for any clinical blood parameter measured fell outside of a diagnostically normal range for healthy individuals across any RFI class. Likewise neither CBC profiles (P < 0.71), PT/S (P < 0.74), nor fibrinogen protein total solids (P < 0.44) were significantly different by RFI class. These data support the hypothesis that RFI status is independent of a confounding influence of sub-clinical infection in heifers on test.