Plasma analytes concentrations of beef heifers diverging in frame size and known feed efficiency group

Abstract Text: Blood plasma analytes might reflect important variation in metabolism and be associated with efficiency of feed utilization and also animal size. Thus, objectives were to evaluate the concentrations of glucose and plasma urea-N in beef heifers from two frame size groups and also divided into two residual feed intake (RFI) groups using two prediction models as measures of feed efficiency. A group of 89 crossbred heifers (initial BW=342±3.4 kg, n=89) was tested over a 106-d period. Heifers were fed a forage-based diet allowing for ad libitum consumption. Feed intake was individually measured using the Insentec feeding system. Heifers were sorted into two groups based on frame score: small to moderate (<5.5, average frame size=4.27±0.132, average BW=317±6.1 kg, n=49), or moderate to large (≥5.5, average frame score=6.45±0.191, average BW=371±11.8, n=40). Body weights were measured and blood was collected on d 1, 14, 28, 44, 59, and 106.  Concentrations of glucose and urea-N were determined in plasma. Residual feed intake was calculated including body weight and gain (RFI_Koch) and by also including ultrasound for body composition (RFI_us). Heifers were divided into low (more efficient) and high (less efficient) RFI_Koch and RFI_us groups. Data were analyzed with the MIXED procedure of SAS using repeated measures. Glucose concentration did not differ between frame size groups (P=0.60) or between RFI_Koch groups (P=0.65). Plasma urea-N concentration was lower (P<0.01) for small to moderate heifers in comparison to moderate to large heifers (14.27 vs. 14.94 mM). Plasma urea-N did not differ among RFI_Koch groups (P=0.44).  There was an interaction (P<0.001) between RFI_us groups and sampling day for both plasma glucose (d 1, 28 and 44) and urea-N (d 1 and 28), as more efficient heifers had higher levels (4.55, 4.30 and 3.64 mM glucose; 16.41 and 15.28 mM urea-N; respectively) if compared to less efficient heifers (4.09, 3.75 and 3.02 mM glucose; 12.76 and 13.17 mM urea-N; respectively).  These results indicate that glucose and urea-N may differ among body composition corrected efficiency groups and that urea-N may differ among frame size groups. Thus, accounting for body composition in the RFI model may result in a better prediction of physiological differences in blood metabolites, and consequently basal metabolism. In addition, supplementary studies will be conducted to further investigate the biological differences of blood analytes in animals differing in feed efficiency.

Keywords: glucose, residual feed intake, urea-N