917
Relationship of metabolic hormones, urea and body composition with feed efficiency in Angus heifers carrying different genetic markers under grazing condition
The objective of this research was to explore potential physiological indicators of residual feed intake (RFI) in two beef cattle groups carrying simultaneously different genetic markers (GM) associated with low and high RFI (V and C groups, respectively) under grazing conditions. Twelve Angus heifers of each group (aged 369 ±28 days, 294 ± 37.4 kg of body weight (BW), at the beginning of the experiment) ranked by BW and RFI in a previous feedlot experiment were randomly assigned to four paddocks of an unrestricted high quality temperate pasture during 57 days. Concentrations of serum IGF-1, leptin and urea at day 1, 21 and 53, ultrasound subcutaneous fat thickness at day 1 and 56, (SFT), subcutaneous 12/13th rib fat depth, intramuscular fat percentage and eye muscle area at day 40 (SBF, IMF and EMA, respectively) as well as estimation of body composition by the urea dilution technique at day 56 (whole body fat content –BF% and whole body protein content – BCP%) were obtained. Data were assessed using a mixed model. Concentrations of IGF-1 did not differ between GM but tended (p <0.078) to be affected by the interaction between GM and sampling date, being lower in V than in C group at day 1 (274.3 vs. 321.5± 22.6 mm/L). Leptin concentrations tended (P=0.09) to be greater for V than C group (2.80 vs. 2.33± 0.24mm/L, respectively) and were neither affected by sampling date nor by their interaction. Serum urea concentrations were not affected by neither GM nor by its interaction with sampling date. The EMA, SFT, SBF, IMF and %BCP did not differ between GM, however estimated BF% and BF:BCP ratio were greater (P= 0.035, P= 0.038, respectively) in V than in C group (23.7 vs. 20.6 ± 1.29%, 1.64 vs. 1.39 ± 0.12). Leptin concentration and BF% were both negatively correlated (r = -0.43 p = 0.037 and r = -0.38, p = 0.071, respectively) with RFI. Our data suggest that leptin concentration and BF% could be used to screen for more efficient females under grazing conditions. Other experiments should be designed to uncover additional indicators underlying variation in RFI under grazing conditions.
Keywords:
residual feed intake, beef cattle, pasture