Ramirez, Justin A (2014-04). Effects of Residual Feed Intake Classification on Temperament, Carcass Composition, and Feeding Behavior Traits in Growing Santa Gertrudis Heifers. Master's Thesis.
Objectives of this study were to evaluate the associations of feed intake, feeding behavior, temperament, and carcass composition with performance and feed efficiency traits in growing heifers. Santa Gertrudis heifers (n = 369) with initial BW of (275.3 +- 37.4 kg) were used in this study. Intake and feeding behavior traits were collected for 70 d using a GrowSafe system while consuming a forage-based diet (ME = 2.19 Mcal/kg DM). Heifers were weighed at 14-d intervals and ultrasound traits measured on d 0 and 70. Residual feed intake was computed by regression of DMI on mid-test BW0^(.75) and ADG, and heifers classified into low, medium, and high RFI (+- 0.50 SD) groups. A 2- population distribution model was fit to log10-transformed non-feeding interval lengths to estimate meal criterion (MC), which was used to compute meal traits (frequency and duration). As expected, RFI was positively correlated with DMI (r = 0.72; P < 0.001), but not with initial BW or ADG. Residual feed intake was negatively correlated with G:F (r = -0.72; P < 0.001) and RG (r = -0.49; P < 0.001). In contrast to previous studies, RFI was not correlated with ultrasound LM area or back-fat thickness measured on day 70 of the trials. Residual feed intake was positively correlated (P < 0.05) with bunk visit (BV) frequency (r = 0.45) and duration (r = 0.35), and meal duration (r = 0.31), but negatively correlated with MC (r = -0.25). The R^(2) of the base RFI model (ADG, mid-test BW0.75) was 0.48, which increased to 0.69 with the inclusion of backfat thickness and feeding behavior traits (BV frequency and duration, MC, meal frequency and duration, and time to bunk). Heifers with low-RFI phenotypes had 21.5 and 23.2% lower (P < 0.001) DMI and G:F compared to high-RFI heifers. Heifers with low RFI had lesser (P < 0.001) BV frequency (64.7 vs 80.6 +- 1.6 events/d) and duration (95.7 vs 113.4 +- 3.6 min/d), higher (P < 0.01) MC (9.45 vs 8.12 +- 0.55 min) and greater (P = 0.39) time to bunk (TTB; 67.9 vs 58.3 +- 3.5 min) compared to high-RFI heifers. Between-animal variation in RFI was not associated with differences in ultrasound measurements of carcass composition in this study. However, variation in feeding behavior traits account for 41% additional variance in DMI beyond that associated with carcass ultrasound, ADG, and mid-test BW0^(.75). The longer MC observed in low-RFI heifers suggests that these heifers take longer to initiate subsequent meals compared to high-RFI heifers, and demonstrate that heifers with divergent RFI have distinctive feeding behavior patterns.