Relationship between feed intake, feeding behaviors, performance, and ultrasound carcass measurements in growing purebred Angus and Hereford bulls.

The objectives of this study were to examine the growth, DMI, and feeding behaviors of Angus and Hereford bulls; identify the relationships between feeding behaviors and variation in DMI and residual feed intake (RFI); and determine the value of feeding behaviors in predicting DMI. Individual DMI was measured in Angus bulls (n=189; initial BW=427±3.4 kg) and Hereford bulls (n=146; initial BW=411±4.1 kg) fed a grower ration for 71 d in 2009, 78 d in 2010, and 74 d in 2011 using a GrowSafe intake monitoring system. Feeding frequency (FF, meals/d), head down duration (HDD, s/d), head down duration per meal (HDDM, HDD/FF, s/meal), average meal size [AMS, kg/(meal·d)], and feeding rate (FR, g/s) were also measured or calculated using behavior data collected by the GrowSafe system. Ultrasound measures of 12th-rib fat thickness (UFT), longissimus muscle area (ULMA), and intramuscular fat (IMF) were determined during the midtest-weight event of every trial. The data from 3 yr were pooled to generate mean differences between the breeds. Residual feed intake was calculated using a linear regression of DMI on ADG and midtest BW0.75 (MMWT). Animals were classified into 3 RFI groups based on their RFI score as Low (>0.5 SD below the mean), Average (±0.5 SD from the mean), or High RFI (>0.5 SD above the mean). Angus bulls in the Low RFI group consumed 17% (P<0.0001) less DM than the bulls in the High RFI group, while in the Hereford bulls there was a 14% (P<0.0001) difference in DMI between Low and High RFI groups. Significant phenotypic correlations were observed between RFI and DMI (0.83, 0.77), G:F (-0.65, -0.51), HDD (0.41, 0.59), HDDM (0.40, 0.53), AMS (0.52, 0.36), and FR (-0.31, -0.51) in Angus and Hereford bulls, respectively. The HDD, HDDM, and FR were significantly correlated with DMI. The feeding behavior traits, HDD, HDDM, and FR when added to the RFI base model, explained 18, 17, and 13%, respectively, of the variation in DMI not explained by ADG and MMWT in Angus bulls. Similarly, in Hereford bulls, HDD, HDDM, and FR explained 35, 26, and 24%, respectively, of the variation in DMI not explained by ADG and MMWT. These data suggest that feeding behaviors are related to DMI of growing Angus and Hereford bulls.