PROCEEDINGS - American Society of Animal Science

NEg on a dry matter basis. The finisher ration was
comprised of 10.76% CP, 5.53% CF, 4.03% fat, 1.81
Mcal/kg NEm, and 1.15 Mcal/kg NEg on a dry matter
basis. Average start and finish weights were collected. DMI
data were collected during the test using the GrowSafe
automated feeding system (GrowSafe Systems Ltd.,
Airdrie, Alberta, Canada). A certified ultrasound technician
using an Aloka 500 real-time unit equipped with a 3.5-MHz
transducer collected all ultrasound data.
Three different measures of feed efficiency were
calculated for each animal. Feed conversion ratio (FCR)
was determined as the ratio of DMI to ADG. Partial
efficiency of growth (PEG) was computed as the ratio of
ADG to DMI for growth (Koch et al., 1963). RFI as defined
by Koch et al. (1963) was calculated for each animal as the
difference between actual intake and intake predicted by the
stepwise linear regression used to determine the order of
inclusion of carcass characteristic and the significance of
trial to reach the final regression model of DMI on ADG,
MMWT (BW 0.75 ), and uFT (Statistix9, 2008). Bulls were
classified using RFI into low (0.5 SD; n=41; RFI=0.887 kg/d) groups (Basarab et al.,
2003).
Data were analyzed by ANOVA fitting RFI group
as the independent variable (Statistix9, 2008). All pairwise
comparisons were made using Tukey HSD (Statistix9,
2008). Relationship between RFI and phenotypic
performance traits and carcass traits were established using
Pearson Correlation (Statistix9, 2008).
Results and Discussion
Favorable differences in FCR, PEG and daily
intake were detected among RFI groups. Low (11.120 kg)
RFI grouped bulls exhibited significantly reduced DMI
compared to marginal (12.091 kg) and high (12.920 kg)
RFI grouped bulls (Table 1). Significant differences in DMI
were also detected among marginal and high RFI grouped
animals (P=0.00). Lancaster et al. (2005) reported a 15%
reduction in feed intake between low and high RFI bulls,
despite no detection of differences in ADG and body
weight. Similar trends existed for FCR. Low grouped bulls
exhibited significantly lower FCR (5.74 kg; P=0.00)
relative to marginal (6.43 kg) and high (6.94 kg) grouped
bulls. Favorable group differences were apparent for PEG.
Low RFI bulls had higher PEG (0.493; P=0.00) than high
RFI bulls (0.320). Marginal RFI bulls also had significantly
higher PEG (0.380; P=0.00) than high RFI bulls. No
significant group differences were detected in on-test
weight, off-test weight, or ADG indicating that selecting for
reduced RFI may result in improved feed efficiency with
minimal impact on growth (Table 1). Residual feed intake
has been shown to be genetically and phenotypically
independent of its component traits (Arthur et al., 2001).
This finding indicates that changes to component traits will
not likely result from selection based upon improved RFI.
Analyses of group means for phenotypic carcass
characteristics are included in Table 2. No significant group
differences were detected in uREA, uFT, or uPIMF.
Comparisons made were consistent with the findings of
Cardin et al. (2008), indicating low RFI grouped bulls had
numerically lower uFT. Robinson and Oddy (2004)
suggested that selection for reduced RFI would likely result
in decreased subcutaneous fat. Additionally, bulls did not
differ based upon RFI group in ultrasound carcass trait
EPDs (Table 3). Lancaster et al. (2008) noted that RFI has
been weakly correlated with twelfth rib fat thickness. As
expected, accounting for body composition in the
regression model removed these differences. Basarab et al.
(2003) reported that including body composition in the
model to determine RFI accounted for more variation in
DMI. Lancaster et al. (2009) reported that adjusting RFI for
carcass composition only minimally impacts animal ranking
in growing animals when compared to using only MMWT
and ADG, yet in finishing animals, the relationship between
body composition and RFI is stronger suggesting it may be
favorable to include body composition in the regression
model to calculate RFI.
Phenotypic correlation indicated that RFI
was significantly and favorably correlated with FCR (0.490;
P=0.00), a finding corroborated by numerous investigations
(Jensen et al., 1992; Herd and Bishop, 2000; Hoque et al.,
2005; Hoque et al., 2006; Cardin et al., 2008). RFI was also
significantly correlated with PEG (-0.898; P=0.00),
indicating that selection for reduced RFI would likely result
in increased PEG. Lancaster et al. (2005) reported similar
findings when RFI was correlated with PEG (-0.85),
indicating bulls with low RFI had higher PEG than both
marginal and high RFI bulls. RFI was significantly
correlated with dry matter intake (0.702; P=0.00). Hoque et
al. (2006) and Arthur et al. (2001) reported correlation
values of 0.72 and 0.69, respectively, for RFI and DMI.
Furthermore, Carstens et al. (2002) and Nkrumah et al.
(2004) reported that RFI was significantly correlated with
ultrasound back fat (0.22 and 0.19 respectively; P