Residual feed intake (RFI) is calculated as the difference between an
animal’s actual feed intake and its expected feed intake based on its
size and growth over a specified test period. Following a single generation of
divergent selection for postweaning RFI, Angus steers and Angus ×
Hereford, Angus×Poll Hereford and Angus × Shorthorn
crossbred steers born in 1996 and 1997 were fed in a feedlot. Cohorts of
steers were slaughtered at the same age and had attained similar
(P>0.05) final liveweights: 467 kg for steers
selected for low RFI (high efficiency; HE, n
= 91) and 459 kg for steers selected for high RFI (low efficiency;
LE, n = 98). The HE and LE steers had similar
(P>0.05) carcass weight (247 and 244 kg),
dressing percentage (53.1 and 53.2%) and eye-muscle area (58.9 and
60.3 cm 2 ). The HE steers had slightly less
subcutaneous fat over the rib than the LE steers (9.2 v.
10.1 mm, P<0.05), and there was a trend towards
less fat over the rump of HE steers (11.5 v. 12.1 mm,
P = 0.10). For meat samples taken from the
M. longissimus dorsi (LD) there were no differences
(P>0.05) between the HE and LE steers in content
of intramuscular fat (5.4 and 5.3% fresh weight), marbling scores, meat
colour and fat colour. There were also no differences
(P>0.05) between HE and LE steers in shear force
and compression values for samples of LD aged for 1 day (4.6 and 4.6 kg shear
force, 1.45 and 1.44 kg compression), or for 14 days (3.8 and 3.5 kg, 1.36 and
1.32 kg). Myofibril fragmentation index (MFI) measures the breakdown of these
structural elements which occurs as an initial step in the process of protein
degradation and meat tenderisation. MFI was lower (i.e. less fragmentation;
P<0.05) in LD samples from HE steers than from LE
steers, both in samples aged for 1 day (67.7 v. 72.5
units) and in samples aged for 14 days (87.8 v. 91.1
units). The rate of decline in MFI between 1 and 14 days post slaughter was
similar in the LD samples from both lines of steers. There were no differences
(P>0.05) between HE and LE steers in the activity
of m-calpain and µ-calpain in LD immediately after slaughter (HE
steers: 1.9 and 2.3 units, LE steers: 1.8 and 2.1 units). The level of
calpastatin in LD from the HE steers was 13% higher than in the LD from
the LE steers (5.2 and 4.6 units respectively,
P<0.05). Rate of myofibril fragmentation was
positively correlated (P<0.01) with the ratios of
both m-calpain and µ-calpain to calpastatin, but not
(P>0.10) with levels of either calpain or
calpastatin. A single generation of divergent selection for RFI produced
differences in calpastatin and myofibril fragmentation that may, with on-going
selection for low RFI, negatively affect meat tenderness.