ABSTRACT: Two hundred sixteen crossbred barrows
and gilts (84.3 kg BW) were used to test the effects
of dietary energy density and lysine:energy ratio
(Lys:ME) on the performance, carcass characteristics,
and pork quality of finishing pigs fed 10 ppm ractopamine.
Pigs were blocked by BW and gender, allotted
to 36 pens (six pigs per pen), and pens were assigned
randomly within blocks to dietary treatments (as-fed
basis) arranged in a 2 3 factorial design, with two
levels of energy (3.30 or 3.48 Mcal/kg) and three Lys:ME
(1.7, 2.4, or 3.1 g lysine/Mcal) levels. Pigs were fed experimental
diets for 28 d, and weights and feed disappearance
were recorded weekly to calculate ADG,
ADFI, and G:F. Upon completion of the feeding trial,
pigs were slaughtered and carcass data were collected
before fabrication. During carcass fabrication, hams
were analyzed for lean composition using a ham electrical
conductivity (TOBEC) unit, and loins were collected,
vacuum-packaged, and boxed for pork quality data collection.
Energy density had no (P 0.22) effect on ADG
or ADFI across the entire 28-d feeding trial; however,
pigs fed 3.48 Mcal of ME were more (P 0.02) efficient
than pigs fed 3.30 Mcal of ME. In addition, ADG and
G:F increased linearly (P 0.01) as Lys:ME increased
from 1.7 to 3.1 g/Mcal. Carcasses of pigs fed 3.48 Mcal
of ME were fatter at the last lumbar vertebrae (P
0.08) and 10th rib (P 0.04), resulting in a lower (P
0.03) predicted fat-free lean yield (FFLY). Conversely,
10th-rib fat thickness decreased linearly (P = 0.02),
and LM depth (P 0.01) and area (P 0.01) increased
linearly, with increasing Lys:ME. Moreover, FFLY (P
0.01) and actual ham lean yield (P 0.01) increased
as Lys:ME increased in the diet. Dietary energy density
had no (P 0.19) effect on pork quality, and Lys:ME
did not (P 0.20) affect muscle pH, drip loss, color, and
firmness scores. Marbling scores, as well as LM lipid
content, decreased linearly (P 0.01) as Lys:ME increased
from 1.7 to 3.1 g/Mcal. There was a linear (P
0.01) increase in shear force of cooked LM chops as
Lys:ME increased in the finishing diet. Results indicate
that 3.30 Mcal of ME/kg (as-fed basis) is sufficient for
optimal performance and carcass leanness in pigs fed
ractopamine. The Lys:ME for optimal performance and
carcass composition seems higher than that currently
used in the swine industry; however, feeding very high
Lys:ME (3.0 g/Mcal, as-fed basis) to ractopamine-fed
pigs may result in decreased marbling and cooked
pork tenderness.