One hundred British and British cross
steers, averaging 631 lb ( initial wt) were used in
a growing and finishing study to evaluate the
effects of unprotected amino acid
supplementation on cattle performance and
carcass characteristics. All diets contained 1%
of a nonprotein nitrogen source, and treatments
were: no additional supplemental protein
(UREA), 2) supplemental protein from soybean
meal (SBM), 3) 13 grams/day of an amino acid
supplement (Low AA), and 4) 26 grams/day of
an amino acid supplement (High AA). The
Low AA treatment supplied 2 grams
methionine, 8 grams lysine, 2 grams threonine,
and 1 gram tryptophan per day, whereas the
High AA treatment provided twice those
amounts. The grower diet was based on whole-plant
sorghum silage, and the finishing diet was
based on rolled corn and corn silage. During
the growing period, gains were higher (P<.05)
for SBM-supplemented steers than for UREA
steers and intermediate for amino acid-supplemented
steers. Intakes were higher for
steers supplemented with Low AA than for
those supplemented with UREA or High AA.
Few significant differences among treatments
were observed in cattle performance during the
finishing period. Hot carcass weights, dressing
percentage, KPH fat, and yield grade were
unaffected by amino acid supplementation. In
this study, supplementing growing and finishing
cattle with unprotected amino acids did not
significantly improve steer performance or
carcass characteristics, suggesting either that
these amino acids were not limiting in these
steers or that not enough of these supplemented
amino acids escaped ruminal degradation to
affect steers’ performance.
