- The objectives of this study were to investigate the effects of
the by-product, ryegrass straw, and the feed additive, monensin, on the
performance of feedlot lambs.
Four pelleted rations were fed ad libitum: Ration 1 - 50% barley,
50% alfalfa hay; Ration 2 - 50% barley, 50% alfalfa hay and 25g/ton
monensin; Ration 3 - 50% barley, 25% alfalfa hay and 25% ryegrass
straw; Ration 4 - 50% barley, 25% Alfalfa hay, 25% ryegrass straw and
25g/ton monensin. The two levels of ryegrass straw (0 and 25%) and
the two levels of monensin (0 and 25g/ton) were arranged factorially
across the four rations. Sixty single-reared lambs sired by Hampshire
rams started the trial shortly after weaning at an average weight of
29.15 kg. Fifteen lambs were fed each ration; five lambs in each of
three pens. Lambs were weighed weekly and terminated from the trial
when they reached 45.45 kg in unshrunk body weight. Two lambs from each
pen were slaughtered and carcass traits obtained. A sample of the rumen
fluid was taken from each slaughtered lamb to determine volatile fatty
acid (VFA) concentrations. Some of the traits of interest were ration
dry matter digestibility (DMD), lamb average daily gain (ADG), feed intake (FI) and feed efficiency (FE: feed/gain), carcass yield grade
(YG), total volatile fatty acid content of rumen fluid (TVFA) and cost
per unit of gain (CUG).
Results indicated that monensin-fed lambs had a lower feed intake
(-7.8%, P<.10), TVFA (-24%, P<.01), and ADG (-7.3%); an improved FE
(+4.7%); an increased DMD (+1.7%) and a similar YG as non-monensin-fed
lambs. Ryegrass straw fed-lambs had a lower DMD (-8.0% P<0.05), TVFA
(-18.3%, P<0.01) and ADG (-7.3%); a poorer FE (-5.4%); an increased
FI (+6.0%) and a similar YG as non-ryegrass straw-fed lambs. The acetic:
propionic acid ratio was 2.72, 3.02, 3.07 and 2.67 for monensin, nonmonensin,
ryegrass straw and non-ryegrass straw-fed lambs, respectively.
CUG was lower (-1.8 for monensin than for non-monensin-fed lambs and
lower (-8.0%) for ryegrass straw than for non-ryegrass straw-fed lambs.
Estimated cost per kg carcass gain was lower (-1.1%) for monensin than
for non-monensin-fed lambs and higher (12.6%) for ryegrass straw than
for non-ryegrass straw-fed lambs.