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Effects Of Monensin and Lasalocid on Mineral Metabolism
of Wethers Fed Bermudagrass Hay
S. M. Williamson, 1,2 E. B. Kegley, 2 D. L. Galloway, 2 T. J. Wistuba, 2 and K. P. Coffey 2
Story in Brief
Twenty-four crossbred wethers (initial BW 85 ± 5.7 lb) were used to evaluate the effects of monensin or lasalocid
on mineral metabolism in growing lambs. Wethers were randomly assigned to one of four treatments: 1) control, 2) 33
mg/d monensin, 3) 33 mg/d lasalocid or 4) immediately harvested. A corn-based supplement (0.2 lb DM) was individually
fed once daily throughout the experiment to administer monensin or lasalocid. Lambs were allowed ad libitum
access to bermudagrass hay and water while housed in individual pens. The average P concentration of the bermudagrass
hay was 0.39%. After 35 d, lambs were moved to metabolism crates for total feed, feces, and urine collection. A
7-d collection period followed a 7-d crate adaptation period. After the collection period, lambs were returned to individual
pens and continued on dietary treatments until d 77 or 78, when lambs were harvested. Fecal Mg excretion was
lower (P < 0.10) and urinary Mg excretion was greater (P < 0.01) for monensin-supplemented lambs when compared to
controls. Magnesium absorption, expressed as g/d (P < 0.10) and as a percentage of intake (P < 0.05), was greater for
the monensin-fed lambs compared to the controls. Although there were no effects of ionophore supplementation on the
retention of P (0.90 g/d and 26%), Ca, or Mg; monensin increased the apparent absorption of Mg in wether lambs.
Introduction
Previous research has shown that feeding ionophores
such as monensin and lasalocid increased the retention of
some minerals, including P, by sheep and cattle (Starnes et
al., 1984). This effect may prove to be important in the environmentally
charged search for ways to increase the retention
of certain minerals, such as P, in food animals and especially
in ruminants grazing waste-amended pastures. While there
has been a great deal of work involving the effect of
ionophores on mineral absorption and retention in ruminants
fed high-concentrate diets, there has been limited research on
the effects of ionophores on mineral absorption and retention
in ruminants fed forage-based diets (Spears et al., 1989).
Another area in need of further research is the explanation
of this increase in mineral retention and particularly to
determine the site of increased mineral deposition. Kirk et al.
(1985a) suggested that monensin may be acting directly on
the animal’s tissues in addition to the digestive system.
Therefore, this experiment evaluated the effect of monensin
and lasalocid supplementation on mineral metabolism and
tissue mineral concentrations of wether lambs fed bermudagrass
hay with a high concentration of P.
Materials and Methods
Twenty-four crossbred wethers (initially averaging 85
± 5.7 lb) were obtained from a single source. Wethers were
blocked by weight and fed bermudagrass hay and a cornbased
supplement. Wethers were randomly assigned to one of
four treatments (six wethers/treatment): 1) control, 2) 33 mg
monensin/d (Rumensin, Elanco Products Corp., Indianapolis,
IN), 3) 33 mg lasalocid/d (Bovatec, Hoffmann-La Roche Inc.,
Nutley, NJ), and 4) immediately harvested at d 0. Lambs from
all four treatments were humanely harvested at the University
of Arkansas Red-Meat Abattoir following industry-accepted
procedures.
Wethers assigned to treatments 1, 2, and 3 were
allowed to adapt to the diet for 35 d. The adaptation diets
included ad libitum access to bermudagrass hay and 0.2 lb
DM/wether of one of three supplements (Table 1). Chemical
composition of the supplements and hay are shown in Table
2. During the adaptation period, wethers were fed in individual,
raised metal pens (3.5 ft x 5 ft). Hay was offered to lambs
at 110% of the previous day’s intake.
Total collection period. On d 35, wethers were placed
in metabolism crates, designed for the total collection of feces
and urine. Lambs had ad libitum access to water and continued
to receive their appropriate supplement. Hay was fed during
the collection phase at 120% of the previous day’s intake.
There was a 7-d crate adjustment period followed by a 7-d
collection period.
Tissue sampling. Wethers were returned to individual
pens after the collection period. Treatments were continued
until d 77 or 78, when wethers were harvested to obtain tissue
samples. Tissue samples and organ weights were obtained
from the heart, longissimus muscle, liver, kidney, rumenreticulum,
and spleen.
1 Currently with the Department of Crop, Soil, and Environmental Sciences, Fayetteville.
2 Department of Animal Science, Fayetteville.
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