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The Role of Selenium Yeast in
Ruminant Feeding Programs |
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In livestock, selenium (Se) deficiency
appears to compromise immune function and is associated with a
higher incidence of health-related problems including mastitis,
retained fetal membranes, uterine infections, cystic ovaries, and
poor conception rates. In some geographic areas, marginal soil Se
concentrations can result in Se deficiency, even when rations are
fortified with 0.3 ppm of supplemental Se, the FDA maximum
inclusion level.
Selenium yeast (Se-yeast) has recently received FDA approval as a
source of supplemental Se for beef and dairy cattle. Numerous
suppliers have suggested Se-yeast has a higher biological
availability than the industry’s universal inorganic Se source,
sodium selenite. The cost of Se-yeast could be at least 10 times
greater than that of inorganic Se. For example, complete
replacement of supplemental inorganic Se with Se-yeast increases
ration cost about $2.00 per ton of treated feed or about 4˘ per
cow per day. Because of the high cost of Se-yeast as a Se source,
its biological and economic benefit must be evaluated carefully
for each livestock operation.
The Biological Role of Selenium and the
Bioavailability of Selenium Sources
The most consistent biological response to enhanced Se status in
animals is improved immune function. Improved immune function can
result in lower metabolic problems, as mentioned previously. The
source of Se supplementation is unlikely to produce significant
growth or milk yield responses and production studies are usually
insensitive to improved Se status of growing or lactating
ruminants.
Many organic trace minerals (e.g., chelates, proteinates) have
higher bioavailabilities than their inorganic counterparts. It is
logical to assume organic Se would also have higher
bioavailability compared with inorganic Se. However, Se has very
specific biological roles that must be considered when measuring
relative bioavailability of Se sources. There are a number of
functional selenoproteins in the body but only the selenoenzyme,
glutathione peroxidase (GSH-Px), has a nutritionally meaningful
role in metabolism. Blood GSH-Px activity or whole blood Se is
commonly used to detect bioavailability differences among Se
sources. The assumption for these methods is that organic Se
(e.g., Se-methionine) is more “bioavailable” compared with
inorganic Se.
Under normal dietary conditions, sodium selenite is readily
absorbed, converted to Se-cysteine, and then incorporated into the
GSH-Px enzyme. In contrast, supplemental Se-methionine is stored
as body protein and later extracted for further conversion to Se-cysteine.
This is a multi-step process that generally slows the movement of
Se to GSH-Px. Data exist showing that, across livestock species
maintained in typical production situations, feeding sodium
selenite and Se-yeast result in similar blood GSH-Px activity.
Weiss (2002) suggested that in typical dairy rations, Se in
Se-yeast may be about 20% more available for promoting GSH-Px
activity than Se from sodium selenite.
About 65% of the Se in Se-yeast is in the form of Se-methionine.
When Se-yeast is fed, the body uses Se-methionine as a source of
methionine for body protein synthesis. Enrichment of meat and milk
proteins with Se when Se-yeast is fed can have useful production
implications but this does not necessarily indicate higher
bioavailability. Se enrichment of milk occurs when Se-yeast is fed
because Se-methionine is used as a methionine source for protein
synthesis. Weiss (2003) summarized nine studies with lactating
ruminants and reported that milk Se concentration was 1.18 times
greater when animals were fed Se-yeast compared with inorganic Se.
Conditions Where Selenium Yeast May Be
Beneficial
Safety and Regulatory Restrictions: Certain markets do not
permit the use of inorganic Se. In addition, because Se-methionine
is primarily directed toward deposition in body tissues, Se-yeast
will be much less likely than sodium selenite to result in
toxicity to the animal if excess Se is consumed.
Presence of Dietary Antagonists: Considerable evidence
shows certain dietary antagonists may reduce the availability of
sodium selenite in ruminants. This may become important when
supplemental mineral intake is low and in low soil-Se geographies.
Sulfur is the most important antagonist in ruminants and has been
shown to result in linear decreases in Se digestibility from
inorganic Se, when sulfur concentrations were within ranges
typically encountered.
Increasing Selenium in Key Tissues: Increasing Se
concentration in the proteins of fetal tissues and in colostral
milk may yield health benefits. As such, there should be
measurable benefits to including Se-yeast in the diets of animals
during gestation and early lactation.
Economic Analysis and Recommended Usage
Rates for Se-Yeast
Complete replacement of supplemental Se from inorganic Se with
Se-yeast increases ration cost about $2.00 per ton of treated feed
or about 4˘ per dairy cow per day. Cost for beef cattle would be
about 2.6˘ per day because of lower feed intake levels. Assuming a
possible 20% greater GSH-Px activity when Se-yeast is fed compared
with inorganic Se, complete replacement of inorganic Se with Se
yeast is probably not cost-effective, even considering FDA limits
on supplemental Se.
The ADM technical service staff offers the following suggestions
for optimizing usage of organic Se from Se-yeast in ruminant
diets:
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Total dietary Se must not exceed the
FDA maximum of 0.3 ppm. There seems to be a consensus among
Se-yeast suppliers that 50% replacement of inorganic Se with
Se-yeast may be an economically appropriate. As such, it is
suggested that no more than 0.15 ppm of Se from Se-yeast be
included in ruminant diets and that no more than 1 mg/day of
supplemental Se as Se-yeast be included in beef rations and no
more than 3 mg/day of supplemental Se be included in the rations
of dairy cattle.
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Late pregnant and early lactation
animals will likely derive the highest economic benefit from the
use of Se-yeast, because Se stores in the fetus and colostrum
will also be increased with probable health benefits for the
offspring.
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Across all classes of growing and
breeding livestock, Se-yeast should probably be restricted to
only to those animals experiencing high stress with potentially
low feed intake.
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Vitamin E and Se have overlapping
roles in cellular protection against the damaging effects of
superoxides in the body. As such, a high level of diet
fortification with vitamin E will likely offset some of the
potential benefits of a more bioavailable Se source. For a cost
that is equal to that of replacing 50% of the inorganic Se with
Se-yeast, the level of supplemental vitamin E in the ration can
be increased 3 to 5 times. In a dairy ration, for example,
supplemental vitamin E could be increased from 1,000 to 3,000 IU
per cow daily at equal cost to replacing 50% of the inorganic
Se.
Differentiating Among the Selenium Yeast Products on the Market
Currently, there are at least five Se-yeast suppliers with
products on the market. All of these products should be equally
safe and all have FDA approval for use in animal feeds. It is the
ADM position that, on the basis of equal Se-methionine levels, all
of these products should have similar biological efficacy.
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Suggested Maximum Feeding
Rates for Se-Yeast |
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Se-Yeast, ppm
in ration |
Se-Yeast, mg/day |
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Dairy |
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Calves |
0.15 |
1 |
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Heifers |
- |
- |
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Close-up Dry Cows |
0.15 |
1 to 2 |
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Lactating cows (<120 DIM) |
0.15 |
3 |
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Lactating cows (>120 DIM) |
- |
- |
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Beef |
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Brood cows (late gestation to
breeding) |
0.15 |
1 to 2 |
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Stocker cattle |
- |
- |
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Feedlot cattle |
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Arrival period |
0.15 |
1 |
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Growing/finishing |
- |
- |
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