There are a number
of factors affecting vitamin stability in animal feeds. These factors may vary
depending on whether the vitamins are naturally occurring in the feed
materials, or added to the diet in a premix form.
The following is a discussion of these factors in either
case, together with the strategies to be adopted for improving the stability
and utilisation of the dietary vitamins by animals.
Symptoms of a vitamin A
deficiency include reproductive problems in both males and females. Photo: Henk
Riswick
Forage plays a significant role in the supply of vitamins
to ruminants. However, the vitamin content of forages is highly variable and
unpredictable due to the following stability factors:
Forage species
Grasses generally have the lowest level of ᵦ-carotene
(146 mg/ kg DM) while legumes have the highest level (438 mg
kg21 DM). The differences in the level of ᵦ-carotene are mainly due to the
ratio of leaf to stem in the plant and capacity of the plant to synthesize
carotene
Stage of maturity
The levels of ᵦ-carotene and 𝛼-tocopherol in the
grasses and legumes are very high in the young stages and
reduce as the plant matures. At maturity, plants may have 10% (in the case of
grasses) to 40% (for legumes) of the value of the carotene of immature plants.
The principal factor responsible for the variation in levels of ᵦ-carotene and 𝛼-tocopherol of
forages in the course of their maturation is the change in the ratio of leaf to
stem because the leaves are considerably richer in these vitamins than the
stems. The formation of stems is accompanied by an increase in the
concentration of DM; there is thus a negative correlation between the DM
content and the level of ᵦ-carotene.
Climatic conditions
For a given stage of growth, a forage is richer in
ᵦ-carotene and 𝛼-tocopherol
when grown under rainy conditions with a low temperature. The positive
influence of these conditions is related to the increase in the leaf-to-stem
ratio which contributes more to the increased level of ᵦ-carotene 𝛼-tocopherol.
Haymaking
Drying crops either on the ground or in barns reduces the
vitamin levels. It was found that in excess of 80% of carotene from clover was
lost during the first 24 hours of sun-drying and was practically zero when the
crop was dried for 4–5 days in the sun. Forages exposed to rain and then dried
in the sun have less ᵦ-carotene than sun-dried forage. Thus, if the forage
rests exposed to the sun for an extended period of time and at the same time is
exposed to several showers, the destruction of ᵦ-carotene is nearly complete.
Ensiling
Stability of some vitamins such as vitamin A may be
adversely affected if mouldy or spoiled silages are fed for long periods.
Symptoms of a vitamin A deficiency include reproductive problems in both males
and females. Pregnant cows may abort, retain their placenta, and develop a
uterine infection or give birth to weak, dead, or blind calves. Bulls with a
vitamin A deficiency produce semen with low numbers of sperm and high numbers
of abnormalities. To prevent moulds from developing in silage, production
practices that preserve quality should be strictly followed. Accepted
silage-production practices include:
- Harvesting at
the proper moisture content (30-35 %)
- Chopping
uniformly at the proper length to allow for better compaction and exclusion
of air
- Silo size
should be matched to herd size to ensure daily removal of silage at a rate
faster than deterioration can occur
- Filling the
silo rapidly
- Packing the
silage sufficiently
- Covering the
horizontal silo immediately during or after filling
- Using silage
additives (such as ammonia, propionic acid, microbial cultures, or
enzymatic silage) may be beneficial in preventing mould growth.
With the advent of intensive livestock production, the
production of synthetic vitamins in a premix form was essential due to the
variable and unpredictable vitamin content of forages.
Stability of the synthetic vitamins may, however, be
influenced by the following factors:
Composition of the premix
Vitamins are quite sensitive to their physical and
chemical environment, with oxidation-reduction reactions due to contact with
trace minerals being the predominant cause of vitamin instability. The type of
trace mineral can have a significant effect on vitamin stability. Free metal
ions, sulfates, carbonates, and oxides (the primary forms of inorganic trace
minerals used) are the most reactive, while chelates (a classification of
organic trace minerals) are the least responsive. Therefore, the current
practice of over-fortification with inorganic trace minerals can inadvertently
have a negative effect on vitamin stability and, thus, negatively impact animal
health and performance indirectly.
Pelleting
Pelleting is typically the most aggressive process
against vitamins due to exposure to heat. There is, however, little or no
vitamin destruction with pelleting temperatures of up to 80°C. At higher
pelleting temperature, chemical modification can enhance the stability of some
vitamins such as vitamin C which is easily oxidised and destroyed in this case.
The esterification of the 2-carbon atom of L-ascorbic acid with phosphate
protects L-ascorbic acid from oxidation. Coatings or encapsulation with
carbohydrate, protein, or ethyl cellulose gives vitamins such as vitamin A and
D3 greater protection against heat, moisture, and pressure during pelleting.
Storage
Premixes
containing vitamins can be stored for about 3 to 4 months. However, storage
time should not exceed 60 days if choline and trace minerals are present in
combination with vitamins in the premix. Also, the use of barriers such as
plastic-lined bags aid in reducing the absorption of moisture, thereby
improving vitamin stability.
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