HAY PRESERVATIVES: PROPIONIC ACID AND MICROBIALS
Limin
Kung, Jr., Ph.D.
Ruminant Nutrition & Microbiology Laboratory
Alfalfa is usually wilted 15% or less moisture prior to
baling for hay. However, harvesting at low moistures increases loss of leaves
(protein) during baling and reduces nutritive value. In addition, long wilting times increases the risk for damage
from rain. In order to
retain more nutrients and prevent rain damage, hay can be baled at
higher moistures but the risk of spoilage from undesirable microbial
metabolism during storage increases dramatically.
Although much is known about the microbiology of silage, less is
known about microbial metabolism in moist hay.
Although bacilli can be associated with spoilage of wet hay (>
20% moisture), most suggest that fungi are the microbes that cause
spoilage problems in moist hay. Moist
hay undergoes several distinct heating cycles when stored in stacks.
The significance of the heating peaks and associations with
either plant or microbial metabolism are poorly understood.
It does appear, however, that if the moist hay does not undergo
some heating, the bales never "dry down" during storage.
Excessive heating of the hay bale can lead to an increase in heat
damaged protein (acid detergent insoluble nitrogen, ADIN).
Heat-damaged protein lowers the protein value of the hay.
Various hay preservatives inhibit mold and may be useful in
reducing drying time and field losses by allowing baling at higher
moisture levels. If
properly done, baling at higher moisture levels can reduce dry matter
loss (from leaf shattering) and potential damage from rain.
The two most commonly used preservatives for high moisture hay
are bacterial inoculants and propionic acid-based solutions.
Propionic acid-based preservatives have been used for many years.
Propionic acid is most effective in inhibiting growth of molds
> yeast > bacteria. Use of concentrated propionic acid has declined primarily
because of it is corrosive and volatile.
In recent years, there has been much interest in
"buffered" propionic acid mixes that have a pH of about 5.5 to
6. These products are based
on ammonium or sodium salts of propionic acid.
Research shows that use of buffered propionic acid was as
effective as unbuffered acid in preserving the quality of moist alfalfa
hay (about 30% DM). In one
study, significant improvements in nutrient yield were found when a
buffered propionic aid product was added to moist (13.8 to 25.2%) large
alfalfa bales. However, the
improvements were not large enough to pay for the preservative.
Researchers at the US Dairy Forage Research Center in Wisconsin
tested several propionic acid-based preservatives and found that at high
application rates (40 lb/ton) these products limited temperature rises
during storage but did not consistently control DM losses or bale
quality.
In order for propionic acid to be effective, correct levels must
be used. Results of a study
are shown below (Table 1). At
least 1% propionic acid was needed to preserve hay with 32% moisture.
Table 1. Effect
of propionic acid on high moisture hay (32% moisture).
|
Treatment
|
Maximum storage temp., oF
|
Dry weight loss,
%
|
In vitro DM
digestion, %
|
|
Control
|
124
|
15.1
|
60.5
|
|
Propionic acid
|
|
|
|
|
.02%
|
127
|
16.7
|
61.8
|
|
.2%
|
115
|
13.2
|
62.2
|
|
.5%
|
104
|
11.7
|
61.0
|
|
1.0%
|
86
|
7.6
|
65.0
|
Knapp et al. 1976.
Recommended levels of propionic acid addition to moist hay are
shown in Table 2. Note that
these are values for a 100% propionic acid solution.
In the market, solutions will vary from 10 to 100% propionic
acid. The use of very dilute products are not recommended because
larger volumes of water are applied to the crop. (Why add more water to your hay?)
Depending on the product and application rate, it will cost $5 to
20/ton of hay to treat with propionic acid-based products.
Seldom does the increase in leaf retention pay for the use of the
additive. However, acid
preservation may be most beneficial when the producer is faced with
potential loss from rain damage. Uniform
distribution of propionic acid is important since "pocketing"
of molds has been observed (only parts of the bale being moldy).
Table 2. Use
of propionic acid for high moisture bales.
% propionic acid1/wet
forage
suggested levels
% moisture
I2
II3
20-25
0.9%
0.5% (10 lb/ton)
26-303
1.13%
1.0% (20 lb/ton)
1100%
propionic acid equivalent
2Mathison
and Baron 1991.
3Walgenbach
as cited by above.
Microbial inoculants can improve silage fermentation and animal
performance. Because of
this success, there has been much interest in using microbial inoculants
to reduce spoilage in moist hay. Advantages
of an efficacious additive would include lower cost, lower liquid
application rate, and use of a biologically safe product
(non-corrosive). Early
studies tested lactobacillus-type products developed for silage with few
positive effects. The logic
was that lactobacilli would produce acid, reduce pH and thus reduce mold
growth. However, this logic
was slightly flawed since many fungi are acid tolerant and many yeasts
assimilate lactic acid. In
addition, the water activity (moisture available for microbial growth)
of moist hay appears to be insufficient to support growth of most
lactobacilli. Although
there is use and farmer testimony that silage inoculants can be used to
preserve moist hay, there is little scientific evidence to support these
claims.
Past research has centered on developing microbial inoculants
specifically for moist hay. Although
some of these products still contain lactic acid bacteria, others
contain Bacillus sp. Bacilli
are spore-forming organisms that grow more readily in an environment
with low water activity and are also quite heat resistant.
More importantly, these inoculants have been screened in the
laboratory for inhibitory effects on storage fungi in hay. The mode of fungal inhibition is unknown, but we do know, for
example, that certain bacilli produce antibiotics and some have
suggested that bacilli produce a lipoprotein that has antifungal
properties. Competition for
nutrients and space, and production of unidentified antifungal compounds
have also been suggested as modes of action.
Caution should be used when interpreting laboratory data since
inhibition of fungi on petri plates, under controlled conditions, does
not always equate to improvements in the stability of a moist bale.
The recent approach in developing microbial inoculants appears
sound but translation of laboratory results to the field have yet to be
proven.
Microbial inoculants have been applied at the baler but there is
some discussion among researchers on whether application might be more
beneficial at mowing. The
rationale for this is to suppress fungal populations that may increase
while hay is drying; especially at the bottom of a moist windrow.
In general, the responses to microbial inoculants in hay have
been poor to inconclusive. One
report showed that addition of a bacteria called Bacillus
pumilus or a silage inoculant (bacteria:S.
faecium and L. plantarum)
to moist hay (17 to 31% moisture) improved color, odor and other visual
assessments but the inoculants had no effect on chemical composition of
the hay. However, more
often than not, others workers (including work done at the University of
Delaware) using a variety of products have shown no response to
microbial inoculation on high moisture hay.
Over several years, researchers at the USDA Dairy Forage Center,
Madison, Wisconsin, have never seen a positive response from any hay
inoculant (R. Walgenbach, personal communication).