Effect of a propionic acid bacterial inoculant on fermentation and aerobic stability of whole-plant corn silage

The effects of a strain of Propionibacterium shermanii, applied with and without lactic acid bacteria (LAB), on the fermentation and aerobic stability of whole-plant corn silage was determined using laboratory-scale silos. The addition of LAB increased the rate of fermentation, and all inoculated silages underwent a more efficient ensiling process than control silage. Only silages made with P. shermanii had measurable levels of propionic acid in the 90-day silages. Corn silages made with P. shermanii were more stable when exposed to air than control or LAB-inoculated silages.


Introduction
Aerobic instability during the feedout phase (i.e., short bunk life) is often a problem with whole-plant corn, sorghum, and winter cereal silages, particularly if the dry matter (DM) content exceeds 3 5%.When the silo is opened, oxygen has unrestricted access to the exposed feeding face.Aerobic microorganisms (i.e., yeast, mold, and bacteria) present in the silage can consume soluble nutrients, including lactic acid, which increases the te mperature and pH of the silage.
If allowed to continue, the deterioration and spoilage can cause a silage to have virtually no nutritional value.A rapid removal of silage and correct sizing of the height and width of the feeding face can help minimize DM losses during the feedout phase.
Presently, there is considerable interest in using biological inoculants to overcome the problem of aerobic instability.These additives should produce substances in the silage that have antimycotic properties, which would inhibit the growth of yeast and mold in silage exposed to air.Propionic acid bacteria can ferment soluble carbo hydrates and lactic acid to propionic acid, which is an eff ective antimycotic agent.Although the production of propionic acid during the fermentation phase is a sound concept, results in a limited number of controlled experiments have been inconsistent.
Our objective was to deter mine the effect of a propionic acid bacterial inoculant on aerobic stability of whole-plant corn silage.

Experimental Procedures
A description of the whole-plant corn used in the trial, including harvest date, chemical composition, and epiphytic microflora, is presented in Table 1.The hybrid was Pioneer 3377, grown under irrigation and harvested at approximately 60 to 70% milk line stage of kernel maturity.The following six treatments were compared: 1) control (no additive), 2) Lallemand inoculant (to supply 700,000 cfu of LAB per gram of crop) (LAB), 3) Propionibacterium shermanii (to supply 1.4 10 cfu per gram of 5 crop) (PS 10 ), 4) treatment 2 + treatment 3, 5) In preparation of the silages, 250 lb of chopped forage were placed on a polyethylene sheet, and the inoculants were applied and mixed thoroughly with the forage.All inoculants were applied as water solutions and used within 2 weeks after being received from the manufacturer.After all silages were prepared, the silos were filled on an alternating schedule, which distributed the time from harvest through silo filling equally across all treatments.The silos were packed with a hydraulic press, which exclude d air and filled all silos to similar densities.There were 21 PVC laboratory-scale silos (4.5 lb capacity) and three polyethylene pail silos (35 lb capacity) per treatment.Three PVC silos per treatment were opened at 1/4, 1/2, 1, 2, 4, 7, and 90 days postfilling.The pail silos were opened at 90 days postfilling, and the silage from each treatment was composited.Aerobic stability was measured over a 10-day period using insulated, 4.5 lb capacity containers and thermocouple wires.

Chemical and Microbial Analyses of the
Pre-ensiled Forage and Silage s.Pre-ensiled forage was analyzed for DM; pH; total nitrogen; buffering capacity; water-solu ble carbohydrates; neutral detergent fibe r; acid detergent fiber; and total epiphytic LAB, yeast, and mold counts.Silage fermented from 6 hours to 7 days was analyzed for pH and lactic acid; end-product silages (90 days postfilling) were analyzed for pH, lactic acid, volatile fatty acids, ethanol, ammonia-nitrogen, and total yeast and mold counts.Silages in the aerobic stability measurements were analyzed for pH and total yeast and mold counts after 2, 4, 6, 8, and 10 days of exposure to air.

Results and Discussion
Although all six corn silages had very rapid rates of fermentation, those made with LAB (Treatments 2, 4, and 6) had lower (P<.01)pH values and higher (P<.01) lactic acid contents during the first 4 days postfilling than control silage (data not shown).
Fermentation characteristics and pH for the corn silages at 90 days postfilling are presented in Table 2.All five inoculated silages ensiled more efficiently than the control silage --as evidenced by higher (P<.05 ) lactic acid contents and lactic to acetic acid ratios and lower (P<.05)acetic acid and ethanol contents.These results are consistent with several previous studies on inoculated corn and sorghum silages (KAES Report of Progress 651, page 101).Only the four silages made with P. shermanii (with or without LAB) contained detectable amounts of propionic acid at 90 days.However, the level of this acid varied considerably among silages for each treatment and ranged from .06 to .33% of the silage DM.
Results of the aerobic stability measurements are shown in Tabl es 3 and 4. Corn silage treated with only PS 10 (Treatment 5) was 6 clearly the most aerobically stable, and this silage also had the highest level of propionic acid (.22%).Corn silage made with only LAB (Treatment 2) had a slightly higher yeast count after the 90-day storage phase than the other five silages and also was the least stable in air.Its temperature exceeded the ambient by about 5EF after 94 hrs of expos ure to air.Control and LAB + PS 10 silages (Treatments 1 and 4, 5 respectively) began to he at and had yeast counts near log 8.00 after 118 to 122 hrs.PS 10 and 10 5 LAB + PS 10 silages (Treatments 3 and 6, re-6 spectively) were the next to undergo aerobic spoilage after 146 to 148 hrs of exposure to air.When aerobic deterioration began in the four silages treated with P. shermani i , it was at a slow rate and not the typical rapid increases in temperature and pH observed in aerobically unstable corn silage.
These results suggest that the strain of P. sherm anii used was capable of competing with other microorganisms in the ensiling process; it produced propionic acid that was detectable in the 90-day corn silages.Silages inoculated with this organism were more stable during the feedout phase than control or LAB-inoculated silages.The Propionibacteriu m was more effective when applied at 10 cfu per gram of 6 ensiled forage and when not in combination with LAB (i.e., L. plantaru m and P. acidilactic i).

Table 4 . pH and Yeast and Mold Counts over Time for the Corn Silages during the 10- day Aerobic Stability Measurements
1*No distinguishable first peak.These si lages exhibited at gradual increase in temperature following the initial temperature rise.