The temperature of silage can help us monitor the ensiling process from production to feedout. Oxygen in the silo disrupts the anaerobic ensiling process and initiates biological and chemical processes that consume nutrients and energy, leading to heat production.
This increases the silage temperature and negatively impacts the silage, both in terms of quantity (dry matter is lost) and quality (the most digestible nutrients are used up first).
Heating during initial ensiling
Heat production is a normal event during the ensiling process. During initial ensiling, a rise of up to 20ºF is common even in a well-managed silo. Depending on ambient temperatures when the silage was produced, temperatures up to 110ºF – or higher in warmer parts of the country – are common.
Prior to ensiling, chopped forage is a living organism and will continue to respire while oxygen is available. Oxygen trapped in the silage mass is the main driver of these biochemical processes causing heating.
At moisture levels of 50 percent and 70 percent, respiration rates are about 30 and 70 percent of maximum, respectively. Respiration rate peaks at 115ºF, but the enzymes responsible are fully deactivated at 130ºF.
Prolonged temperatures more than 105ºF can cause protein damage in the crop, impacting the availability of amino acids. This occurs slowly below 100ºF, doubling with each 25ºF increment above that threshold. Prolonged elevated temperatures may lead to extensive browning and decrease intake and digestibility.
Proteolysis (protein breakdown) is also affected by temperature. The rates of protein degradation double for every 18ºF increase between 50 and 100ºF. A rapid, efficient fermentation reduces proteolysis, so using research-proven inoculants with fast-growing homolactic bacteria is recommended practice, especially in warm, humid ambient conditions.
Growth rates of the lactic acid bacteria (LAB) essential to the initial ensiling fermentation are affected by temperature, availability of sugars, redox potential (anaerobiosis) and moisture levels. They grow most rapidly at temperatures between 80 and 100ºF. Below 80ºF, growth is slower, although most fermentations should be complete between seven to 10 days.
Although generally thought of as anaerobes, most LAB can grow under aerobic conditions, consuming molecular oxygen and helping to create anaerobic conditions in the plant mass.
Oxygen is removed due to respiration, LAB activity or silage packing. Then the temperature should slowly decrease. After fermentation is completed, the silage stabilizes. During the storage phase, the temperature should be between 70 and 90ºF.
A recent silo survey found core temperatures of 90ºF after 90 days of storage. A lot depends on the amount of silage present and the storage structure. Heat loss is faster from bags or towers than from a large bunker or drive-over pile.
Heating during storage and feedout
Secondary heating may occur in the silo because of aerobic deterioration. Silages affected may have a distinct smell (like ethanol, must or mold) and quickly heat up. However, temperature should not be used as the only indicator of aerobic spoilage.
If there is steam coming out of the pile when silage is removed from the face, it could just be retained heat coupled with cold ambient temperatures.
Researchers reported the temperature of corn silage in a horizontal silo or bunker rose from 54 to 68ºF with increasing depth from the face during winter. In the summer, the temperature decreased from 77 to 68ºF as the depth increased.
The silage at depth was stable at 68ºF. The use of silage surface temperature as the only reference for silage stability during feedout can be misleading.
In another study, the highest temperatures during storage occurred at the sampling point closest to the open surface, just 2 inches. Heating had two peaks, first at 24 to 36 hours post-filling and the second peak three to five days post-filling. The temperature then declined steadily due to the end of growth of spoilage organisms and heat loss.
Oxygen also penetrated 8 to 10 inches after the substrates near the surface had been consumed, leading to losses in silage dry matter and quality. Researchers observed “normal” fermentation temperatures only at a depth of 14 inches or more from the exposed surface.
During peak heating, aerobically unstable silages may reach 120ºF or higher due to the growth of spoilage yeasts. Look for:
- High temperatures at the aerobic interface (from 4 inches to up to 3 feet deep, depending on the material and packing)
- Molds at, or close to, the surface
- An abnormal smell to the silage
Invest in short (6 inches) and long (3 feet) temperature probes to track temperatures close to the surface as well as within and behind the aerobic interface.
Silage stability can also be monitored by tracking pH using relatively inexpensive pH strips. High silage pH (greater than 5.0), a musty smell and a temperature above 110ºF strongly suggest the silage is aerobically unstable and spoiling.
Growth of spoilage yeasts usually leads to higher temperatures (85 to 110ºF), but deterioration can occur at temperatures as low as 40ºF (Figure 1).
Silages with adequate concentrations of antimycotic (yeast and mold-killing) acids – such as acetic and propionic acid – can remain stable even during times of high ambient temperatures.
Using a propionic acid product at low levels (1 to 3 pounds per ton) showed variable results. Researchers consistently recommended higher application rates.
A more cost-effective solution is to use a silage inoculant with high dose rate of Lactobacillus buchneri 40788, which is reviewed by the FDA and allowed to claim efficacy in preventing heating and spoilage.
The use of the high dose rate L. buchneri 40788 in combination with a strong homolactic LAB (such as Pediococcus pentosaceus) produces a rapid initial pH drop followed by acetic acid production. The low pH from the initial fermentation increases the toxicity of the acetic acid to the spoilage yeasts.
Negative impacts of spoilage yeasts
Silages with a population of 100,000 colony-forming units (CFU) of spoilage yeasts per gram or more are particularly prone to aerobic spoilage, but silages with smaller populations have also been reported to deteriorate. During feedout, oxygen can penetrate up to 3 feet behind the surface, creating the aerobic interface.
Oxygen allows yeast to become active, using sugars and lactic acid for growth, increasing yeast numbers, raising pH and producing heat, with temperatures rising as high as 140ºF. Molds and other opportunistic microbes can then exploit the higher- pH environment, producing a second temperature spike.
Most yeasts grow at temperatures between 30 and 100ºF, with few well adapted for growth at temperatures above 115ºF. Under ideal conditions, yeasts can double their population in approximately two hours.
A sample of silage with 100,000 CFU of yeasts per gram could end up with 6,400,000 after 12 hours, rising to more than 100 million after 20 hours.
Researchers have proposed the use of the temperature from the mid-line of the silo face at 8 inches depth as a reference value for the aerobic stability of the silage at different points along the face.
They also found the temperature of peripheral sampling points were more than 9ºF higher than the reference temperature at the center, the pH was higher than 4.5 and yeast count higher than 100,000 CFU per gram (Table 1).
Remember management basics
The basics of silage management are important to minimizing spoilage and ration heating. Correctly harvest the crop, use an inoculant proven with independent research, quickly fill and pack well. During feedout, keep a tidy face.
Don’t pile silage ahead of feeding and manage removal rates to keep ahead of air infiltration. PD
Bob Charley has a Ph.D. in applied microbiology from the University of Strathclyde in Glasgow, Scotland. Renato Schmidt has a Ph.D. in animal nutrition from University of Delaware and is employed by Lallemand Animal Nutrition as a forage products specialist.
References omitted but are available upon request. Click here to email an editor.
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Bob Charley
- Forage Products Manager
- Lallemand Animal Nutrition
- Email Bob Charley
