By Bill Mahanna and Ev Thomas
The natural microbial (epiphytic) populations that exist on the fresh crop at harvest exert a tremendous influence on the stability and feeding value of the resulting ensiled feed. Factors such as temperature, humidity, solar radiation, plant maturity, moisture, length of wilting time and soil contamination during harvest all influence the type and quantity (colony forming units or cfu/gram of forage) of epiphytes populating the crop.
Which Organisms Dominate?
The ultimate goal of ensiling is to stabilize the crop via the action of lactic acid bacteria (LAB). This reduces pH through the efficient conversion of sugars to lactic acid.
As dairies transitioned to larger bunkers and drive-over piles, it also created a greater need to reduce aerobic deterioration on the face of the silage during feedout.
Total epiphyte counts can vary from near zero to several million cfu/gram of fresh crop. The microflora-on fresh plants are primarily gram-negative, aerobic (oxygen-loving) species. The preferred gram-positive, anaerobic LAB that drive the fermentation process are very much in the minority. Furthermore, not all of the small population of LAB is desirable because most are Leuconostics species which are inefficient at converting sugars, lack acid-tolerance and can’t reduce pH below about 5.0.
Without going into the hundreds of epiphytic populations, the ones most problematic to forage and high-moisture grain are yeast, molds and soil contaminants introduced during harvest such as gram-positive, sporeforming bacilli and clostridia. Crops such as corn silage and high-moisture corn, especially if stressed by drought or early frost, can have very high yeast counts. The proliferation of yeasts in silage re-exposed to oxygen at feedout can have a negative effect on dry matter loss, heating and palatability.
In the presence of oxygen, certain yeast species have the ability to metabolize lactic acid, causing an elevation in silage pH which reduces the inhibitory effect on other heatgenerating spoilage organisms such as mold, bacilli and acetobacter species. Yeast and acetobacter can also produce aromatic compounds such as esters, aldehydes and ethyl acetate (smells like fingernail polish) which can significantly reduce feed palatability. University research has shown that the impact of yeast can be minimized by proper harvest moisture, silage compaction/ feedout methods and the use of silage inoculants containing viable strains of Lactobacillus buchneri.
Mold spores are virtually everywhere and easily survive over winter in soil and plant residues. Common field fungi (primarily Aspergillus and Fusarium spp.) are capable of producing recognizable toxins including aflatoxin, vomitoxin (DON), fumonisin, zearalenone and T-2. Estimates are that 70 to 90 percent of all mycotoxins are already on the plant prior to harvest, and no silage additive or inoculant is capable of degrading these preformed toxins.
However, producers can exert management influence over storage fungi like Penicillium (toxin-producer), and nontoxin producers such as Mucorand Monila mold species. These molds do not typically infect the crop prior to harvest, but their soil-borne spores can contaminate the fresh forage during harvest. Ensuring proper harvest moisture, silage compaction and feed-out methods can help reduce aerobic conditions conducive to the growth of these storage molds.
Clostridia are well-known for their ability to degrade proteins and produce butyric acid. Reducing soil contamination levels (ash) in legumes and grasses in addition to ensiling at higher dry matters such as 40 to 50 percent reduces the chances of clostridia problems. Clostridia take a month or two to grow and establish populations, so if forced to ensile wet silages, it is best to feed them immediately before they initiate their destructive process. Producers are trending towards higher dry matter legume/grass silages from having learned this lesson the hard way.
How to change the process
Researchers at the U.S. Dairy Forage Research Center showed that epiphyte counts were elevated with warmer temperatures, longer wilting times and if rainfall occurred during wilting of legume forage. While wide-swathing aids in rapid wilting of legume/grass forages, the greater exposure to solar radiation can have a negative effect on LAB counts. Research on several fungicide products are in agreement that fungicides do not appear to negatively impact LAB populations or viability. Finally, the process of harvesting tends to quickly raise LAB counts presumably because of the availability of nutrientrich plant juices.
Moisture of the crop at harvest also dictates which epiphytes dominate, exemplified by clostridia preferring a high-moisture environment. You can easily observe the influence of harvest moisture on which silage microbes dominate by looking at their metabolism end-products (volatile fatty acids and ammonia-N) across different moisture ranges. In general, wetter silages undergo a more extensive fermentation; have a slightly lower pH, more ammonia-N and typically exhibit higher acetic acid levels (primarily from higher yeast and heterofermentative bacterial growth).
Drier silages undergo less extensive fermentation, have a slightly higher pH, less ammonia-N and typically lower acetic and butyric acid. Silage management is critical with drier silage to minimize porosity. The lower ammonia-N (soluble protein) in drier silages should be factored into diets to be sure that rumen bacteria have adequate nitrogen that used to be provided from the higher soluble protein found in wetter legume/grass silages. The benefit of inoculation is overwhelming epiphytes with highly competitive LAB strains which dominate and direct the fermentation process to a more consistent endpoint, despite differences in harvest moisture.
The U.S. Dairy Forage Research Center conducted studies showing when a silage inoculant was added at a rate that was at least 10 percent of the epiphytic population, the inoculant always improved fermentation. When the inoculant was applied at less than 1 percent of the epiphytic population, the inoculant produced no significant changes in fermentation.
May Need Additional Help
Given the current value of forage and its importance in the diet, many producers today have decided to take a more proactive approach to protecting their forage investment rather than relying on what Mother Nature provided on the crop. The 2012 Hoard’s Dairyman Market Study reported that surveyed producers used an additive on corn silage 84 percent of the time and hay silage in 68 percent of situations.
The use of a research-proven silage inoculant designed with multiple strains to sequence the silage through the process of pH decline as well as reduce yeast growth during feedout is one tool to complement rapid harvest. Attention should also be given to silage compaction (silage density) and face management.
Used by permission from the June 2013 issue of Hoard’s Dairyman.
Copyright 2013 by W.D. Hoard & Sons Company, Fort Atkinson, Wisconsin.