Introduction to Leaf Silage - Part 6
Author : Louise Jakobsen
As promised, we go a little more into depth of silage smells;
We have borrowed this information from an article posted by Progressive Forage and is based on commercial silage production for livestock, but the principles are the same.
There is an assumption that silages with a fruity or sweet odour are synonymous to a well-fermented, stable feed. This is not usually the case since this smell is often due to high alcohol (ethanol) concentrations that are produced mainly by yeasts but also some types of bacteria. The alcohol may react with acids in the silage, producing esters and adding to the fruity aroma. Since yeasts are the main cause of heating events in silages when exposed to air, these fruity-smelling silages can be quite unstable at feed-out.
Silages that are aerobically unstable may also present a musty or mouldy smell, and may have visible mould growth. At this point, the silage has already heated because of the prior activity of spoilage yeasts, so significant losses have already taken place, as described above.
Mouldy silage should be discarded since it might be contaminated with mycotoxins, which can cause serious production, health and fertility issues, in addition to already being lower in nutritive value due to the prior growth of yeasts.
An earthy odour in silage is a sign of bacillus growth and likely a high pH. This type of material has undergone a different form of aerobic spoilage, involving severe heating driven by the growth of thermogenic bacilli.
Material with this aroma should be discarded, as most of the nutritional value has been lost and there is potential for negative effects if fed.
Another sharp odour in silages is caused by high concentration of acetic acid; in this case, it is easy to detect since it is associated with the smell of vinegar. If there is a high amount of lactic acid, the silage should feed well and be aerobically stable; if not, then the silage may not be stable and a drop in feed intake may be observed.
This type of fermentation profile is characteristic of a natural or “wild” fermentation and is also found in very wet (corn) silages.
PUTRID AND FISHY SMELLS
It is not unusual to get foul, putrid smells in wet cereal and grass silages, and particularly in alfalfa silage. This smell is most associated with clostridia, which utilize soluble sugars or organic (lactic) acids to produce butyric acid (which has a rancid butter smell). This silage has a slimy, dark green to black appearance.
Clostridia grow in the absence of oxygen and in high moisture, but are acid-intolerant. Therefore, forages with more than 35 percent DM will hardly ever undergo a clostridial fermentation, while achieving a fast, efficient initial fermentation (and pH drop) will kill off these nasties before they can even get going.
Corn silage and haylage ensiled at high DM contents (more than 45 percent and 50 percent, respectively) can present a burnt tobacco smell due to a reaction where proteins bind with sugars in the presence of excessive heating. In the early stages of this process, the odour is more like sweet tobacco, which is of less concern.
WHAT'S THE 'RIGHT' SMELL?
Ideally, well-fermented silage should not have a strong odour because the main organic acid from the fermentation – lactic acid – is nearly odourless.