Yeasts and Molds

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Yeasts and Molds

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CattleNetwork.com - Stanley,KS

Yeasts and MoldsBoth are fungi: yeasts growing as single cell

organisms while molds grow as multicellular filaments. Both occur

widely in soil and water and on vegetation, increasing in numbers on

vegetation as the crop ages or gets damaged (e.g. frost, hail,

drought) and during wilting. In addition to being able to grow on

free sugars, both yeasts and molds secrete extracellular enzymes

which break down the complex plant materials into simple sugars

which can then be used for growth.

Many of the yeasts found on plant material contain carotenoid (orange

to red) pigments to protect them against UV exposure and so can be

responsible for some of the colors seen on silage faces. While

yeasts can grow aerobically, they can also grow fermentatively

(anaerobically), with ethanol being one of the major products. Other

products that yeasts can produce in anaerobic growth conditions

include n-propanol, iso-pentanol, acetic, propionic, butyric and iso-

butyric acids, as well as small amounts of lactic acid. In the

presence of air, yeasts will oxidize sugars fully, producing carbon

dioxide and water and generating heat. Many yeasts can also use

lactic acid for growth, again oxidizing it fully and generating

heat. Yeasts are responsible for the vast majority (>95%) of heating

silages: a yeast population >100,000 CFU/ gram in the silage will

almost certainly mean that the silage will heat as it is exposed to

air during feedout. Yeast growth can be inhibited by acetic acid.

The conditions normally associated with stable silage, low pH and

anaerobic conditions, do not favor growth of molds. Generally they

are only a problem where air exposure has occurred, e.g. at the top

and

on the sides of bunkers or piles, where there have been air leaks

into

the silage, where packing has been poor (e.g. localized lumps of

moldy

silage), at surfaces left exposed during filling and at the surface

of the

silage during feedout. As the silage moves towards the surface, if

there

are high numbers of yeasts present these can grow on the lactic acid

present, raising the pH and the silage temperature, promoting the

subsequent growth of molds. Mold growth is undesirable, since the

molds will fully oxidize both sugars and lactic acid, and will also

break

down (hydrolyze) and fully oxidize cellulose and other cell wall

components, resulting in huge dry matter and energy losses. In

addition, many of the molds commonly found in silages can produce

mycotoxins, which can cause significant health and/ or reproductive

problems and dramatically reduce performance. Finally, molds

produce spores that become airborne when the silage is disturbed and

can cause respiratory problems if they are inhaled (both for the cows

and for the producer and farm workers). Mold growth can be

inhibited by propionic acid.