Getting oxygen out of the silage is the key because yeasts do not grow in low-oxygen environments. Regardless of the weather, once oxygen is added back to the silage, these organisms or their spores begin to grow again. However, their growth is expedited by warmer temperatures. When the temperature exceeds 50 to 60ºF, and especially when temperatures exceed 80 to 90ºF, yeasts grow exponentially, and the consequences of their growth become more important. Obviously, the population density of the yeasts or spores also will help determine the initial rate at which they begin to grow.

The heating we feel in the silo or in the feedbunk is primarily a result of yeast growth. The heat is given off when these organisms use sugar or starch in the plant material for their growth. Unfortunately, these nutrients are part of the feed energy we would like for rumen bacteria to convert into energy which the cow can use for milk production or growth. Therefore, yeast growth causes energy loss in silage.

In addition, part of the soluble protein in silage is used during yeast growth and lost prior to feeding, which reduces the protein available for rumen bacteria. As an added insult, when yeasts break down protein they produce ammonia. Unfortunately, both heated silage and the smell of ammonia strongly reduce the palatability and resulting intake of silage by the dairy cow. Unwanted yeast growth produces more yeast cells, which accelerates nutrient losses, heating and production of ammonia. Finally, feeding silage in a total mixed ration (TMR) can make the problem worse in hot weather because silage yeasts get a fresh supply of oxygen and grains (containing soluble starch and proteins) once the TMR is mixed.

Check your silo frequently to see the amount being fed each day is enough to minimize the problem in the silo. If the heating is considerable, use more silage each day, if at all possible, and perhaps pay more attention to keeping a clean bunk face on bunker silos. If the TMR has heated greatly within a few hours of feeding, you may need to feed less at a time and mix and feed TMR more often (perhaps going from one to two feedings per day or from two to three feedings per day) for a time period.

Another method may be to offer more feed at night, when the temperatures are cooler, to encourage cows to eat more. Also, there are TMR and silage additives that retard the growth of yeasts and molds as their principal objective, and these may be cost-effective if they minimize loss of DMI by your lactating cows.

Whatever management changes you choose to use, check to be sure your approach is working by regularly monitoring DMI and feedbunk temperatures at two, four and eight hours after feeding.  PD

—From Penn State Dairy Digest, July 2006

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