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Virginia Cooperative Extension - Knowledge for the CommonWealth

Get a Better Handle on Forage Quality

Dairy Pipeline: November 2006

Bob James
Extension Dairy Scientist, Dairy Nutrition
(540) 231-4770;

Have you ever started the herd on a new silage crop and experienced an intake and milk production crash? Producers commonly evaluate silage on the basis of DM%, CP%, ADF% and NDF%. Generally, lower fiber means higher energy content and more milk. However, sometimes these traditional measures are not sufficient to determine how a herd might respond to a silage crop. Fermentation analysis can provide valuable information and help determine expected response to a forage or it’s suitability for a given class of livestock on the farm.

Silage depends on fermentation of carbohydrates (primarily sugars and starch) to produce acids which in turn preserve the crop. Rapid filling, exclusion of air by good packing and a plentiful supply of readily available carbohydrates are essential for good fermentation. Goals for silage fermentation are shown in the table below.

What does the fermentation analysis tell us about a silage? A high pH commonly means a poor fermentation has occurred and the silage may not keep very well. It’s also associated with higher losses of nutrients during fermentation. High pH is more common with mature forages or those ensiled too dry.

Lactic acid is a strong acid that should predominate in the silage. This is the acid responsible for the rapid drop in pH. T

he importance of acetic acid is not as clear. Lactobacillus buchneri is included in some microbial additives and it produces acetic acid which helps to extend bunk life after silage is fed. If L. buchneri is not added, higher acetic acid levels might be associated with depressed intake.

Propionic and butyric acids should not be detectable. Butyric acid indicates an undesirable fermentation has occurred. This silage usually stinks!

High levels of ammonia are frequently associated with a poor fermentation. The silage may have been too wet, poorly packed or the silo filled too slowly. It usually indicates excessive breakdown of protein.

What can be done with a poorly fermented silage? Silage with high levels of butyric acid should not be fed to the milking herd and especially not to close up or fresh cows as it will result in low intake and increased ketosis. If it must be fed to the milking herd it should be fed to lower producing, later lactation cows and diluted with a higher quality silage. Older heifers may be more likely candidates to receive this forage. Dry silage with a low pH may be less digestible and have lower palatability. Sometimes the addition of wet brewers grains will help “perk” this silage up and improve intake of the ration. Fermentation analysis is available from many forage testing laboratories at an added cost above the basic analysis.

Legume silage
Corn silage
4.0 - 4.8
3.7 - 4.2
Lactic acid
6 - 8%
3 - 5%
Lactic acid (% of acids)
Acetic acid
<10% of total N
<10% of total N

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