Estimating the Weight of Forage in a Forage Wagon
byDaniel W. Wiersma and Brian J. Holmes
Marshfield Agricultural Research Station
Biological Systems Engineering
Most dairy farmers do not
have a drive-over scale available on their farms. Frequently it would be
useful to know the weight of forage harvested from a field. Measuring
alfalfa or corn silage yield is necessary to adjust management practices,
to maintain crop inventories, and for crop reporting purposes. Sometimes
growers sell hay from a field and require an estimate of the quantity
removed or sold.
Wisconsin Forage Wagon Density Study
Researchers at the
Marshfield Agricultural Research Station conducted a study to estimate the
weight of loaded forage boxes. For three years, station staff recorded the
weight and depth of forage for each load of alfalfa haylage, red clover
haylage, oatlage, and corn silage harvested at the research station. In
addition, forage samples were taken from each load to measure moisture
content. Data was summarized by crop species and cutting number during the
How can the total weight of a loaded forage wagon be calculated?
When the density of
haylage or corn silage in a forage box is know, an estimate of the total
forage weight in a chopper box can be calculated by multiplying the
chopper box volume by the forage density. Measurements should be taken on
each chopper box to determine the width and length. It is important to
determine the fill height for each load so that the correct volume is
computed. If loads are always filled to a uniform height in the forage
box, an average height measurement can be used.
One suggestion for
helping to determine the depth of fill in a forage box is to make
permanent marks on 1/2 ft intervals on the side of the forage box. Then
the operator can quickly view the forage level and write it down for
The following example
demonstrates the method of calculating forage weight in a forage wagon.
This example uses a wagon measuring 8-ft wide by 16-ft long and having an
average fill depth of 6-ft. In addition, this example will assume an
average forage density of 5.0 lbs DM/ft3 and a forage dry matter of 40%.
1. Calculate the volume of forage in a forage box.
ft wide x 16 ft long x 6 ft high =768 ft3
2. Calculate the dry weight of forage in a forage wagon.
lbs DM/ft3 x 768 ft3
3. Calculate the wet weight of forage in a forage wagon.
lbs dry forage ¸ 0.40 %DM
How accurate is this calculation?
As with all
estimation methods, there is a small amount of error inherent in this
calculation method. In this study, the average density was measured as 5.0
lbs DM/ft3. The standard deviation of forage density in this study was 0.8
lbs. This means that the actual forage density for all species ranged from
4.2 lbs DM/ft3 to 5.8 lbs DM/ft3. In the example, the true weight of dry
forage in the wagon ranges from 3,225 to 4,450 lbs.
calculation is better than a guess, it does not provide the accuracy
required for sale of forage or yield checks. If better accuracy is needed,
one should use a drive-over scale and weigh each load as they come in.