Estimating how much silage is on a forage wagon

Farmers are often faced with the challenge of figuring out how much forage is in a chopper wagon. The situation arises when forage is being sold from one producer to another or when an estimate of yield is needed for accounting or crop insurance records. Often, a phone call comes to the Extension office at some point during the estimation process.

I've always maintained that getting an accurate estimate of the amount of forage on a wagon is like trying to nail Jell-O to a tree -------- it's hard to do. Recently, researchers at the University of Wisconsin Marshfield Agricultural Experiment Station weighed and measured haylage/silage produced on the farm for three years. Average depth of forage was estimated as each load of forage was pulled across the scale. Samples of forage from each load was analyzed for moisture content.

Table 1 gives the average silage dry matter density (pounds per cubic foot) on the wagons. For different types of forage, the range is from 4.6 (grass silage) to 5.7 (first cut haylage) with an average of about 5.0 pounds per cubic foot. Surprisingly, forage density did not vary greatly with forage type or moisture.

Working through an example

With this information, we can now estimate the load on a wagon by multiplying volume times density. Let's look at an example. A wagon measuring 16 feet long by 7.25 feet wide and filled to a depth of 6 feet has a volume of 696 cubic feet (16 x 7.25 x 6). To determine dry matter on the wagon, multiply cubic feet times 5 pounds of dry matter per cubic foot. This comes to 3480 pounds of dry matter (696 x 5).

To calculate "as fed" weight, divide the dry matter content (as a decimal) into the dry matter pounds. If our example forage is 60% moisture (40% dry matter) alfalfa haylage, we would divide 3480 pounds by 0.40 to get 8700 pounds as fed.

If we dig deep into the gray matter of the brain, somewhere back there is a statement that says 95% of the measured results will fall within two standard deviations on either side of the average of a bell-shaped curve. The column labeled "SD" in Table 1 refers to the standard deviation. If we assume an average standard deviation of 0.8 in our example, this translates to a potential weight range of 2366 to 4594 pounds of dry matter. Clearly, this is a large variation and one that could result in gross miscalculations over a large number of loads.

Comparing results to other methods

I compared the results of our example forage wagon to a chart provided by a silo inoculant company and the method used by the Farm Service Agency (FSA). The chart from the inoculant company estimated total dry matter weight on the same wagon at 5600 pounds. This figure grossly over estimates the weight and doesn't even fall within the range calculated from the Marshfield research data. Using the FSA method of 12.5 pounds of as fed forage per cubic foot, the estimate comes to exactly that calculated with the Marshfield data (3480 pounds). However, the two methods begin to deviate as forage moisture moves away from 60%.

Weighing wagons is still the best bet

Clearly, estimating the weight of forage on a wagon is not an exact science. When forage is being sold based on the number of wagon loads, weighing all or a portion of the loads across a scale still provides the most accurate and fairest method on which to base transactions. If estimates must be made, errors can be kept to a minimum by accurately determining cubic feet of forage in the wagon and basing estimates on dry matter rather than as fed forage.