Thoughts for Drought: Forage Inventorying

By Zachary Curtis

Periods of drought are challenging to any producer, but they can be especially stressful for livestock producers who must maintain minimum forage inventory levels to carry their herd over winter. Taking time during the growing season to verify current forage inventory levels, estimate incoming forage quantities, and act accordingly can help ease your mind when home-grown forage yields decline. This article is the first part in a series of preparing for and planning for managing forages in times of moisture stress.

The first step in planning for dry weather is to estimate the current forage inventory on your farm. You should consider all forages available on your farm, whether it be silages, hay, or pasture forage reserves. Resources available across the Internet and from your local Extension office can help you accurately determine the amount of forage in your silo, pasture, and hay storage barns. Forage estimates are best conducted on a 100% dry matter basis in order to standardize inventories regardless of forage moisture content (ie. 13% moisture dry hay versus 63% moisture corn silage). If you haven't needed to take a forage analysis of your feed and don't know the moisture, know that you can request a simple dry matter analysis from most forage labs, or you can use a Koster Tester or microwave to get a faster result of your forage moisture content. Standardizing on a 100% dry matter basis will be important in the later part of our series when we discuss estimating animal forage requirements based on their body weight.

When standardizing forage dry matter, remember that Moisture + Dry Matter must always equal 100%. Therefore, to calculate one value from the other, simply subtract the known value from 100% to calculate the unknown value. For example, a freshly-made bale of hay at 18% moisture would contain 82% dry matter (100% - 18% = 82%). After calculating the dry matter content of our forage, we can multiply the "wet weight" of that forage by its dry matter content to figure out the total quantity of forage dry matter available to feed our animals. In our example, if a producer baled 100 tons of 3x3x8 square bales at 82% dry matter, we would find that they only produced 82 tons of "dry matter" (100 x 82% = 82). The remaining 18 tons of forage was just water.

Estimating Silage Inventories

Silage inventory can be difficult to estimate due to the variability of packing density. Fortunately, several land-grant universities across the country have worked to develop tables and calculators that address these challenges as you move between bunker silos, upright silos, and drive-over silage piles.

Inventorying Bunker Silos requires you to know the length, width, and height of the existing forage mass. Note that many producers may "crown" the top of their bunker silo to help shed rainwater; the attached spreadsheet will help account for that crown. Also, consider the "toe" of any unopened bunker silos that slope up towards the maximum height of your bunker silo. Bring along a helper to take accurate measurements of each pile dimension. Use caution when approaching the face of a bunker silo in case of collapse of the face. Most bunker silo calculators will use an average estimate of packing density to translate the silage volume to silage weight, which is roughly 14-15 pounds of dry matter per cubic foot for most silages. This calculator from the University of Wisconsin can help you determine the amount of forage in your bunker silo.

Inventorying Upright Silos requires you to know the diameter of the silo and the height of the remaining silage. They present their own unique challenge, as the density of the silage increases as you move further down the silo. Many reference tables will account for this and will show a slight increase in silage quantity per cubic foot as the measured height approaches ground level. This chart from Michigan State University provides easy reference for estimating silage contained in upright silos.

Silo Capacity: Tons of Corn or Grass Silage (68% Moisture) in Settled Unopened Silos

Note: When a silo is partially unloaded from the top, the remaining silage is more tightly packed and heavier than the same volume in an unopened silo. Therefore, compute the weight remaining as follows:

1. Use the table to find the original contents before the silo was opened.
2. Estimate depth of silage removed and determine in weight from table.
3. Subtract tonnage removed from original contents to find tonnage remaining.

Example

• 50' of settled silage in a 20' silo weight 389 T.
• Weight removed in 32'=205 Tons
• 389 T. (original contents) - 205 T. (removed in 32') = 184 T. (remaining in 18')

Ag-bag silos are relatively simple to inventory compared to other silo types because their packing density is consistent, and their dimensions are easy to calculate. Assuming proper bagger setup and operation, you should find a uniform silage density along the entire length of the bag. The most difficult aspect of ag-bag inventory may be to determine your estimated density based on the type of silage in the bag. The two tables below from the University of Wisconsin can help you calculate ag-bag silage reserves, as well as adjust values if your density is slightly higher or lower than the 13 pounds per cubic foot shown in table 1.

Table 1. Capacities of Silage Bags at 13 lbs DM/ft³ Density

Table 2. Multiplier to Adjust Table 1 Capacities to a Different Density.

 Drive-Over Piles are the most complex silo to inventory. Drive-over piles require a sharp assessment of the pile height, pile length, pile slope, and the shape of the footprint of the pile. Having an experienced helper, such as your nutritionist or another consultant, to work with can help you accurately determine what areas may contain more or less silage than what you would expect at a quick glance. Drive-over pile calculators will use a similar density estimate as bunker silos, which is roughly 14-15 pounds of dry matter per cubic foot for most silages. This calculator from the University of Wisconsin uses multiple measurements to determine the estimated silage capacity of your drive-over piles.

Estimating Pasture Inventories

Pasture inventories can be difficult to estimate due to animal grazing habits, forage species variability, and varying fertility levels across pasture paddocks. Additionally, pasture forages are not collected and baled like hay, which is makes it more difficult to count and weigh. Inventorying pastures can be a time consuming process, but can be valuable during drought conditions to know if and when you’ll need to start feeding stored forages.

Many methods to estimate pasture inventory exist, including hand clipping, utilizing a pasture ruler, or utilizing a rising plate meter. Hand clipping is precise but will be the most time consuming and impractical. A pasture ruler will evaluate forages based on plant height and its relationship to yield. The rising plate meter will look at both height and density. For more information on how to perform these tests, check out Determining Pasture Yield.

When assessing pasture yields, be sure to also look at the composition of the available forage. Is it a majority of desirable, palatable grasses? Are there some weeds in there? Is it mostly weeds that the livestock avoid? Just like with soil testing, those oddball areas should be avoided when doing a pasture evaluation for available forage. Don't collect inventory data from areas that contain heavy amounts of weeds or species that the livestock aren't eating on an annual basis. If there is heavy weed presence, consider some methods of weed control after the moisture returns. This will vary based on the time of the year and the weeds present.

Estimating Hay Inventories

Dry hay (or wrapped baleage) inventories can be estimated by simply multiplying the average weight per bale by the total number of bales in each "lot" of hay harvested. It is best to include as many bales as possible when determining an average bale weight, especially if baleage production occurred over the course of several hours and the crop has wide variations in moisture content.

Reserving time in your schedule to conduct a forage inventory survey is the first step in preparing for and mitigating periods of drought stress. Although drought conditions can lead to decreased forage yields, there are many strategies we can employ to lessen the effects of low yields. Knowing what to expect from the coming growing season will help you take the first steps in developing an effective plan for your farm.