Understanding the relationship between corn grain and forage yield is important to dairy producers and grain farmers, who often contract with each other for corn forage production. Arriving at a fair and equitable price for corn forage is difficult due to the number of factors involved that are dynamic and biologically variable.

The bushels of corn grain in 1 ton of forage (i.e. grain-to-forage ratio or grain equivalents) is often used in contracts. However, grain equivalents can be quite variable, to the extent that one predetermined value should not be used in contracts between growers and dairymen. This variability is due to genetics, environment and management.

For example, corn hybrids with the leafy or BMR trait have lower grain yield than bioengineered hybrids. Some growing seasons can be too wet (or dry) or too cool (or warm) to maximize corn grain yield in a field even though stover yield is maximized. Management decisions like late planting date, plant population and nitrogen rate can influence the amount of grain produced, while adjusting cutter bar height directly influences grain-to-forage ratios.

One of the earliest publications on grain equivalents in corn forage was written in 1972. The researchers found the number of bushels in a ton of corn forage increased as grain yield of the field increased (Table 1).

Corn open-pollinated varieties and hybrids released to farmers through the 1900s had whole-plant yield increases of 0.05 ton of dry matter per acre per year, while stover yield increased 0.02 ton of dry matter per acre per year. More of the forage yield increase is due to an increasing grain yield. Starch content increased 0.15% per year.

Calculating value of modern corn forage fields

Some grain growers want to calculate the forage price based on corn grain yield (as the alternative harvestable crop), and some dairy producers want to calculate the price based on alternative forages (primarily alfalfa as the alternative forage source). In either case, the final price is affected by supply and demand of corn grain within a region.

Since 1997, the University of Wisconsin corn agronomy program has been using “paired” plots in experiments investigating the effect of management factors like hybrid, crop rotation, plant density, planting date, nitrogen rate, fungicide and row spacing on corn forage, and subsequent grain performance in the same plot. In these studies, forage yield and quality were measured in four of the eight rows in the plot. The four remaining rows were left for later grain yield and quality measurements.

Anything that affects grain or forage yield will affect grain equivalents. Depending upon grain yield level, grain equivalents per ton of corn forage ranged from 5.1 to 8.6 bushels per ton forage at 65% moisture (Table 1). To calculate the value of an acre of corn forage, the amount of grain equivalents is multiplied by the corn price and the forage yield.

For example, if corn yielded 170 bushels per acre, the grain equivalent is 7.8 bushels per ton of corn forage at 65% moisture. An average cornfield yield is 25 tons forage per acre. If corn is priced at $3.50 per bushel, the value of the field is 7.8 bushels per ton x $3.50 per bushel x 25 tons per acre = $683 per acre or $27 per ton forage. Further negotiation would be needed over fertilizer removal and harvest, ensiling and storage costs.

Factors that affect the grain equivalent calculation

Harvest timing can affect grain yield in the forage. Kernel milkline is a good indicator of development and remaining potential grain yield. For example, grain yield can still increase 5% to 12% when the kernel is at 50% kernel milk. No further yield increases occur after “black layer” formation at the kernel tip.

Moisture content in forage and grain has a major influence on this relationship and needs to be considered to accurately determine fair forage prices. For a field that has a grain yield of 150 bushels per acre, the grain equivalents range from 20.3 bushels per ton at 0% moisture to 6.4 bushels per ton at 70% moisture.

Environment can significantly affect the amount of grain in corn forage. Drought can reduce plant stature and affect pollination, reducing both grain and forage yield. Sometimes early drought can reduce plant stature, but rains eventually occur, relieving stress, and record grain yields are realized, like the 2005 growing season. Following the 2005 growing season, farmers would sometimes talk about corn forage being “hot” when fed to livestock, meaning that feed had grain equivalents that were too high in the forage. Depending upon year, grain equivalents have ranged from 6.4 to 9.4 at 150 bushels per acre grain yield level.

Hybrid types evaluated have included BMR, leafy, bioengineered and conventional hybrids. In a small study of six hybrids, grain equivalents average 7.5 bushels per ton averaged across locations and years during the years tested. However, the range among hybrids for grain equivalents was 6 bushels per ton (minimum hybrid = 4.5 bushels per ton, maximum hybrid = 10.5 bushels per ton). Some locations produced consistently higher grain equivalents than others. BMR hybrids had significantly lower grain equivalents than conventional or bioengineered hybrids.

A new way to calculate grain equivalents in corn forage

Ideally, a field of corn intended for corn forage use is planned and managed to maximize grain equivalents. Often, corn forage is harvested from the poorest fields. Fields that have been late-planted, weedy, affected by drought or flooding are the ones that end up being harvested for forage. We have measured wide differences among experimental treatments, and the range is economically significant.

A better approach might be to pay for the grain yield produced and adjusted for fertilizer removal and soil erosion benefits. Corn forage is routinely tested for starch content at harvest and could be back-calculated to determine grain equivalents on a field-by-field or load-by-load basis (see starch method in Table 1). This would allow for a much more accurate estimation of corn grain produced in a field, regardless of circumstance, and would be a fairer method for payment.

Clearly, corn grain-to-forage ratios are changing over time. Forage moisture, hybrid and environment significantly affect grain equivalents and must be considered when negotiating a contract. Dairy producers and corn farmers need to understand the grain equivalent relationship when acres are contracted for forage production. This relationship is dynamic and, as we are learning, quite variable – to the extent that one predetermined grain equivalent value should probably not be used in contracts.  

A Microsoft Excel spreadsheet for calculating corn silage value is available at Corn silage pricing decision aid.

Author note: In this article, it is technically more accurate to describe the material we work with as corn “forage” because we do not put the samples through an ensiling process to produce “silage.”

References omitted but are available upon request. Click here to email an editor.

Joseph G. Lauer is with the Department of Agronomy at the University of Wisconsin – Madison. Email Joseph G. Lauer.