Ensiling Soybeans

Believe it or not, soybeans were originally introduced into Canada (Ontario) as a forage crop. For some areas in the US (New York and Vermont) forage soybeans are a standard part of the forage program, especially for some larger dairy operations. In fact, specially-bred forage-type soybeans are available in the northeastern USA, some growing 5 to 6 feet tall.

Can you use grain-type varieties for silage?

The short answer is yes. However, variety selection is as important for forage soybeans as it is for grain soybeans. Typically, forage varieties are longer season, taller and have more vegetative growth and less grain than grain varieties.

A study at New Liskeard, Ontario looked at the forage production from one adapted grain-type (Alta), one non-adapted grain-type (Hanlon in 1999, Wizard in 2000) and one forage-type soybean (Donegal). This study showed that forage yields of grain varieties were 30% less than that of forage varieties. However, some producers in soybean forage country do use grain varieties with success. Grain varieties typically have higher crude protein levels and better digestibility.

Frost Tolerance

Most producers will wait to the last minute (after the frost) before making the decision of whether or not to ensile soybeans. If we do have an early killing frost, options depend on how much of the plant is frozen.

Here are few guidelines:

  • If the leaves turned before the frost, and the beans are yellow, then they will continue to dry, even if they are now soft. These may make grain, though they will be small.
  • if only the tops of the plants were frozen, the beans on the lower unfrosted parts continue to mature. However, ensiling is still a possibility as long as the moisture content is between 60-65%.
  • If the entire plant freezes and leaves and beans were all green at frost time, the beans will dry, but will be green and unmarketable for grain. In this case, as long as there is enough moisture (60-65%) in the plant, chopping for silage is definitely a good option. If they are given a chance to dry completely, they won’t make silage.

What stage should you harvest for hay or silage?

Soybean Silage

It’s never an easy job determining optimum harvest times. Like grain crops, maturity can vary within a single field. Stage of maturity is important for determining cutting date, and moisture content is important for determining chopping and storage dates.

For soybeans, the optimum timing for cutting is right before the pods are full. The lower leaves are often just starting to turn yellow at this time as well (just before R7). Waiting until complete maturity results in forage of lower digestibility and can lead to fermentation problems due to the high oil content of the seeds.

Depending on the stage of maturity, the fat content of the beans could interfere with the bacteria responsible for fermentation. If you do miss the optimum timing, you can still properly ensile soybeans by mixing them with 1/3 corn silage to add the soluble sugars needed to keep fermentation running smoothly. The corn will also dilute the fat content in the soybeans, thereby reducing the soybean buffering capacity.

The moisture recommendation for chopping is similar to that of alfalfa and corn silage. You want to aim for a moisture content of 60-65%. Wetter forages will have more effluent and a higher risk of poor or butyric acid type fermentation. To achieve 60-65% moisture, most stands will need to be wilted before chopping. Because of the high fat content and moderate amount of plant sugars, soybean silage has a higher natural buffering capacity than corn silage. This can potentially result in fermentation problems (eg. Development of high pH’s and mold growth). Where ideal harvesting conditions are not met, inoculants may help reduce ensiling problems.

Soybean Hay

For those not interested in silage, soybean hay quality can be comparable to early-bloom alfalfa. However, this practice is not generally recommended because of slow drying time. Additionally, soybean hay is often dusty and the stems are very brittle increasing feed bunk refusal. The use of conditioners is recommended to increase drying times thus reducing the chance of re-wetting and potential mold development. According to Alvaro Garcia of South Dakota State University, lactating dairy cows and growing heifers have similar performance when given either soybean hay or alfalfa forage. Palatability is not usually a problem (unless the forage is moldy). Soybeans should be harvested at earlier stages for hay than for silage. This will ensure minimal leaf loss, maximizing crude protein levels. At this maturity stage, digestibility of the pods exceeds that of stems by as much as 25%, contributing significantly to the overall nutritive value of the whole plant. Conditioning will achieve a similar dehydration rate of stems and leaves and bring total moisture below 25%. Rake no more than you must, as leaves and pods tend to shatter easily.

Nutritive Comparisons

Reports of soybean silage palatability have been variable, however, overall it appears to be good. Those that have reported palatability issues are typically livestock operations that are feeding soybean silage as a minor portion of the diet. Because the protein, fiber, energy and fat vary greatly between crops, it is important to have representative feed tests done on each of your soybean harvests and a proper ration developed. As soybeans advance from seed development to seed maturing, they will increase in crude protein (18% to 19%), increase in fat (9% to 11%), and decrease in digestibility (46% NDF to 40%). As fat content increases, less soybean forage should be fed. A general rule of thumb for soybean forage intake is that it should be limited to 10 lbs of dry matter if it contains 10% fat. This keeps the total fat levels from soybeans under 1 lb. (450 g). Do not feed soybean forage in combination with other sources of vegetable fat (e.g. Roasted soybeans).

Table 1. Nutrient composition of soybean silage
  Range %
Crude Protein 16.0 - 20.6
Neutral detergent fiber 38.3 - 48.3
Acid detergent fiber 27.3 - 37.3
Acid detergent lignin 6.0 - 7.4
Calcium 1.36 - 1.49
Phosphorus 0.26 - 0.31
Two varieties averaged over two years
Adapted from Coffey et al. 1995. ARPAS 11:74