Soybean Silage

Drought Contingency Plan:

Using Soybeans as Forage – Silage or Hay

Clifton L. Willms, Ph.D., P.A.S.

Cattle Nutritionist

Most soybean producers plant soybeans for the purpose of harvesting the seeds. However, under certain conditions (e.g. early frost or drought) producers have sometimes found it advantageous to harvest soybeans as forage. The purpose of this article is to provide information regarding the optimal harvesting of soybeans for forage.

Bottom Line Recommendation:

If soybeans are harvested as a forage crop, either as silage or hay, they should be harvested in the R6 growth stage. This optimizes dry matter yield per acre and nutritive value. The R6 growth stage is known as the full seed stage, but prior to leaves yellowing and falling off.

Growth Stages:

	Agronomists (plant physiologists) have developed well-defined growth stages for soybeans. There are several vegetative and reproductive growth stages. Below is a partial list of the reproductive stages for reference.
	For a complete description of vegetative and reproductive stages see (http://ohioline.ag.ohio-state.edu/b827/b827_54.html)

*Growth stages of soybeans*
Growth Stage	Stage Title	Definition
R2	Full Bloom	Flower present at node immediately below the uppermost node with a completely unrolled leaf.
R3	Beginning pod	Pod is .5 cm long at one of the four uppermost nodes with a completely unrolled leaf.
R4	Full Pod	Pod is 2 cm long at one of the four uppermost nodes with a completely unrolled leaf.
R5	Beginning seed	Beans beginning to develop at one of the four uppermost nodes with a completely unrolled leaf.
R6	Full Seed	Pod containing full size green beans at one of the four uppermost nodes with a completely unrolled leaf.
R7	Beginning maturity	Pods yellowing; 50% of leaves yellow; physiological maturity

Forage Yield:

	Research consistently demonstrates that the optimum growth stage for harvesting whole plant soybeans for forage (silage or hay) is in the R6 stage. At this stage the pods are filled and some of the leaves are starting to turn yellow, but before leaves start to fall off.
	Whole plant soybean forage yields increase until the leaves start to turn yellow and falling.
	Averaged across two varieties of different maturities (maturity groups IV and V) and two years of production, harvesting soybean silage at the R4 stage increased DM tonnage 60% compared to harvesting at the R2 stage. Harvesting at the R6 stage increased tonnage another 30% over harvesting at the R4 stage. This was a result of both more plant growth as well as increased DM content of the plants. (Coffey et al. 1995. ARPAS 11:74.).
	Yields for whole plant soybean silage reported in the above study at the R6 stage of growth is shown in the table below.

Soybean silage yield harvested at the R6 growth stage
	*Range*	*Average*
Silage harvested, tons/acre
Fresh	8.5 to 13.2	10.7
Dry matter	2.7 to 3.5	3.2
Silage, % DM	25.8 to 36.7	30.1
Two varieties (maturity group IV and V) averaged over two years of production. Adapted from Coffey et al. 1995. ARPAS 11:74.

Forage nutritional quality:

	In vitro dry matter digestibility (IVDMD) of whole plant soybeans remains relatively constant at approximately 60% DM digestibility across various reproductive growth stages (R1, R3, R5, R6 and R7). Leaves and pods were much more digestible (roughly 68 to 72% IVDMD) than stems (roughly 40 to 46% IVDMD). Stem digestibility decreased drastically beginning with pod development. (Munoz et al. 1983. Agron. J. 75:147.)
	In contrast to other forage crops, the nutrient content and forage quality of whole plant soybeans does not change as drastically with advancing maturity because the seed is much higher in protein and energy (fat). Even though the stems have relatively low nutritional value, the seeds are protein and energy rich.
	In contrast, silage harvested at the R4 stage had lower digestibility than silages harvested at the R2 or R6 stages. This seems reasonable since most of the structural carbohydrates are formed by the R4 stage, but the yield of pod material increases substantially through the R7 stage. At R6 the digestibility of pods exceed that of stems by as much as 25 percentage units and contribute significantly to the overall nutritive value of the whole plant. (Coffey et al. 1995. ARPAS 11:81.)

*Nutrient content of soybean silage*
	*Range*
Crude protein, %	16.0 to 20.6
Neutral Detergent Fiber, %	38.3 to 48.3
Acid Detergent Fiber, %	27.3 to 37.3
Acid Detergent Lignin	6.0 to 7.4
Calcium, %	1.36 to 1.49
Phosphorus, %	0.26 to 0.31
Two varieties (maturity group IV and V) averaged over two years and growth stages R2, R4, and R6
Adapted from Coffey et al. 1995. ARPAS 11:74.

Direct cut soybeans make very wet silage. Average DM content of soybean silage (averaged across two varieties and two years) harvested at R2, R4, and R6 growth stages were 22.1%, 25.7%, and 30.1%, respectively.

Palatability:

	If whole plant soybeans are put up as hay, it is important to chop or grind the hay to improve palatability of the stems. Stems are quite coarse and will be refused if not ground.
	In one study, chopped soybean straw was consumed by mature ewes at 1.67% BW compared to ground grass hay at 2.46% BW. Ground soybean straw consumption was intermediate at 1.98% BW. (Gupta et al. 1973. Agron. J. 65:538.)
	When sheep were given a choice, they preferred corn silage 3:1 to R4 soybean silage and 5:3 over R6 soybean silage. (Coffey et al. 1995. ARPAS 11:81.)
	Soybean silage harvested in the R2 and R4 stages had various degrees of mold, a displeasing odor, and a dark green and black color. Soybean silage harvested in the R6 stage rarely had mold, retained a color similar to freshly harvested forage, and had a pleasing odor. (Coffey et al. 1995. ARPAS 11:81.)

Silage Inoculants and ensiling:

	Differences in ruminal fermentation characteristics between inoculated and non-inoculated soybean silage have been inconsistent (Coffey et al. 1995. ARPAS 11:81.).
	Inoculated soybean silages tended to have a shorter lag time compared to non-inoculated silages. (Coffey et al. 1995. ARPAS 11:81.). When measuring in vitro digestibility, lag time refers to the time before differences in digestion can be measured. This time frame is when the forage becomes hydrated in the rumen, microbes get attached, and digestion starts. Hence, a shorter lag time is preferable when measuring forage digestion.
	Inoculated silages harvested in the R4 and R6 stages had reduced ethanol concentrations. (Coffey et al. 1995. ARPAS 11:74.). Ethanol is an undesirable end product of silage fermentation.
	While there were minimal differences in ruminal fermentation criteria, the above studies did not evaluate dry matter recovery or quantify respiration loss differences between inoculated and non-inoculated soybean silage. However, it was observed that there was more mold, unpleasant odors, and dark coloration of R2 and R4 silages.
	In the opinion of this nutritionist, the nutritional profile of soybean silage fits the criteria where one has a good opportunity to benefit from the use of an inoculant under most conditions.

	Soybean silage is low in dry matter (high in moisture). This increases the risk of seepage and high dry matter losses.
	Protein content is high (> 15% CP) and this buffers the silage such that pH remains higher compared to other feed grain silages (e.g. corn silage). A high pH increases the time required for the fermentation process, increases DM losses due to fermentation, and increases the risk of secondary fermentations that give putrid odor and poor palatability.
	Use an inoculant that has appropriate research and quality control. Several inoculant products have been tested and compared on different types of silages. (KSU Cattleman’s Day. 1989. Report of Progress 567.)

It also seems appropriate that thought should go into the type of silo structure to be used. Because of the high protein content of soybean silage, face management becomes a critical issue. Excessive exposure of the face will create additional DM losses. A narrow concrete bunker silo or a bag seems to work well.

Since the objective of most soybean producers is for seed rather than forage for ruminants, the decision whether or not to harvest soybeans for forage should be delayed as much as possible. It is fortuitous that the optimum time for forage harvest of soybeans is the R6 stage of growth.