Soybeans as a forage crop

Introduction

Soybeans were originally introduced to Ontario as a forage crop for cattle. So, this year with some late planted soybeans, with a high demand for forages, and with some alfalfa fields being planted to soybeans after first cut, it seems timely to explore the options of using soybeans as a forage crop.

The soybean may be grown as an annual hay or as a pasture crop; it may be ensiled or fed green. The soybean is one of the few annual legumes suitable for the production of hay, and can therefore substitute for this purpose in the event of an alfalfa failure. In three to four months after seeding it produces hay equal in quality to alfalfa hay. It is usually recommended to feed it with other kinds of hay.

Several management factors were compared for yield and quality of the forage and were reported in the Journal of Agronomy in 1992. Row spacing, population, varieties and stage of development at harvest were all compared and are summarized in Table 1.

Soybeans produced for forage are basically grown with the same management considerations as for grain. The biggest consideration comes with time of harvest, since quality and yield are impacted by that decision.

1. Narrow rows (7 inch) and high populations are preferred over wide rows and low populations. Plants grown at high densities (over 125,000 ppa) produce stems with smaller diameter. Course stems are not as digestible and are left uneaten by livestock.

2. Variety selection does impact on yield and quality. Varieties studied do not match those used in this area, however, dry matter yields generally increased for later maturing varieties.

3. Maturity stage at time of harvest has the greatest effect on yield and quality. Dry matter yields increased with each advance in maturity stage. Crude protein levels decreased from late vegetative stage to late reproductive stage, but as the pods matured, crude protein increased.

4. Soybean forage harvested between R6 and R7 stage (when leaves begin to turn yellow, but before they begin to fall off the plant - usually in mid September in our area) is comparable to alfalfa hay harvested at an early bloom stage of development for crude protein, ADF, and NDF. Since soybeans seeds have a higher oil content, the ether extract (EE) content of the soybean forage is significantly higher than for alfalfa forage. The nutritional impact at first would appear desirable, however feeding large amounts of vegetable fats to ruminant livestock has not always proven beneficial. Increasing the EE content of the ration of lactating cows can increase milk production, but can also decrease intake and reduce fiber digestion. For this reason, it is recommended that the EE concentrations of rations shouldn't exceed 5%, which means that soybean forage harvested at R7 stage should be limited to no more than 50 percent of the total ration dry matter.

Soybean ensilage has sometime been mixed with corn silage and feeding trials have indicated little difference between in value between corn-soybean silage and corn silage. When mixed, it is recommended that about 2 or 3 parts corn and 1 part soybeans makes a well-balanced silage that keeps well and is readily eaten by cattle, and produces no negative effect on milk quality.

There are a few precautions to watch for, with forage soybeans. When white mold is a problem, the palatability will be reduced. Some moulds can lead to toxin production and should be tested before incorporation into a ration.

The other limitation for some soybeans being used as a forage will be the herbicide used for weed control. The following herbicides have specifically given label restrictions that limit their use for forage soybeans. Check the label of any herbicides used before harvesting and feeding the soybeans as a forage.

Herbicides

Assure
Excel (60 day restriction)
Excel + Basagran
Fusilade
Hoegrass
Pinnacle
Poast
Pursuit
Sencor

The table below gives an indication of Whole Plant Nutrient Analysis (DM basis) at 3 different stages of growth. This information comes from Southwestern Ontario harvest research where harvest occurred during the first week of September. The sample differences indicate differences in maturity.