Forage Soybeans for Grazing, Hay, and Silage

Nutritional Quality

During the 1920s and 1930s, forage soybean produc­tion—and the nutritional value of hay and silage—were investigated extensively. These earlier trials concluded that soybeans should be drilled at high populations (1.5 2.0 bu./ acre of average size seed) to obtain small stem diameters. A high plant population would promote thinner-stemmed soybeans, which would be easier to condition and to ensure proper curing. The stem is the least desirable plant com­ponent because it is considered to be coarse and fibrous. Soybeans may be harvested for hay at any stage of growth. Because summer rainfall patterns in Florida are erratic, conditioning the stems may be necessary to cure hay in a timely manner.

Unlike most legume crops used for hay, both foliage and pods provide digestible protein. When harvested just before leaf yellowing at maturity, the soybean plant has pods that are high in protein and oil. The quality of forage soybeans varies depending on the variety, stage of growth, age, and harvesting losses. Contrary to other forage legumes, soybean's percent quantity of protein and digestible energy material per acre increase after flowering until near maturity (Table 1). From the standpoint of producing the highest quantity and quality of forage, the best time to cut is at 90% pod fill or just before the leaves begin to yellow and fall. One of the main advantages of soybean forage is the flexibility of harvest dates because its quality is good over a long period. Tonnage is sacrificed when harvested early.

The soybean plant maintains its high quality over an extended period because of the high protein and energy contents of the developing seed. The mature seed has about 40% protein and 20% oil. When the seeds reach about 90% pod fill before leaf drop, about one-third of the plant weight is in the seed. As the seeds develop, they more than compensate for the loss in quality in the stem and other plant parts.

There is a trade-off in soybean production between tonnage and the speed at which the hay dries. When the pod is at full size and the seed is still small, a hay conditioner may be used to crush the stems and reduce the drying time. Minimal handling of the forage will reduce loss due to shattering. Consider tedding hay earlier in the day when moisture in the plant is sufficient to reduce leaf loss. With good drying conditions, drying time for conditioned hay is about 2–3 days. When the seed are larger, the conditioner may crush or pop the seed out of the pod. At this point, conditioning soybeans is not practical and drying time is greatly increased because of the seed's size. Soybeans with full-sized seeds that have not been conditioned may take 4–6 days to cure properly.

Animals may not eat some soybean hay components, and losses from this refusal may range from 0% to 20%. When the whole plant is ground, animals eat it readily, but this is not considered to be an economically viable practice.

Soybean Silage

When producing silage from soybean, water retention in the seed is not a problem, and the lower palatability of the stems is solved by chopping and the subsequent fermenta­tion process.

The best stage to harvest soybeans for silage is near matu­rity, before any leaf loss. At this point, soluble carbohydrates required for proper ensiling are low. When ensiled alone, soybean silage has an unpleasant odor and emits free ammonia and butyric acid, which are characteristics of undesirable ensiling fermentation.

For proper fermentation to occur with soybean silage, a source of soluble, readily fermentable carbohydrates, such as ground corn grain or molasses, should be added. A minimum of 10% of the total silage on a dry-matter basis should be from these carbohydrate sources. Another option is to blend with corn or sorghum silage (i.e., 50:50). Excellent quality silage has been made from intercropped sorghum and soybean forage, from layering sorghum and soybean forage harvested from different fields, and from mixing corn grain or ground ear corn with the soybean forage.

Be sure to add a commercial inoculant for proper ensiling. A number of improved commercial inoculants are superior for use when making legume silage.

Variety Selection

As with soybeans grown for grain, the highest forage yields are obtained from planting full-season soybean varieties (Maturity Groups 6, 7, or 8) from May 10 to June 15. Maturity group is important to consider because it indicates photoperiod response. The beginning of the reproductive period with flowering and the date of maturation are affected by day length more than the actual age of the plant. Varieties differ in their response to day length. Varieties selected for hay production require matching the expected optimum growth stage with the highest probability of dry weather. Drier periods are more common in late spring and in October. For June harvest, early Maturity Groups 4 and early 5 planted in March are probably the best choice. For an October harvest, Maturity Groups 8 and 9 varieties that were planted in June are probably best. For silage produc­tion, it is not so important to match growth stage and dry weather, so variety selection is less critical.

Nematode resistance and forage yield performance data are very important for variety selection. To reduce the stem size, high plant populations (1.5–2.0 bu./acre) should be drilled or planted in narrow rows. Conventional or no-till planting should be followed. Appropriate soybean inoculants need to be applied to the seed prior to planting in fields not previously planted to soybean. Apply inoculant using a sticker and limit exposing the inoculated seed to sunlight.

The Florida soybean breeding program has concentrated on the development of the long-juvenile trait in soybean. This trait is a single recessive gene that allows soybeans to have a similar period of vegetative growth regardless of planting date. This gene has been incorporated into soybean lines that have resistance to multiple pests. The resulting lines exhibit excellent agronomic performance as well as the ability to be planted over a broad range of planting dates. For example, in northern Florida, the long juvenile soybeans can be planted from April to July with a minimal effect on yield ability. When the long-juvenile soybean is planted following corn in July, the soybean will remain vegetative throughout the summer and flower in late August or September. Beginning pod stage will occur in October, during the driest month of the year, when a window of opportunity exists to harvest soybeans for hay. Hinson Long-Juvenile soybean was released by the University of Florida in 2001 and is a well-adapted forage soybean variety for grazing, hay, or silage production. It has good resistance to southern root-knot nematode and pod and stem blight. It is presently being used by dairy and beef cattle producers and—to a limited extent—by wildlife enthusiasts. Seed of this variety are available by contacting the Florida Foundation Seed Producers, Inc. in Greenwood, Florida, or Barenburg US.

Today with the development of Roundup-Ready® soybeans, there are forage-type varieties available on the market. We have limited information on their performance; however, they have done well in Georgia trials.

Weed Control

There are fewer control measures for weeds in forage than in grain soybean production. Many herbicides have restrictions for use over the top of soybeans when grazed or used for forage. To have limited weed problems, fields with low-weed pressure should be selected and pre-emergent herbicides should be used to control weeds until the crop canopy shades the weeds. The high population and narrow rows will greatly aid in weed control.

For more information, visit the EDIS Publication SS-AGR-05, Weed Management in Soybeans, at https://edis.ifas.ufl.edu/wg010.

Insect Pests

There are several obstacles to managing insect pests on soybeans grown for grazing, hay or silage. Foremost among these problems is the small number of registered insecticides that permit the use of the crop for grazing or hay following application. Pre-harvest intervals range from three to 20 days for the insecticides registered for use on soybeans grown for forage or grazing. This means that, treatments of a number of insecticides cannot be made just before harvesting for hay at the pod fill stage. This is a period in soybean development when they are highly attractive to a number of insect pests, including stink bugs and corn earworms.

Table 2 gives the common names of the active ingredients and an example of a registered product for soybeans that are grazed or cut for hay or silage. Suggested use rates for the commonly encountered insect pests on soybeans in Florida are also given. Remember to read all pesticide labels carefully, and follow label recommendations.

Carbaryl (Sevin 4F) may be used on soybeans within 14 days of grazing or being cut for hay. It may be used up to four times per season but no more than once every seven days.

Diflubenzuron (Dimilin 2L) is a restricted-use insecticide that may be used within 21 days of harvest. A maximum of two applications may be made per season.

Dimethoate (Dimethoate 4EC) has the shortest pre-harvest interval of any of the insecticides registered for use on soybeans that are grazed or cut for forage. Dimethoate may be used to within five days of feeding or grazing.

Methomyl (Lannate LV) is a restricted-use insecticide that may be used to within three days of harvest as forage and 12 days of cutting for hay.

Methyl parathion (Declare 4EC) is a restricted-use insecti­cide that has a pre-grazing interval of 20 days.

Economics of Soybean Forage

Soybeans may be grown for use as high-protein forage with quality similar to alfalfa. When harvested at maximum dry matter per acre (when seeds are nearly full size and leaves have not begun to fall), the weight of the dry forage is about three times the mature seed. Therefore, a 30 bu./acre grain crop would yield about 2.7 tons/acre of dry forage. Yield trials at the UF/IFAS North Florida Research and Education Center in Quincy, FL, have averaged around 3 tons/acre dry matter from June and July plantings. At $100/ton for dry forage and $6/bu for soybeans (current price is near $5.00/ bu), an acre is worth about 50% more as a forage crop.

Summary

Soybeans may be grown for use as a high-protein forage for grazing, haying or ensiling. Adapted varieties should provide forage with qualities similar to alfalfa. Inexpensive seed makes this summer legume an excellent choice for livestock and wildlife. Concerns about weed and pest control measures should be minimized with careful management of this forage as outlined in this publication. For further information on recommended soybean varieties contact your local UF/IFAS Extension office (http://sfyl.ifas.ufl.edu/find-your-local-office/).