Using Soybeans for Forage
Soybeans have been used as a forage crop since they were originally brought to the United States. Their use as a forage declined after the early 1940 as producers adopted other forages that had higher yield. During the 90's, several new forage type varieties with improved yields were developed. With the recent increase in the price of nitrogen fertilizer, there has been renewed interest in the use of these soybean varieties as a forage. The new forage varieties are taller and later maturing than those used for grain production. Like the grain varieties, the forage varieties are susceptible to Asian rust and other diseases common to soybeans, but most of the products approved for treating the grain varieties have not been approved for use on forages. The research that has been conducted comparing varieties indicates that there is considerable variation among varieties in yield and nutrient composition of the resulting forage, so variety selection is important. Although soybeans have approximately 20% fat which provides a good deal of energy, soybean forage only has less than 2% fat and higher concentrations of fiber which result in lower energy concentrations than other legumes such as alfalfa.
One of the challenges with soybean forage is preserving the nutrients. Although soybeans can be harvested as hay, there is considerable leaf loss since the leaves dry faster than the stem and fall off when the dry forage is raked and baled. Most producers elect to harvest as silage or baleage which also has its challenges. Soybean forage, like all legume forage, has limited concentrations of sugar and practically no starch which results in a slow, prolonged fermentation. Because less acid is produced, the resulting silage has a higher pH than corn or sorghum silage which makes it more susceptible to secondary fermentation when exposed to oxygen. Soybeans should be cut with some type of mower-conditioned to mash and break the stems to increase the drying rate. The forage should be wilted to approximately 40 to 50% DM before chopping and chopped to no longer than ½ inch theoretical length of cut so it can be packed tightly. Attempting to ensile wet forage, less than 35% DM, typically results in clostridia fermentation that can produce toxins or at best high concentrations of butyric acid which will reduce acceptability and intake when fed. The chopped forage should be treated with a bacterial innoculate that has been proven to work on legumes. If the forage is preserved as a baleage, it should also be wilted, inoculated, and wrapped as soon as possible to promote good fermentation. As with all forages, the ensiled forage should be sampled and tested before feeding.
There is limited research on the performance of lactating dairy cows fed diets containing forage from soybeans. Canadian researchers recently reported that the results of a trial comparing alfalfa silage with soybean silage fed to lactating dairy cows. The experimental diets contained 36% of the DM from either alfalfa or soybean silage and were not adjusted for differences in nutrient content of the test forages. This resulted in slightly higher NDF concentrations in the diet containing soybean silage compared with the diet containing alfalfa silage; 36.7 vs. 34.1% of DM, respectively. Cows fed the diet containing soybean silage consumed less DM (51.8 vs. 55.3 lb/d) and produced less milk (78.3 vs. 82.0 lb/d) that had higher concentrations of fat (3.78 vs. 3.58 %) and MUN (15.7 vs. 14.0 mg.dL) than that observed for cows fed diets containing alfalfa silage, respectively. Energy-corrected milk yield (71.7 lb/d) and dairy efficiency (1.54 lb milk/lb DMI) were similar for both treatments. The lower intake observed when the cows were fed the diets containing soybean silage was due to the higher NDF and slower rate of digestion of the soybean silage compared with that of the alfalfa silage.
Soybean forage offers another option that may be useful for some producers to consider as they look at alternative forages. If you plan to harvest soybean forage, it is important to wilt the forage and take every step to promote good fermentation which is much more of a challenge than ensiling corn or forage sorghum. Diets should be formulated to compensate for the higher NDF content and lower energy content of the soybean forage. When these steps are taken, milk production can be comparable to that observed with alfalfa silage. If the diet is not adjusted for the higher NDF and lower energy content, DM intake and milk yield may be lower than expected. Soybean forage offers another option that may be useful for some producers to consider as they look at alternative forages.
Dr. John K. Bernard
Dairy Research & Extension
Animal & Dairy Science Dept.