by Asher Wright
Soybeans (Glycine max (L.) Merrill) have a long history of being cultivated as a forage crop. Historically soybeans were popular forage for nutritious hay and silage and it was not until the 1940’s that soybean production shifted its focus to the bean. According to Morse et al. (1950) this shift (when more acreage was planted for beans than for forage) occurred first in the Corn Belt in 1935 and later for the entire U.S.A.in 1941. In 1929, 63 percent of the total acreage for soybeans was planted for forage, in 1943, 21 percent, and in 1948, 10 percent (Morse et al., 1950). This shift was initially due to the need for a high protein feed source for animal production. As soybean oil prices began to soar in the 1960’s, value as an oilseed and protein crop far out weighed it’s value as forage. Though soybeans are now cultivated almost exclusively as a high-protein or oilseed crop their value as forage is being reconsidered and even implemented in certain parts of the world. So why would a producer plant soybean for a forage?
Grazing soybeans as a forage
For the purpose of this paper all management considerations will be in the context of beef cattle production. During the finishing phase of beef production (when a yearling calf is taken to a finished beef product), high protein and high energy feed are paramount for adequate gains, meat quality, and, ultimately, profit. This is easily achieved when animals are in a feedlot receiving a concentrated diet (a diet higher in energy). However for producers attempting to finish cattle on forage, the type of forage and time it is grazed can have major impacts on animal performance and meat quality. Forage production and quality are impacted by the time of year (Figure 1). For example, alfalfa (Medicago sativa) is considered cool season perennial forage with the majority of its growth occurring in the spring and fall. With exception to regions with a very cool growing season year around, alfalfa production and quality (along with all other cool season perennials) declines during the summer. Soybean and other warm season annuals come into play during summer months. With the underlying goal to provide the highest quality forage at all times, mixing perennials and annuals is a perfect model. In March animals begin grazing alfalfa and continue until late June when they are moved to forage soybeans. Specific dates will fluctuate based on region, year, and forage type.
Figure 1. Cool season perennial forage vs. warm season annual forage production
Soybeans can be grazed full bloom (R2) until near maturity (R7). The most rapid change in fiber and CP is during stage R5 to R7 (Hintz et al., 1994). Similar to other forages, research (Hintz et al., 1992; Hintz et al., 1994; Sheaffer et al., 2001) indicates that forage-tissue fiber increases over time while crude protein (CP) ultimately decreases over time. This is true with the steam and leaf. The pod has the opposite trend (Hintz et al., 1994) to decrease fiber and increase CP, especially during R5 to R7. This is what Sheaffer et al. (2001) meant by “maintain high quality over time”. This gives the producer flexibility with management decisions regarding soybeans. Should I graze, cut, or both?
Forage soybeans are great for ratoon cropping. They can be grazed and then harvested for hay or silage, or grazed multiple times. Dr. Atkinson of Southern Illinois University was quoted by Eagle Seed, producers of two forage soybean varieties, that in the right growing conditions, three grazing’s would not be out of the question. With a well-designed management-intensive grazing (MIG) system, soybean can be grazed two and possibly three times depending on climate and growing zone. This can help eliminate the question of, “which plant growth stage will be the most nutritious”, as cattle will be grazing multiple plant growth stages during the season. Grazing soybeans gives the producer flexibility and the opportunity to provide animals with premium forage through the hotter months of the summer.
Harvesting soybeans for hay or silage
Like grazing soybeans, harvesting for hay or silage can occur from full bloom (R2) until near maturity (R7). However, much of the research (Hintz et al., 1994; Blount et al., 2009) indicates that the best time to cut for hay or silage, to optimize nutritional quality and dry matter yield is between the R6 and R7 growth stage (Hintz et al., 1992; Munoz et al., 1983). These stages are characterized as “full seed” (pod containing full size green beans at one of the four uppermost nodes with completely unrolled leaf) and “beginning maturity” (pods yellowing; 50% of leaves yellow; physiological maturity) respectively. Optimization vs. maximization is a common question and will be addressed later in the paper.
If cutting soybean for hay, long curing times are necessary, which is primarily due to the larger stems. In general, the large fibrous stems are the biggest management issue with regards to soybean hay or silage. Blount et al. (2009) recommends conditioning the stems in order to cure the hay in a timely manner. As mentioned earlier, soybean maintains high quality overtime (Sheaffer et al., 2001). Which enables flexibility with hay harvesting times, a very beneficial characteristic. There is a trade off between DM yield and the speed of curing. When beans are at R5 (beans beginning to develop at one of the four uppermost nodes, with completely unrolled leaf) hay can be conditioned because the beans are small enough to not be pushed out of the pods by the conditioning process. When the crop reaches R6 and R7 the conditioner may crush or pop the seeds out of the pod, so conditioning is not recommended. Not conditioning will greatly increase the curing time to 5 or 6 days (Blount et al., 2009). The increase in curing time is a risk, especially in regions with higher chances of late summer rains. It is common knowledge that when hay is rained on, nutritional quality will be reduced. Whether to cut at R5 or wait until R6 or R7 is dependent on the producers region and what quality of hay or silage is desired. If ensiling Blount et al. (2009) suggests adding a soluble carbohydrate (i.e. corn grain or molasses) at the rate of 10% of DM. Which helps combat high levels of ammonia and butyrate, common byproducts of soybean silage. Whether to ensile or make hay is a management question depending on cost, available equipment, climate, and plan for feeding. In general, silage will have a higher feed value (Table 1) in terms of nutrition.
Nutritional quality and yield
Quality refers to how well the plant can provide for the nutrient demands of a growing animal. So what is forage quality comprised of and what would be considered good forage quality? Other than vitamins and minerals, forage quality often refers to fiber content and crude protein. Fiber is broken down into three fractions, gravimetrically quantified, consisting of Neutral Detergent Fiber (NDF), Acid Detergent Fiber (ADF), and Acid Detergent Lignin (ADL). The actual quantities of fiber lie in the differences, with NDF being the entire cell wall content of the plant and the difference between ADF and NDF representing hemicellulose and pectin. The difference between ADL and ADF is the quantity of cellulose. NDF is typically considered more when assessing quality, and as NDF increases, “forage quality” and ultimately animal performance will decrease. Crude protein takes into account cellular protein and non-protein nitrogen; it is the total N x 6.25. According to Nutrient Requirements of Beef Cattle, finishing steers at 2.5 lbs. of gain per day requires an NDF of 30% or less and a CP of 12.5 % (NRC, 2000).
Much of the literature forage soybean discusses different maturity groups, which to plant, what row spacing and seeding rate to plant at and when to harvest. The research reviewed (Seiter et al., 2004; Hintz et al., 1994; Sheaffer et al., 2011) demonstrated that plant growth stage at harvest is the major factor in quality. The issue of plant maturity is very important with soybean because unlike other legume crops when soybean is harvested just prior to leave yellowing and maturity, the pods are high in protein and oil, which adds to the overall “quality”.
As mentioned earlier, plant maturity is the major factor in fiber content and composition as well. Hintz et al. (1994) found that the greatest change in fiber occurred between R5 and R7. As plants matured NDF and ADF increased, with the greatest shift during R5 to R7. The opposite is true for the pod, which adds to the overall quality. The pod decreased in fiber composition during R5 to R7. During these stages, forage samples sent to the lab that include the bean, will have a much better (closer to 30%) NDF and ADF quantity then stem and leaf alone. According to the research of Sheaffer (2011), harvesting in R6 or R7 will provide the most optimal NDF and ADF quantities because the mature beans are in the pods but leaves have not begin to drop yet.
Seiter et al. (2004) also indicated that plant maturity was the primary factor affecting fiber content. However in one year of their two-year study, row spacing had a significant affect on fiber composition. The wider row spacing (76 cm) compared to the narrow row spacing (18 cm) resulted in larger stem diameter, a possible cause of the increased fiber content. In their study, R5 at 76 cm row spacing resulted in 417 g/kg ADF and 487 g/kg NDF compared to 324 g/kg ADF and 421 g/kg NDF for R5 at 18 cm row spacing. Based on previous research (Blount et al., 2009; Seiter et al., 2004; Sheaffer et al., 2001) a more narrow row spacing is recommended. Though yield is not affected, fiber composition is greater at the larger row spacing, mainly due to increased fiber in the stems. Based on the research of Lundry et al. (2008) Common NDF and ADF values during ideal harvest (R5-R7) will be 30% – 38% and 32% – 38%, respectively.
Other than fiber, crude protein (CP) is a primary indicator of adequate nutrition for growing cattle. As mentioned earlier, 12.5% CP or slightly greater, is ideal for growing and finishing beef cattle. In general, a legume crop will have no problem meeting this, and in most cases exceed it (Table 1). Crude protein may become a metabolic cost to the animal when in too high of a concentration. This is why Blount et al. (2009) recommend mixing sorghum and soybean or corn and soybean when making silage. Energy is always the limiting nutrient in the rumen and balancing energy and CP is paramount to efficient gains and preventing waste.
Like fiber, CP was only affected by forage maturity (Sheaffer et al., 2004). Hintz et al (1992) found that CP was only affected by maturity as well, but they also concluded that a smaller row spacing resulted in less CP (8 g/kg less). This is in agreement with the work of Seiter et al. (2004) who found a wide row spacing (76 cm vs. 18 cm) resulted in lower CP (139 g/kg vs. 155 g/kg) concentrations. This was only true for one of the two years of their study and they concluded that CP is unpredictable due to high year variability. In general the leaves of soybean will average 20 to 22% CP (Lundry et al., 2008).
Though nutritional quality is very important, optimum yield is also very important. This is especially true for the economic side of the equation. Like all crops yield, soybean forage, is affected by precipitation, soil type, and pest pressure. This was demonstrated by the work of Rao et al. (2005) and Nielsen (2011) who both concluded that precipitation distribution played a major roll in yield and that yield was positively correlated to plant water use. When all of these variables are controlled, however, soybean forage yield will follow the same trend with fiber and CP and will increase with plant maturity (Table 2). All studies reviewed (Hintz et al. 1992; Sheaffer et al., 2001; Seiter et al., 2004; Bilgili et al., 2005) found this to be true.
The other major factor affecting soybean forage yield, is row spacing. All of the extension papers reviewed recommended smaller row spacing. Research in Turkey by Acikgoz et al. (2009) found that the narrowest row spacing, regardless of number of seeds planted, resulted in a higher percentage of plants reaching maturity; 68.3% at 20 cm and 54.4%, 48.5%, and 44.8% with increasing rate of 40 cm, 60 cm, and 80 cm, respectively. This is in agreement with previous work by Hintz et al. (1992) who determined that a 20 cm row spacing produced more forage than 76 cm. This contrasts later work by Hintz et al. (1994) who determined row spacing had little affect on yield. From a management perspective and in general, row spacing has been shown to result in higher yields. Based on recommendations by state extension services, narrow row spacing should be implemented to maximize forage yield.
Table 1. Nutrient composition of forage soybean silage and hay, adapted from Dr. Atkinson’s research at Southern Illinois University, www.eagleseed.com/articles.html.
Table 2. Effect of harvest date on soybean forage quality and quantity, Blount et al., 2009.
To optimize or maximize
Soybean is a unique forage crop because it not only produces quality forage but also is an energy and protein dense bean that can be consumed by grazing animals. So the question becomes, at what point should I graze or harvest? If grazing, graze earlier when leaf tissue is high in CP and low in fiber. Manage animals in a way that gives rest to areas that have already been grazed, and another grazing will be easy to achieve. As stated earlier, forage of soybeans will decrease in quality over time until the beans begin to develop in the pods. This should mainly be considered if harvesting for hay or silage. Thus the optimal time to harvest for hay is between R5 and R7. This leads to the issue of combining the stem with the silage, feed refusal, and an ultimate waste of the high-protein, high-energy bean as mentioned by Blount et al. (2009). There is the possibility to chop the forage for a more uniform mixture, but this has been shown to be costly (Blount et al., 2009). Overall optimization vs. maximization will depend on the producers operation, the needs of the animals, and whether harvesting is even an option. Optimize for quality and maximize for yield.
Selecting the correct variety
With plant growth stage playing a large role in quality, selected the proper maturity group variety is crucial for obtaining the best yield and quality possible. Maturity group is an indicator of photoperiod response and must be taken into account. Photoperiod response means that the plant will follow a vegetative and reproductive schedule based on the daylight as opposed to age of plant. For hay production it is important to match highest optimal quality with the greatest chance of a dry weather window. Blount et al. (2009) suggest growing full season (Maturity Group 6, 7, and 8) varieties with the long juvenile traits in the Southern U.S.A. In the Southeast this enables the producer to plant between April and June with little affect on yield. If grazing or harvesting for silage, matching the driest part of the year with optimal maturity is less important, thus variety selection is less important. However, if harvesting for hay, it is important to match maturity with harvest date.
Soybeans will outperform other broadleaf forage legumes such as field pea or vetch (Bilgili et al., 2005) as a forage crop. Soybeans have the potential to provide similar feeding value as ensiled alfalfa, a real opportunity for producers. Based on research and the work of Acikgoz et al. (2009) soybeans for forage will produce their highest yield at a similar seeding rate as for grain (about 900,000 seeds/hectare) but with a more narrow spacing (20 cm or less).
When it comes to yield, forage soybean harvested between R5 and R7 have the real potential to pay off. This is because dried forage will weigh about 3 times more than the mature seed. Based on the work of Blount et al. (2009) using current prices (100$/ton forage and 6$ bu/soybeans) and yields from a 3-year trial in Florida, a 3-ton/acre yield would accrue 50% more profit than the bean alone. This assumes a marketing outlet and price points, but in general, forage soybeans harvested for hay or silage can pay off.
Beyond agronomy and economics, soybeans provide a number of ecological benefits as well. They produce nitrogen through their symbiosis with Rhizobium, provide an excellent wildlife fodder to encourage on-farm biodiversity, and give the producer an opportunity to double crop and keep soil covered after a cereal crop is harvested in early summer. In general, adapted varieties of soybean provide high quality forage for grazing animals during the hottest months of the year. Soybean silage or hay is on par with alfalfa. With affordable seed costs forage soybean is a real opportunity for beef cattle producers everywhere.
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