Fresh soybean forage
Currently soybean is rarely grazed so there is limited information regarding this utilisation in contemporary livestock farming. Much of the experience on fresh soybean forage was collected in the USA in the first half of the 20th century when it was relatively popular (Morse et al., 1952).
In a cow preference study in Australia, cows grazed soybean for a significantly greater time (approximately 70%) than cowpeas (Vigna unguiculata) and lablab (Lablab purpureus) (Horadagoda et al., 2009).
Due to its relatively high fibre content, soybean forage is of variable but moderate OM digestibility, comprised between 60 and 70%.
In the USA, pasturing cattle on soybean was not recommended because it damages the growing plant. However, mixtures of Sudan grass and soybean were popular as midsummer pastures for dairy cattle (Morse et al., 1952). In a contemporary trial in Australia, dairy cows grazing soybean forage had a higher feed intake than cows grazing kikuyu (Pennisetum clandestinum) pastures. However, due to the lower DM content of soybean pasture, the total DM intake was not different between the two groups of cows, and milk production was not improved by soybean (Clark et al., 2014).
Pasturing lambs and breeding ewes on soybean forage used to be a popular practice in the USA, particularly in the Corn Belt (Morse et al., 1952).
Soybean silage is a valuable protein source that can be used to replace expensive feeds such as soybean meal in ruminant diets (Gobetti et al., 2011).
Ensiled mixtures of maize and soybean silage used to be popular in the USA for feeding dairy cattle (Morse et al., 1952). In a more recent trial, dairy heifers fed on sorghum-soybean silage had a daily gain of 380 g/d, but degraded DM intake and FCR, together with lower nutrient digestibilities values, compared to heifers fed alfalfa hay, oatlage or wet corn gluten feed (Jaster et al., 1984). In Brazil, crossbred (Holstein x Zebu) steers were fed on diets including up to 40% soybean silage as a protein supplement to maize silage and concentrate. Adding an inoculant and/or molasses to the silage increased DM and NDF intake as well as weight gain compared to untreated soybean silages (Rigueira et al., 2008). Nellore steers were fed up to 60% soybean silage and 40% concentrate for 84 days. Soybean silage slightly reduced DM intake (2.09 vs. 2.17 kg/d) and slightly increased average daily gain (1.21 vs. 1.17 kg/d) in comparison to a maize silage diet (Souza et al., 2008).
In Japan, a pure soybean silage, included at up to 17% of the diet (DM basis) for sheep, successfully replaced wheat bran in a maize silage-based diet (Touno et al., 2014). In Brazil, soybean silage (60% of the diet DM) supplementing sugarcane tops (20%) and concentrate (20%) in a diet for lambs resulted in a daily weight gain of 133 g/d. Compared with maize silage, soybean silage yielded the highest protein intake, the highest digestiblity of nutrients and resulted in the highest daily weight gain in finishing lambs (Lima et al., 2013). In Cuba, a mixture of 40:60 sorghum and soybean forages yielded a high grade silage with a higher ME than the same mixture before ensiling (11.2 vs. 10.3 MJ/ kg) (Lima et al., 2011).
In the USA up to the 1950s, soybean hay used to be widely fed to ruminants, though it was not recommended to feed it alone as it would cause digestive upsets. The presence of woody and coarse stems could make it unpalatable but heavier rates of seeding as well as a timely harvest could alleviate this problem (Baxter et al., 1982; Morse et al., 1952). There have been very few trials concerning soybean hay conducted after the 1950s, in or beyond the USA.
In the USA, soybean hay cut before it becomes coarse was considered to be equal to alfalfa and red clover for milk production and for the maintenance of weight in dairy cattle. Hay cut when the pods were completely formed was superior to hay cut at the earlier stages for milk production (Morse et al., 1952). In jersey cows fed isonitrogenous diets containing, as sole forage, soybean hay cut at bloom (protein 14% of DM) or at early pod stage (protein 15% of DM), there were no significant difference in body weight, milk yield or milk fat content (Hubbell et al., 1986).
In the USA, soybean hay was found suitable for feeding beef cattle and compared favourably with alfalfa and clover (Morse et al., 1952).
In the USA, soybean hay used to be fed to fattening lambs, but only fine hay was suitable for young lambs. Breeding ewes fed only soybean hay performed similarly (lamb weight, ewe weight, health and dairy performance) as ewes fed alfalfa hay (Morse et al., 1952).
Soybean straw is considered as a roughage but with a better nutritional value than rice straw. It is suitable for cattle both as fresh and ensiled material. The most practical ways of utilizing soybean straw in dairy cattle feeding systems are as a roughage source supplemented with protein sources or concentrate feeds, or as supplemental roughage. To improve the nutritive value of soybean straw and pods, treating it chemically with urea and spraying it with a urea/molasses solution have been suggested (Sruamsiri, 2007).
The palatability of untreated soybean straw is low because of its relatively hard stem (Sruamsiri, 2007).
OM digestibility of soybean straw ranges from 42 to 64%.
In the USA, soybean straw given without a supplement to weaned beef cattle heifers did not maintain their weight, but when supplemented with maize grain there was a small weight gain of 110 g/d (Bagley et al., 1989). In India, Murrah buffaloes heifers fed soybean straw ad libitum as the sole diet had a daily weight gain of 316 g/d, but supplementation was recommended for better performance (Kumar et al., 1995). Soybean straw fed to growing calves replaced 50% or 100% of sorghum straw in diets with concentrates, supporting the same growth performance (Adangale et al., 2009). In the USA, soybean straw treated with various alkalis had a higher in vitro DM and OM digestibility. Alkali-treated soybean straw fed alone to heifers and steers (202 kg) for 141 days allowed the same daily weight gain as fescue hay (Felix et al., 1993; Felix et al., 1997).
DM intake of soybean straw fed alone to sheep ranged from 55.76 g/kg W0.75 (Brazil, Carneiro et al., 1998) to 62.84 g/kg W0.75 (India, Singh et al., 2009). Soybean straw offered as sole feed to growing sheep (India, Singh et al., 2009) or supplemented with 8.5% soybean meal for mature ewes (United States, Gupta et al., 1973) met the maintenance requirements in both cases.
Soybean straw offered as sole feed to growing goats met the maintenance requirements (India, Singh et al., 2009). DM intake of soybean straw fed alone ranged from 35.60 g/kg W0.75 (Brazil, Carneiro et al., 1998) to 53.21 g/kg W0.75 (India, Singh et al., 2009). In India, soybean straw fed ad libitum with a concentrate to growing kids led to a higher forage intake and lower concentrate intake compared to sorghum stover (Baswade et al., 2007). In Nigeria, soybean straw included at up to 30% of the DM, replacing maize mill waste, in the diets of growing goats significantly increased DM intake (516 vs. 465 g/d) and daily weight gain (57.8 vs. 34.7 g/d) (Arigbede et al., 2008). In India, a complete diet comprised of 60% soybean straw and 40% concentrate maintained the body weight of 26 kg adult goats (Kale et al., 2009). The same diet allowed a daily weight gain of 48.6 g/d in growing kids (Rajmane et al., 2000).