Sorghum forage can be used in different ways, as pasture, green chop, hay, silage and crop residues (straw, stubble and leaves). Due to its relatively low protein content, it usually requires protein supplementation as well as minerals and vitamins (Undersander et al., 2003). bmr varieties, developed since the 1990s, are supposed to have a higher fibre digestibility that makes them competitive with maize (Brouk et al., 2011).
Due to the vastness of the topic, the following recommendations for ruminants do not constitute an exhaustive overview of utilization of sorghum forage throughout the world and are more a sampling of reported scientific work on the subject. Readers are invited to consult their local extension services for proper advice on the best way to use sorghum forage in their specific circumstances.
For forage sorghums grown in Western Europe, intake is good at the beginning at the cycle but decreases rapidly as soon as the inflorescence grows. Intake is higher for regrowth: for instance, intake at bolting stage in the 1st cycle is 21.5 g DM/kg LW while intake at regrowth is 24.0 g DM/kg (Vignau-Loustau et al., 2008).
Digestibility and energy value
As an example of the variation in the nutritive value, OM digestibility and ME values for fresh whole sorghum grown in Western Europe were 71 % and 9.5 MK/kg DM at bolting, decreased to 61 % and 8.4 MJ/kg DM at heading and more or less stabilised after that stage. The differences between a 8-week regrowth grown in normal conditions or during a drought were -12 percentage points for OM digestibilty (70 to 58 %) and -1.4 MJ/kg (9.4 to 8 MK/kg DM) (INRA, 2007).
When compared to maize silage, sorghum silage is more fibrous and less digestible. In temperate regions, whole crop sorghum silage has a nutritive value higher than that of fodder grasses and legumes, and close to that of a medium-quality maize silage (Vignau-Loustau et al., 2008). It has been noted that equations based solely on ADF for predicting digestibility are likely to underestimate the value of bmr varieties (Brouk et al., 2011).
Pasture and stubble
While silage is the main use of forage sorghum (at least in temperate and developed countries), pasture and stubble grazing are common practices in some areas. In Australia, for instance, fodder sorghums are also used for autumn grazing by dairy and beef cattle to fill in a feed shortage between summer and winter grazing crops. The grain sorghums are valuable for grazing after the grain has been harvested and the crop residues (stubble, dropped seed-heads and regrowth, plus weeds) provide good autumn and winter roughage (Cook et al., 2005).
In Venezuela, cows grazing ad libitum a mixture of Sorghum bicolor and Pennisetum purpureum with a concentrate supplement produced in average 17.0 kg of 4 % fat-corrected milk during the first 6 months of 2nd or 3rd lactation. (Bodisco et al., 1976).
In Queensland, Australia, yearling steers grazing sweet sorghum at the rate of 1.25 head/ha had average daily gain of 0.86 kg/head/d over 7 months (experiment from 1965 cited by Cook et al., 2005).
Several trials have taken place in Brazil that demonstrated that sorghum pastures can support growth though being slightly inferior to other grasses. 13-month old Charolais, Nelore and crossbred steers (1.75 t/ha) grazing fertilized sorghum pasture gained around 50 kg in 3 months (0.61 kg/head/d) (Neumann et al., 2005). 15-month old crossbred heifers gained less weight grazing sorghum than Pennisetum americanum (0.71 vs 0.78 kg/head/d) (Coser et al., 1983). In a comparison of several grasses including Sorghum bicolor, Pennisetum purpureum, Brachiaria plantaginea and Pennisetum americanum, steers grazing sorghum had the second best daily gain (1.121 kg/d) after Pennisetum americanum, but no significant differences were found in terms of stocking rate and liveweight gain. It was concluded that a properly managed summer sorghum pasture could be used as an alternative source of feed for intensive beef cattle production systems (Restle et al., 2002).
Fresh sorghum forage is highly palatable to goats but they preferred it chopped (Abdel-Moneim et al., 1999). In Sudan, a comparison of cut-and-carry alfalfa and sorghum forages fed to sheep and cattle found that alfalfa intake was higher than sorghum intake. DM digestibility of alfalfa and sorghum was similar in cattle but alfalfa was more digestible in sheep. Supplementation of sorghum grain and molasses increased the digestibility of sorghum forage in both animal species (Ahmed et al., 1983).
Because of its salt-tolerance, sorghum forage can be valuable in saline soils, for instance in coastal areas and other salt-affected regions of North Africa of the Middle-East. In Oman, sorghum hay grown under high salinity levels (3, 6 and 9 dS/m) may be used for feeding Omani sheep without adverse effects on health or performance. Sheep ate similar quantities of sorghum and Rhodes grass (Chloris gayana) hay, but their daily weight gain tended to be lower with sorghum. DM and crude protein digestibilities were lower with sorghum than with Rhodes grass, but fibre digestibilities were similar (Al-Khalasi et al., 2010).
In Egypt, sorghum grass cultivated in strongly saline soils (10 dS/m) and irrigated with water containing 4000 to 7000 ppm total salts contained enough nutrients to cover the nutritional requirements of sheep. Yields were also promising and sorghum forage could be able to solve summer and autumn feed shortages while increasing the economical value of marginal saline resources (Fahmy et al., 2010). In Pakistan, sorghum hay (regular and bmr) cultivated in saline land and harvested 80 days after sowing could profitably replace wheat straw as the base fodder for sheep. Regular sorghum was almost twice productive than bmr sorghum (13.2 vs 7.5 t/ha) (Khanum et al., 2010).
In a trial with dairy goats, sorghum hay could replace alfalfa hay without impairing milk yield (Andrade et al., 1996). No differences in digestibility of medium quality (sorghum hay) and high quality (oat hay) forages between Rambouillet sheep and Angora goats were observed but Angora goats, who have higher protein requirements due to mohair growth, responded with increased intake to higher levels of supplemental protein (Huston et al., 1988).
Most trials with sorghum silage have taken place in developed and temperate countries and have examined the merits of sorghum relatively to maize for dairy cows.
Sorghum silage is often promoted as a replacement for maize silage for lactating and dry dairy cows and there is an extensive literature about this. The conclusion so far is that utilisation of sorghum forage as a total replacement for maize silage in lactating diets is possible in some cases. Studies have reported similar milk production from sorghum silages, and particularly with bmr sorghums, as compared to maize silage. In France, an example of a successful utilization of sorghum (from a non-bmr cultivar) has been described as follows: in a 15-week trial, dairy cows were fed sorghum or maize silage ad libitum with 2.2 kg/d of concentrate (and 50 g/d of urea for the maize treatment). Sorghum was more palatable than maize (+ 15 % DM intake), milk production was similar (about 20 kg/d) and fat content was higher with the sorghum treatment (4.26 vs 4.01 %). However, the authors noted that, in a previous trial, drought had caused improper fecundation and bad silage quality, and that milk yield had been much lower (-16 %) than for the control diet (Emile et al., 2005). In the USA, cows fed 65 % silage (maize, alfalfa, bmr and regular sorghum), bmr sorghum silage resulted in a milk yield lower than that allowed by maize silage, similar to that obtained with alfalfa and higher to that obtained with regular sorghum. In a second experiment, bmr silage fed at 35 % in the diet resulted again in higher yield than regular silage but did as well as maize (Aydin et al., 1999). However, there are large variations in nutrient profiles between varieties and it is also possible to lose production, so total substitution of maize silage with sorghum silage, even from bmr varieties, should be studied carefully (McCollum et al., 2005; Brouk et al., 2011).
Growing and beef cattle
The use of sorghum silage in beef cattle has been much less investigated than in dairy cattle. It is used by commercial farms in the USA where it is considered to offer better control of weight gain in growing and dry cattle (Brouk et al., 2011). In a 2-year trial in Texas, bmr sorghum silage was tested in finishing diets based on steam-flaked maize grain and was fed either at 10 % (diet DM), substituting totally for maize silage, or at 7.5 % to contribute the same amount of NDF as the maize. There were no differences in feed intake, daily gain, feed efficiency, or carcass traits among the treatments (McCollum et al., 2005).
In sheep fed sorghum silage in Brazil, the optimum level of concentrate (maize grain, soybean meal and wheat bran) in the diet was between 30 and 45 %. This amount increased the availability of forage DM, the nutritional value of the diet, promoting animal productivity (Simon et al., 2008).
Straw and stover
Sorghum straws have a poor nutritive value with an OM digestibility lower than 50 % (Blümmel et al., 2003) and animals must be supplemented with protein-rich concentrates. For instance, in Nigeria, the best levels of supplementation for cottonseed cake supplementation were respectively of 60 g/d for sheep and 48 g/d for goats. Goats digested sorghum straw better than sheep (Alhassan et al., 1986). In the production of sweet sorghum syrup, it is the leaves, not the stems, that are left on the field (Madibela et al., 2002).
Another way to improve the nutritive value of sorghum straw is to treat it. In India, urea, liquid ammonia and even animal urine were found to improve the degradability of straw, because these treatments increase the nitrogen content of the feed and the efficiency of the microbial population in the rumen (Dhuria et al., 2007). NaOH treatment was effective to increase the in vitro DM digestibility (by 20 percentage points) of sorghum straw (Escobar et al., 1985). In Kenya, treatment with 5 % NaOH of maize/sorghum straw reduced fibre by about 15 %, increased in vitro DM digestibility by 8.5 percentage points and intake by about 60 %. However, these enhancements failed to increase performance (average daily gain) in sheep and goats (Tessema et al., 1984). In India, fine grinding followed by treatment with lime (10 g Ca(OH)2/100 g DM) increased the in sacco DM degradability after 48 h of incubation from 54.1 to 89.2 % (Gandi et al., 1997).
In dual purpose sorghum varieties, grain yield and forage quality are not inversely related, but stover yield is often insufficient. Genotypes promoting high digestible OM intake are only suitable for farmers with sufficient amounts of stover to allow the feeding of animals to appetite. When only restricted amounts of stover are available, it is more relevant to choose genotypes that promote high digestibility under restricted feed intake (Blümmel et al., 2003).