Grain Sorghum Forages: What's New in Genetics and Management?
There has been renewed interest in forage sorghum and sorghum-sudangrass attributable to the 2012 drought and declining water in the Ogallala Aquifer (South Dakota to Texas). The advantage of these forages is their adaptability to high temperatures and requiring about 33% less water than corn. Sorghums are diverse cultivars ranging from shorter (3-5 feet) grain (milo) sorghums to taller (8-13 feet), higher-tonnage forage sorghums that have stems and leaves similar to corn. Forage sorghums have varying grain-to-stover ratios, ranging from no grain with male sterile to upwards of 40% grain depending upon variety. Sudangrass grows 4-7 feet, has much smaller leaves and stem diameter and can be harvested as early as 45 days after planting. The smaller stems allow for faster drying than other sorghums for those interested in harvesting as hay. Sorghum-sudangrass hybrids are intermediate between forage sorghum and sudangrass, with leaf-to-stem ratios driving their nutritive value and regrowth contributing to total yield potential. There are also brown midrib (BMR) versions of forage sorghums, sudangrass and sorghum-sudangrass hybrids which have reduced lignin in both the stem and leaves, resulting in higher fiber digestibility. However, similar to corn, there is a slight yield drag (10%) in BMR genetics compared to conventional genetics (Mahanna and Thomas, 2013).
Forage sorghums are typically harvested for silage when grain is about mid-dough maturity to optimize yield, quality, berry starch digestibility and adequate plant dry matter for ensiling. Non-heading varieties usually require a killing frost for the plant to reach adequate dry matter to prevent excessive levels of effluent. Post-frost harvesting can result in lower yield and quality due to leaf loss and lodging. Sudangrass and sorghum-sudangrass are harvested before reaching 3 feet tall, allowing for two to three cuttings per year. These crops must be field wilted to achieve proper ensiling moisture.
There are several published research studies with sorghum silages claiming similar milk production when dairy cows are fed a ration containing BMR sorghum silage versus a ration containing corn silage. However, the cows in these studies are typically late-lactation and/or low-producing cows (57-75 pounds per day). In one short-term study, cows fed the BMR forage sorghum silage consumed 2 pounds more dry matter per day than those fed corn silage, yet the cows on the corn silage treatment gained 7.5 pounds more body weight. This would indicate more energy among the corn silage treatment despite similar milk production. The other issue is that the corn silage in these trials didn‘t represent typical fiber and starch content levels. In one study, the corn silage contained 55% NDF, which was similar to the level of NDF in the BMR forage sorghum. Obviously, there was little starch in the corn silage to dilute the NDF. Similarly, another trial comparing BMR forage sorghum to corn silage used corn silage containing 46% NDF and only 20% starch. These trial details may help put in perspective the claims that BMR forage sorghum has 85-100% the feeding value of corn silage. Perhaps this is true for the poorest corn silage, but certainly not compared to typical corn silage. Research has yet to be conducted comparing BMR sorghum to BMR corn silage. In the end, it is not just who wins in milk production, but which forage yields the most starch and digestible fiber resulting in the highest income over feed cost which, unfortunately, is not reported in most studies. Despite advances in sorghum breeding, the variability in plant height (yield), dry matter, standability, starch content and starch digestibility has held back wider adoption of sorghum silage. However, for producers dealing with dwindling water supplies, BMR forage sorghum may have a place, especially for heifers, dry and late-lactation cows which have lower nutrient requirements (Mahanna and Thomas, 2013).