Whether grazed, harvested for hay or cut for silage, warm-season annual grasses are the kings of forage production. Common species such as forage sorghum, sudangrass, sorghum-sudangrass hybrids and millets grow best under warmer temperatures, with peak performance at 75° to 90°F.
All species are highly productive with sudangrass on the lower end, producing 3 to 5 tons per acre, and forage sorghum recording yields up to 11 tons per acre.
While all three species are able to be used in a variety of different ways, knowing your end goal can make selecting one species over the other a bit easier. Forage sorghum has the highest growth, typically reaching heights of 8 to 13 feet. Stems and leaves are similar in size to corn, so drying down can be difficult.
With this in mind, the primary use of forage sorghum is silage, where quality can be expected to be slightly lower than corn silage. Grazing forage sorghum also is an option, but expect plenty of waste. Animals will strip leaves off the large stems and either leave them standing or trample them to the ground.
Sudangrass is on the other end of the spectrum from forage sorghum with fine stems and leaves. This characteristic makes sudangrass a great option for hay production, allowing for better drydown than the other two species. Sudangrass also has a better potential to regrow after defoliation as long as enough stubble is left behind. Producers looking for multiple grazing events should keep sudangrass in mind for this reason.
A mix between forage sorghum and sudangrass, sorghum-sudan hybrids embody the middle ground between the two parent species. While producing more tonnage than sudangrass, sorghum-sudan hybrids are smaller stemmed than a straight forage sorghum crop, allowing for a wider variety of end-use options.
Knowing what your final goal is during planting can be helpful when managing for this species. Producers interested in grazing or hay production may choose to plant at a higher rate, increasing stand counts and decreasing stem size. This makes stems a more palatable option for grazers and will improve drydown times for hay. If silage or greenchop are the end goal, a lighter seeding rate may be used resulting in a thicker stem.
Forage sorghum, sudangrass and sorghum-sudan hybrids all are producers of prussic acid, so properly managing harvest of these species is important. Sudangrass produces the lowest levels of this compound and has minimal risk for animal death, while forage sorghum has the highest levels. Sorghum-sudan hybrids are once again in the middle.
Prussic acid is released from a compound called dhurrin, found in highest concentrations in young plants and new shoots. During the curing process for hay and silage, much of the original prussic acid is lost, so even if the original plant contained toxic levels, the final product is seldom an issue.
The highest risk occurs during grazing or harvesting for greenchop. For both of these harvest methods, allow plants to grow to a height of 16 to 20 inches for sudangrass and 18 to 24 inches for sorghum and sorghum-sudan hybrids before harvest.
If coming back for harvest or grazing of regrowth or plants regrowing after a frost or drought, remember that new shoots will once again have high concentrations of prussic acid. This new growth will be the first thing animals seek out, so make sure this new growth gets to appropriate heights before grazing or harvest.
Another risk for summer annual grasses is nitrate poisoning. This is especially true in plants that were overfertilized or planted in a nitrogen-rich environment, such as a previously used stock pen with lots of manure present.
Additionally, drought conditions can cause plants to stockpile nitrogen, increasing toxicity. Nitrates usually are highest in the base of the plant's stalk, so if this is a concern, graze so the bottom third of the stalk remains.
The ensiling process reduces nitrate levels of 40% to 60%, so nitrate poisoning from silage is rare. For hay or greenchop, limit feed amounts to dilute nitrate levels across a total ration to limit nitrogen intake.
With a proper understanding of the end goal and taking steps to mitigate risks from prussic acid and nitrates, summer annual grasses can be an invaluable part of your summer forage systems.
Beckman is a Nebraska Extension educator.