Ben Beckman, Nebraska Extension Educator

Whether grazed, harvested for hay, or cut for silage, warm season annual grasses are the kings of forage production.  Common species like forage sorghum, sudangrass, sorghum/sudangrass hybrids, and millets grow best under warmer temperatures, with peak performance at 75-90°F.  All species are highly productive with sudangrass on the lower end producing 3-5 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 8-13 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 is also 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 dry down than the other two species.  Sudangrass also has a better potential to regrow following 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 dry down times for hay.  If silage or green chop are the end goal, a lighter seeding rate may be used resulting in a thicker stem.

Forage sorghum, sudangrass, and sorghum/sudan hybrids are all 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. 

Highest risk occurs during grazing or harvesting for green chop.  For both of these harvest methods, allow plants to grow to a height of 16-20 inches for sudangrass and 18-24 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 over fertilized or planted in a nitrogen rich environment, like previously used stock pen with lots of manure present.  Additionally, drought conditions can cause plants to stockpile nitrogen, increasing toxicity.  Nitrates are usually highest in the base of the plant’s stalk, so if this is a concern, graze so the bottom 1/3 of the stalk remains.  The ensiling process reduces nitrate levels 40-60%, so nitrate poisoning from silage is rare.  For hay or green chop, limit feed small 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.

Interviews with the authors of BeefWatch newsletter articles become available throughout the month of publication and are accessible at https://go.unl.edu/podcast.