MSU Extension Cover Crops

Forage sorghum, sudangrass and sorghum-sudangrass hybrids (sometimes abbreviated as sudex) are exceptionally heat and drought tolerant annual crops that are often used as cover crops. They also have a long history as livestock feed harvested as hay, haylage or pasture. This makes them a prime candidate for dual purpose use as both cover crop and forage. Dual purpose cover crops can help crop growers recoup the cost of planting a cover while providing much needed feed for livestock. Because roots are not harvested, many of the soil improvement benefits remain. Frost-killed pasture can be stockpiled for winter grazing similar to corn stover, while also continuing to provide ground cover and erosion protection.

For more information on using the sorghum family as a dual-purpose cover crop, check out the following video and continue reading below.

While sorghum family cover crops are well suited as dual purpose forages, there are a few crucial management differences between these two cropping goals. Whenever planning to harvest or graze any cover crop, it is essential to make sure this will not violate the terms of crop insurance contracts or incentive payments. It is also critical to check the pesticide application history for the field over the last 18 months to be sure grazing or forage harvest restrictions are not present. Any such restrictions will be stated on the pesticide label.

Be realistic about potential forage yield. Seeding date greatly affects potential sorghum family forage yields. These plants are tolerant of summer drought but require hot temperatures for optimum growth. Cool nights less than 50 degrees will greatly slow their growth rate and they are killed by frost. Yield potential as a forage in a cover cropping situation will therefore greatly depend on planting date and weather. When planted late in the season as cover crops – for example, after wheat harvest – forage yield will probably be no more than half of what might have been possible with June planting at the recommended time for a dedicated forage crop. The sorghum family will take at least eight weeks to reach machine harvestable yields, so they are not suitable for planting inside an eight-week window before expected first frost for your location. If planning to graze instead of machine harvest, lower yield potential in less than eight weeks may be acceptable.

Recommended seeding rates for forage uses are typically greater than the same species when used solely as a cover crop. If optimizing forage yield is a main goal, consider increasing seeding rates to 10-15 pounds per acre for sorghum, 20-30 pounds per acre for sudangrass and 30-60 pounds per acre for sudex.

A modest amount of nitrogen fertilizer at planting may be economical to speed up growth rates and enable greater forage yields in short late-season growth windows. Keep in mind that sorghum family plants can accumulate toxic levels of nitrate under the same conditions that cause nitrate poisoning in other crops like corn silage. These include a good supply of soil nitrogen along with any weather conditions that reduce growth rates, such as drought or cool, cloudy weather. Therefore, avoid applying more than 25-50 pounds nitrogen per acre to sorghum family cover crops intended for late season harvest because growth will be already be slower due to cool weather late in the season. Nitrite poisoning can kill animals quickly. It can occur on pasture and the toxin is still present in hay. Fermenting forages as haylage or baleage can reduce nitrate levels by about half but levels can still be high enough to kill. If nitrate accumulation is suspected, always test forage for nitrate before feeding it.

All species in the sorghum family can cause prussic acid poisoning in livestock. Forage sorghum has the greatest toxicity potential and sudangrass the least, with hybrid sudex intermediate. Prussic acid is also called hydrocyanic acid, otherwise known as cyanide. Poisoning occurs when enzymes naturally present in the plant tissue convert a secondary compound called dhurrin into toxic cyanide gas whenever the plant wilts. Therefore, producers grazing sorghum-family forages must be prepared to remove livestock from pasture immediately if conditions cause sudden wilting of the forage. Potentially dangerous wilting can be caused by frost, drought, cutting, chopping, trampling or even just by chewing.

Rumen microbes can detoxify some prussic acid if animals are gradually introduced to the forage and are not consuming prussic acid at a rate faster than it can be broken down. Death can still occur in livestock used to sorghum if prussic acid levels spike due to frost or if hungry animals eat sudden large doses of toxic forage. Toxicity potential is greatest in seedlings, lush dark green new leaves, frosted forage and drought-wilted forage.

High levels of soil nitrogen fertility and low fertility of phosphorus and potassium can increase toxicity. Because of the concentration of cyanide in new leaves, sorghum forages should never be grazed or fed as green chop until plants are at least 18 to 24 inches tall. This is approximately belly high on a mature cow. Read more from Michigan State University Extension about controlling the risk of prussic acid poisoning due to frost.

Fortunately, prussic acid dissipates after machine harvest, so harvested forages are considered safe to feed after hay is fully cured (three weeks) or haylage/baleage is fully fermented (six to eight weeks). Be aware that cyanide gas is heavier than air and can accumulate in low, poorly ventilated areas. If storing sorghum-family silage in upright silos or bunkers, take the same precautions that apply to toxic silo gases for any crop. Ventilate the feed room and use caution if entering the silo.

Resources

• MSU Forage Connection website
• Recommended Hay and Pasture Forages for Michigan , MSU Extension bulletin E3309

For more information, contact forage and cover crop specialist Kim Cassida at cassida@msu.edu.