Planting Forage Sorghum, Sudangrass and Sorghum-Sudangrass Hybrids
More than just an emergency forage crop, forage sorghums and sudangrass crosses are highly digestible and high-yielding.
Summer annual grasses will perform well in dry conditions, but also have the potential to produce high-quality forage under optimum growing conditions. These crops may help stretch forage supplies for producers who anticipate inventory shortages. Early June is an ideal time to establish these forage crops for both dry and normal growing conditions.
Species and cultivar selection
Each year, Penn State University conducts forage variety trials on warm-season annual grasses to help producers identify high-yielding, high-quality varieties. Consult the trial results at their website or contact your local extension office to determine which varieties perform best in your region. Select hybrids that are proven to have high fiber digestibility levels to support high animal production levels.
Hybrids containing the brown midrib (BMR) genetic mutation will contain higher levels of fiber digestibility than non-BMR hybrids. Brachytic dwarf-type hybrids will have shorter internodes and higher leaf-to-stem ratios than conventional-type hybrids. Male-sterile species will not produce a seedhead and may preserve more sugar and energy in the leaves and stems than conventional-type species. Starting in 2023, several seed companies plan to offer dhurrin-free sorghum species, which would eliminate the risk of prussic acid poisoning following drought or frost.
Producers should consider the differences in warm-season annual grass species. Sorghum species will grow until maturity or a killing frost, and will not regrow after grazing or mechanical harvest. Sudangrass species will regrow after each grazing or harvest period until they are killed by frost if the growing conditions permit. Sorghum-sudangrass hybrids will exhibit behavior of both species and can be harvested multiple times before a killing frost.
Planting
Plant sorghums in well-drained soils with temperatures above 60° F at planting depth for rapid seedling development, typically around two weeks after the ideal corn-planting window. Seed sorghum-sudangrass species at 25-30 pounds (lb.) per acre, and forage sorghums at 7-9 lb. per acre at a depth of 1-1.5 inches (in.). Maintain soil pH between 6.5 and 7 for best results.
If soil moisture is adequate, stands may be successfully established up to July 4 for forage sorghum and July 15 in some areas for sudangrass crosses. Higher plant densities of sorghum-sudan will lead to thinner stem diameters, which should help decrease animal refusal of stem material at feeding. Use caution in planting high rates of forage sorghums, however, as thinner stems will increase the risk of lodging as the seedhead develops.
Fertility
Phosphorous and potassium requirements are similar for these forages as for corn silage. Sorghum-sudan hybrids are also significant users of nitrogen (N). Sorghum species remove about 7 lb. of nitrogen per ton of yield, and average Pennsylvania conditions produce about 10-15 tons per acre. Sorghum-sudan or forage sorghums generally require about 100-150 lb. of nitrogen over the life of the stand. When planting sorghum species after a failed corn crop, remember to credit any nitrogen applied for the corn crop to the sorghum crop. Producers should also consider applying sulfur at 10% of the nitrogen application rate to encourage nitrogen conversion to plant protein.
Forage sorghums are best managed similar to corn, with a nitrogen side-dress application made when it is about 8-12 in. tall. Sorghum-sudan should receive split applications of nitrogen to maximize fertilizer efficiency. This is typically 100 lb. at planting, and a second application of 50 lb. after first cutting. Base this second application on moisture levels and yield potential. Be very cautious of high nitrogen fertility if significant rainfall is not expected in the long-range forecast, or the crop will contain elevated nitrate levels at harvest.
Manure can be used as a nitrogen source, but is not recommended. Manure nitrogen mineralization rates often do not match the growth curve of sorghum species, and will not provide the high demand for nitrogen of rapidly growing plants. In addition, traveling on stubble after first cutting with a manure spreader will damage crowns and result in injury and poor regrowth.
Weed control
If you are no-tilling into an alfalfa or grass sod, a burn-down treatment with glyphosate or Gramoxone® will be necessary. Weeds are rarely of concern in sorghum stands after establishment, as rapid growth and frequent harvesting tends to outcompete any weeds.
If grassy weeds are a concern at establishment, consider the use of safened seed. Safened seed is treated with a herbicide antidote to allow the plant to metabolize S-metolachlor or alachlor (Group 15) herbicides. Consult the Penn State Agronomy Guide for recommended herbicides to use with sorghum species. When planting into a failed corn crop, always consult the label of any herbicides previously applied to be sure they will not cause crop failure in the succeeding sorghum crop.
More information on weed management in sorghum can be found in “Sorghum Herbicide Options.”
Mechanical harvest
Harvest sudangrass and some sorghum-sudan cultivars early and frequently. Begin considering harvest when plants are 30 in. tall. Do not delay cutting. These plants will rapidly grow to 3 feet (ft.), and can quickly attain 4 ft. or more if not managed properly. Do not let these stands get ahead of your planned cutting height. Under optimum growing conditions, a second cutting may be ready for harvest in as little as 25 days. Mow at a cutting height of at least 4 in. to encourage rapid regrowth. Do not harvest stands that are less than 2 ft. in height.
This crop contains high levels of moisture and will require wide windrow management to ensure rapid dry-down. Mow the crop in as wide of a windrow as possible to hasten dry-down and retain crop nutrient values. Limit excessive wheel traffic in sorghum-sudangrass fields, as the damage to the crowns will slow the regrowth of the following cutting.
Grazing
Begin grazing pastures interseeded with sorghum-sudangrass when the crop reaches 18-24 in. tall. Grazing a crop that is too short increases the risk of prussic acid poisoning. Remove animals from the area when 6-8 in. of stubble remain. Allow the pasture to rest for about 30 days before allowing animals to return. Using rotational grazing to manage your herd will allow for a more consistent grazing and recovery cycle for your sorghum-sudangrass stands.
For information regarding harvest principles of summer annuals, please see “Harvesting and Feeding Warm-Season Annuals for Forage.”
More details about summer-annual grass forage management can be found in the article “Summer-Annual Grasses for Supplemental or Emergency Forage.”
Editor’s note: Zachary Curtis is an extension educator in agronomy and natural resources. Leanna Duppstadt is an extension educator in field and forage crops. These recommendations are provided for Pennsylvania. Please check with your local extension office for resources adapted to your area.
