Planting Forage Sorghum, Sudangrass, and Sorghum-Sudangrass
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 to 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.
Establishment
Species and Cultivar Selection
Each year, Penn State University conducts Warm-Season Annual Grass forage variety trials to help producers identify high-yielding, high-quality varieties. Consult the Forage Variety Trails Reports or contact your local Penn State 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 fiber digestibility levels 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 do not produce viable pollen, and so their seedhead does not fill grain. This results in more sugar and energy in the leaves and stems than conventional-type species, improved standability, and eliminates the need for kernel processing. Recently, several seed companies have begun offering dhurrin-free sorghum-sudangrass species, which would eliminate the risk of prussic acid poisoning following drought or frost in grazing systems. Prussic acid is less of a concern in mechanically-harvested systems, as the ensiling process helps to reduce prussic acid levels.
Producers should also 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. Sorghum species are the preferred species for silage production because their full yield is achieved in one cutting. 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 degrees at planting depth for rapid seedling development, typically around two weeks after the ideal corn planting window. Producers should think about establishing these species prior to drought concerns, so that adequate soil moisture is present for uniform imbibition and germination. Your seed dealer may have variety-specific seeding rate suggestions, but typically, sorghum-sudangrass species are seeded at 25-30 pounds per acre, and forage sorghums at 7-9 pounds per acre at a depth of 1 to 1.5 inches. Maintain soil pH between 6.5 and 7 for best results. If soil moisture is adequate, stands may be successfully established up to July 4th for forage sorghum and July 15th in some areas for sudangrass crosses. Higher plant densities of sorghum-sudangrass will lead to thinner stem diameters, which should help decrease animal refusal of stem material at feeding. However, thinner stems will decrease forage quality as the stem contains less of the digestible pith. Use caution in planting high rates of forage sorghums, because thinner stems will increase the risk of lodging as the seed head develops.
Fertility
Nitrogen, phosphorus, and potassium requirements are similar for these forages as for corn silage. Sorghum species remove about 7 pounds of nitrogen per ton of yield. In intensively-managed systems (20 tons/acre at 65% moisture), sorghum-sudan or forage sorghums generally need no more than 150 lbs./acre of nitrogen. If the expected yield is less than 15 tons/acre at 65% moisture, which is more likely for most PA producers, due to the late seeding regime and low temperatures in the fall, target 100 lbs./acre of nitrogen fertilization. Split nitrogen applications into pre-seeding and post-first-cut applications, balancing management intensity and N use efficiency. Optimizing potassium fertilization will foster sorghum's already superior drought tolerance.
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 sidedress application made when it is about 8-12 inches tall. Sorghum-sudan should receive split applications of nitrogen as well to maximize fertilizer efficiency. This is typically one hundred pounds at planting and a second application of fifty pounds 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 producers should consider the source. Liquid manures with a high ammonia-nitrogen (NH4-N) content, such as dairy or swine manure, can provide readily available nitrogen to feed a fast-growing warm season annual crop. Take care to utilize incorporation via tillage or rainfall to capture the volatile ammonia nitrogen after application. Solid manures, such as poultry litter or penpack, contain higher levels of organic nitrogen (org-N) and their nitrogen mineralization rates often do not match the growth curve of sorghum species. Use caution when traveling on stubble after first cutting with a manure spreader, as heavy traffic will damage crowns and result in injury and poor regrowth.
Weed Control
If you are no-tilling into an alfalfa or grass sod, a burndown 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 an 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.
Post-emergent herbicide options are limited to broadleaf weed chemistries such as dicamba, Huskie, Peak, and Starane Ultra. Be careful of herbicide language, as some herbicides are limited to grain sorghum or forage sorghum.
More information on weed management in sorghum can be found in "Sorghum Herbicide Options."
Harvest
Mechanical Harvest
Harvest sudangrass and some sorghum-sudan cultivars early and frequently. Begin considering harvest when plants are 30 inches tall. Do not delay cutting. These plants will rapidly grow to 3 feet and can quickly attain 4 feet or more if not managed properly. Do not let these stands get ahead of your planned cutting height. Under optimum growing conditions second cutting may be ready for harvest in as little as 25 days. Mow at a cutting height of 6-8 inches to encourage rapid regrowth. Do not harvest stands that are less than 2 feet 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.
For information regarding harvest principles of summer annuals, please see "Harvesting and Feeding Warm-Season Annuals for Forage."
Grazing
Begin grazing pastures interseeded with sorghum-sudangrass when the crop reaches 18-24 inches tall. Grazing a crop that is too short increases the risk of prussic acid poisoning. Remove animals from the area when 6-8 inches of stubble remain. Allow the pasture to rest for about 4 weeks before allowing animals to return. Utilize strip-grazing within your rotational grazing framework to allow for a more consistent grazing and recovery cycle for your sorghum-sudangrass stands.
More details about summer-annual grass forage management can be found in the article "Summer-Annual Grasses for Supplemental or Emergency Forage."
