What Is Sudan Grass and How Is It Used?

Sudangrass is a fast-growing, warm-season annual grass in the sorghum family, cultivated primarily as livestock forage but increasingly valued as a cover crop, a soil-health tool, and even a feedstock for biogas production. Genetically, it sits at the crossroads of cultivated grain sorghum and a wild relative, giving it an unusual combination of high biomass output and resilience in hot, dry conditions. Its versatility explains why it shows up on farms ranging from arid rangeland to intensively managed dairy operations, and why researchers keep finding new roles for it.

What Sudangrass Actually Is

Sudangrass (Sorghum sudanense) is not simply a wild sorghum or a weedy cousin of grain sorghum. Genome sequencing has confirmed that it originated as a natural hybrid between cultivated grain sorghum and its wild relative S. bicolor ssp. verticilliflorum. Despite that wild parentage, the sudangrass genome turns out to be more similar to commercial U.S. sorghums than to wild or traditionally cultivated African sorghums.¹ That matters because it means sudangrass inherited many of the agronomic traits breeders have selected for in grain sorghum, like heat tolerance and efficient water use, while retaining the vigorous vegetative growth of its wild ancestor.

The plant typically grows between one and two meters tall, with narrow leaves and a fine stem compared to forage sorghum. It tillers aggressively, meaning a single plant sends up multiple shoots from its base, which is part of why it regrows quickly after cutting. When people talk about “sorghum-sudangrass hybrids,” they mean deliberate crosses between sudangrass and forage sorghum, bred for even higher yields. Pure sudangrass and these hybrids are related but not identical, and the distinction matters for both forage quality and toxicity risk.

Why It Thrives Where Other Forages Struggle

The biggest selling point of sudangrass is its ability to produce large amounts of biomass on limited water. Under drip irrigation in arid conditions, plants given frequent, light watering produced roughly 15 to 17 percent more fresh and dry biomass than those subjected to heavier water stress, and optimizing nitrogen alongside water efficiency pushed yields even higher.² The takeaway for growers is that sudangrass responds well to deficit irrigation strategies, where you deliberately give the crop less water than it could use but time the applications carefully to keep productivity up. That makes it a practical choice in regions where water is expensive or restricted.

Beyond drought tolerance, sudangrass establishes fast. You can plant it after a spring grain harvest and still get a meaningful forage crop before fall, which is why it fills the “summer annual” niche on many farms. It handles heat that would stall cool-season grasses like fescue or orchardgrass, keeping pastures productive during the months when perennial forages go dormant or grow slowly.

Forage Quality and the Maturity Tradeoff

Sudangrass can be excellent forage, but the window between “high quality” and “mediocre” is narrower than many growers expect. Crude protein in a young vegetative stand can start around 20 percent and drop to roughly 10 percent as the plant matures toward heading.³ At the same time, fiber content climbs and digestibility falls, with the steepest decline happening once the plant transitions out of its purely vegetative stage. Research tracking digestibility over time found little change during the first couple of months of growth while plants remained vegetative, followed by a sharp drop-off of roughly half a percentage point per day once reproductive development began.³

Studies comparing sudangrass varieties confirm this pattern broadly. As maturity advances, dry matter and fiber go up while crude protein and calculated digestible nutrients go down.⁴ The crude fiber increase in sudangrass stems and leaves has been documented alongside a general decline in protein content as sampling dates progress.⁵ All of this means timing your harvest or turning cattle onto the pasture at the right growth stage is one of the most consequential management decisions with this crop.

How Harvest Management Shapes Yield and Regrowth

Sudangrass is cut-and-come-again forage, regrowing from basal buds and axillary buds after each harvest. But how you cut it determines how much comes back. Research on sudangrass hybrids has consistently found that less frequent cutting and shorter stubble produce the highest total forage yields within a season.⁶ That sounds like a simple formula: cut seldom and cut low. But the picture gets more complicated when you look at cumulative regrowth.

Plants that had been cut repeatedly showed dramatically reduced subsequent yields. In one trial, plants cut four times after a May planting produced less than 4,000 kilograms of dry matter per hectare afterward, while uncut plants or those cut only once produced up to 18,000 kilograms per hectare.⁷ Each additional harvest draws down the plant’s stored energy reserves, particularly in the crown and lower stem, and there is a point of diminishing returns where each successive cutting gets thinner and thinner.

Stubble height matters as much as cutting frequency. Raising the cutting height from about 8 centimeters to 25 centimeters increased dry matter production by over 1,200 kilograms per hectare within a single 30-day regrowth period in most harvests tested.⁸ Leaving taller stubble preserves more axillary buds and leaf area, giving the plant a head start on regrowth. The practical guideline most extension services recommend is to cut or graze no lower than about 15 to 20 centimeters and allow the stand to recover to at least 45 to 60 centimeters before the next pass.

Brown Midrib Varieties and What They Change

If you have spent any time shopping for sudangrass or sorghum-sudangrass seed, you have probably seen “BMR” on the label. Brown midrib varieties carry a mutation that reduces lignin content in the plant’s cell walls, and reduced lignin generally means higher digestibility for ruminant animals. In sudangrass specifically, BMR varieties showed higher digestible nutrient content compared to conventional varieties, though dry matter yield was comparable across both types at any given maturity stage.⁴

In grazing trials with stocker cattle, the BMR sorghum-sudangrass hybrids delivered higher average daily gains and greater total gain per hectare at light to moderate stocking rates. At heavier stocking rates, though, a conventional photoperiod-sensitive variety maintained weight gain better.⁹ Cattle on these forages were capable of gaining over a kilogram per head per day and up to 437 kilograms per hectare over an 84-day grazing season, depending on variety and stocking rate.⁹ Those are strong numbers for a summer annual forage, competitive with more expensive perennial pasture systems.

The BMR trait also has an unexpected side effect involving insect pests. Fall armyworm larvae fed continuously on fresh BMR leaves ended up with lower pupal and adult body mass compared to those fed on conventional sorghum-sudangrass plants.¹⁰ Reduced lignin may change the balance of defensive compounds or nutrient availability in ways that make the plant a poorer host for certain chewing insects. This does not make BMR varieties insect-proof, but it is an interesting additional benefit beyond the digestibility improvement.

Prussic Acid and Nitrate Risks

The one serious safety concern with sudangrass and its hybrids is their potential to accumulate hydrogen cyanide, commonly called prussic acid, and nitrates. Both can be toxic or lethal to livestock under the wrong conditions. All sorghum-family plants produce dhurrin, a compound that releases hydrogen cyanide when plant cells are damaged, whether by chewing, frost, wilting, or drought stress. Young, rapidly growing tissue has the highest concentrations, which is why the risk is greatest in short regrowth after cutting, new shoots after a drought-breaking rain, or frost-damaged stands.

Pure sudangrass generally carries lower hydrogen cyanide levels than sorghum-sudangrass hybrids. Comparative research found that the average HCN content of sorghum-sudangrass hybrid forage was higher than that of Piper sudangrass, and in almost all cases the nitrate nitrogen content followed the same pattern.¹¹ The genome study mentioned earlier noted that sudangrass has lower dhurrin content compared to grain sorghum, which aligns with its reputation as the safer member of the sorghum family for grazing.¹

Practical management to reduce the risk includes waiting until plants are at least 45 to 60 centimeters tall before grazing, avoiding turning hungry cattle onto fresh regrowth, and not grazing for several days after a frost. Hay-making also reduces risk because much of the HCN volatilizes during the drying process, though nitrate levels remain unchanged in dried forage. Ensiling reduces both HCN and nitrate to some degree. If you are growing sorghum-sudangrass hybrids rather than pure sudangrass, the toxicity precautions become even more important.

Cover Cropping and Soil Health Benefits

Sudangrass has earned a strong reputation among vegetable growers and organic farmers as a summer cover crop, and the reasons go beyond simply covering bare ground. Its dense, fibrous root system breaks up compacted soil layers, and the sheer volume of aboveground biomass it produces adds organic matter when incorporated. But the more interesting benefits are chemical.

Sudangrass roots release compounds that suppress certain weed species. Research on green foxtail, a common warm-season weed, found that sudangrass shoot residues reduced emergence by up to 76 percent and root exudates alone suppressed it by up to 55 percent. When the two worked together, the combination delayed foxtail emergence by 4 to 12 days beyond what either effect achieved alone, suggesting a synergistic relationship between the decomposing residue and root chemistry.¹²

Sudangrass and its sorghum relatives also help manage plant-parasitic nematodes. When grown and incorporated as a green manure, sorghum varieties acted as poor hosts for root-knot nematodes, functioning both as trap crops that lure nematodes into roots where they cannot reproduce well, and as biofumigants that release hydrogen cyanide into the soil during decomposition.¹³ The same dhurrin that poses a risk to livestock becomes an asset in the soil, where the cyanide released during decomposition helps knock back nematode populations before a susceptible cash crop is planted.

How It Compares to Other Summer Forages

Farmers considering sudangrass often weigh it against warm-season perennial options or other annuals. One direct comparison trial pitted a sorghum-sudangrass hybrid against eastern gamagrass, a native perennial, for summer pasture. The sorghum-sudangrass provided higher average daily gains for cattle in two out of three years, but eastern gamagrass supported roughly twice as many animal-days per hectare, meaning it carried more animals over the season and produced greater total gain per hectare in most years.¹⁴ Calving rates were identical at 89 percent on both forages, so reproductive performance was not compromised either way.

That comparison illustrates a recurring theme: sorghum-sudangrass and pure sudangrass can push individual animal performance, but carrying capacity and persistence across seasons depend heavily on management, stocking rate, and whether you are willing to replant every year. Perennial alternatives avoid the annual establishment cost but rarely match the peak quality of a well-managed sudangrass stand during the height of summer.

Pest Resistance Built Into the Plant

Sudangrass carries genetic resistance to certain insect pests that affect the broader sorghum family. Greenbug, a damaging aphid pest of sorghum and small grains, has been a particular focus of breeding work. Genetic analysis of several sudangrass lines from Ukraine and Russia identified accessions carrying combinations of dominant and recessive resistance genes, with two dominant genes designated Sgr14 and Sgr15 characterized in one cultivar.¹⁵ These resistance genes are valuable not just for sudangrass production itself but as a genetic resource that breeders can move into grain sorghum through crossing.

Bioenergy and Industrial Applications

Sudangrass has attracted interest as a biogas feedstock because of its high fresh biomass yield per hectare and relatively low input requirements. Researchers have evaluated it as an energy crop for anaerobic digestion, the process that converts organic matter into methane-rich biogas.¹⁶ Its fast growth cycle means multiple harvests per season are feasible in warm climates, and the same deficit-irrigation strategies used for forage production can keep water costs manageable in drier regions. Sudangrass will not replace maize as the dominant biogas crop anytime soon, but it fills a niche on marginal land or in rotations where a high-biomass summer crop is needed without the water and fertility demands of corn silage.

Cleaning Contaminated Soil

One of the more unexpected uses for sudangrass is phytoremediation, using living plants to pull pollutants out of the ground. In research on strontium-contaminated soil, sudangrass achieved an annual removal rate of about 23 percent of the strontium from soil containing 500 milligrams per kilogram. When paired with specific microbial groups that enhanced uptake, strontium accumulation in the grass increased by half a fold to fourfold compared to grass alone, and the researchers estimated that the best grass-microbe combination could theoretically clean contaminated soil in about three years.¹⁷

Strontium is a particularly tricky contaminant because it behaves chemically like calcium and gets taken up by plants through the same pathways, which is exactly what makes a high-biomass, calcium-hungry grass like sudangrass effective at extracting it. The forage produced from contaminated land is obviously not suitable for livestock, but the ability to harvest and remove contaminated biomass offers a lower-cost alternative to excavating and replacing soil. This is still an area of active research, and field-scale applications remain limited, but the combination of sudangrass’s massive root system, fast growth, and tolerance of poor soils makes it a natural candidate for this kind of work.