Sorghum-Sudan Grass (SSG) is a warm-season annual forage prized for its rapid growth and high biomass production, making it a popular choice for hay. This hybrid grass is utilized in multi-cut systems to provide feed during the summer slump when cool-season grasses are less productive. The precise timing of the harvest is the most important factor determining the quality, palatability, and safety of the resulting hay. Cutting at the correct stage balances the goals of maximizing tonnage and maintaining high nutritional value.
Optimal Timing for the Initial Harvest
The initial cutting of Sorghum-Sudan Grass is scheduled to achieve the highest forage quality before the plant begins rapid reproductive development. For hay production, the target height for the first cut is generally between 30 and 40 inches. Harvesting at this stage ensures the stems are not excessively thick, which aids in the drying process required for hay.
The corresponding growth stage should be the late vegetative or early boot stage, just before the seed head emerges from the upper leaf sheath. Cutting at this point maximizes the concentration of crude protein and overall digestibility. As the plant matures and grows taller, its cell walls accumulate lignin, which increases structural rigidity but significantly reduces the digestibility of the forage.
Delaying the cut until the plant reaches 50 to 60 inches increases the total dry matter yield, but this gain comes at a cost to the hay’s nutritional profile. As the plant height increases, the crude protein content can decrease by over 50 percent, making the forage less suitable for high-demand livestock like lactating dairy cows. Therefore, the first cut is a compromise, sacrificing some potential tonnage to secure a higher-quality feed product.
Managing Toxicity Risks Related to Cutting
Safety considerations dictate the cutting time due to the potential for Sorghum-Sudan Grass to accumulate toxic compounds. The two main risks are prussic acid poisoning and nitrate accumulation. Prussic acid, or hydrogen cyanide, is formed when the plant’s natural cyanogenic glucosides are rapidly converted following cellular damage.
Conditions that stunt or stress plant growth can cause a buildup of these toxins, particularly drought, high nitrogen fertilization, and frost damage. When cutting, it is recommended to leave a taller stubble, typically 6 to 8 inches, to mitigate this risk. Prussic acid is concentrated in the youngest leaves and the lower portions of the plant stalk, so a higher cutting height removes less of the toxic material.
Environmental stressors mandate specific waiting periods before a safe harvest can occur. Following a killing frost, a waiting period of at least seven to ten days is necessary to allow the toxic compounds to volatize and dissipate from the drying plant material. Similarly, a severe drought-ending rainfall requires waiting approximately four days before cutting, as the rapid regrowth immediately following the rain can contain high levels of prussic acid. High nitrate levels, often concentrated in the lower stalk, are exacerbated by large nitrogen applications followed by dry weather.
Strategies for Subsequent Cuttings
Management of subsequent harvests differs from the initial cut because the plant is already established and the goal shifts to maintaining vegetative regrowth. After the first harvest, subsequent cuts should occur when the forage reaches a height of approximately 20 to 30 inches. The focus is on harvesting the younger, more digestible plant material before it becomes overly mature.
The typical interval between cuts is 30 to 45 days, depending on environmental conditions like moisture and temperature, which determine the speed of regrowth. Harvesting at these intervals keeps the plant in its vegetative state, ensuring a consistent supply of higher-quality forage across multiple cuts. The strategy for subsequent cuts depends on leaving sufficient stubble to fuel the next cycle of growth.
Leaving at least 6 inches of stubble is necessary because the plant relies on the nodes remaining on the stalk for the development of new tillers and basal buds. A stubble that is too short will deplete the plant’s carbohydrate reserves, resulting in slower or weaker regrowth for the next cutting. Maintaining a healthy stubble height is important for the final cut of the season to ensure the plant has adequate reserves before the onset of cold weather.