Whether grazing dairy or beef cattle, warm-season annual plants can provide nutrition during the hot months in midsummer when warm-season perennial plants go into semidormancy and lose most of their nutritive value.
Commonly grown summer annual grasses include pearl millet, forage sorghum, sorghum-sudangrass hybrids, sudangrass and crabgrass. Each of these forages have different growth habits and are adapted to different soil types and environmental conditions. Warm-season annuals require soil preparation, planting and fertilization every year.
Pearl millet
“Pearl millet is adapted to sandy, acidic soils and is very drought-tolerant,” said Dr. Dennis Hancock, USDA-ARS (formerly with University of Georgia). “It is a tall growing, erect plant with several stems from a central crown. The forage plant requires at least 6 to 8 inches of stubble to regrow, but regrowth rate and animal performance is best if a 9- to 12-inch stubble height is maintained. Grazing should begin when plants reach 20 to 24 inches of height and the crop can make good-quality hay if cut when plants reach a height of 2 to 3 feet. If harvested prior to boot stage, total digestible nutrients [TDN] in pearl millet can range from 52 to 58 percent. Crude protein [CP], prior to advanced maturity stages, will range from 8 to 11 percent.”
Plant pearl millet when soil temperature is at least 65ºF or warmer. Recommended seeding is 4 to 25 pounds per acre. There is some evidence to suggest that seeding rates at the high end of the range will produce a higher leaf-to-stem ratio, which translates to higher-quality forage. The gain in quality may not compensate for the expense of the higher seeding rate, however.
“Pearl millet does not produce prussic acid, which gives it a distinct advantage over forage sorghum, sorghum-sudangrass hybrids and sudangrass. It is possible to graze or harvest pearl millet at any growth stage and during drought without the risk of prussic acid poisoning. Pearl millet can have high nitrate levels, however,” said Hancock.
Forage sorghum
Forage sorghum is closely related to grain sorghum but has different growth characteristics. Varieties of forage sorghum range from 5-foot-tall semidwarfs to types that may reach 12 feet in height. Recent advances in forage breeding have resulted in higher forage quality with brown midrib (BMR) types, earlier maturing lines and photoperiod-sensitive plants, which remain in the vegetative stage throughout the season.
“Historically, forage sorghum production was limited to warm regions, but that’s changing as earlier-maturity genetics become available,” said Dr. Gregory Roth, Penn State University Extension. “Minimum temperature required for sorghum growth is about 60 degrees, and highest yields occur when mean temperatures are between 75 and 80 degrees. Forage sorghum is better adapted to feeding as silage rather than grazing, since it normally does not regrow after harvest.”
Prussic acid poisoning and nitrate toxicity can occur when grazing sorghum. Possibility of occurrence is greatest when sorghum is grazed to a height of less than 24 to 30 inches. Poisoning is also a concern immediately after a killing frost or on regrowth of sorghum killed by an early frost.
Sorghum-sudangrass hybrids and sudangrass
“Sorghum-sudangrass hybrids are crosses between sorghum and true sudangrass,” said Dr. Chris Teutsch, University of Kentucky. “These hybrids are annual grasses that resemble sudangrass but have coarser stems, taller growth and produce higher yields. Both the hybrids and sudangrass regrow after grazing, unless they are limited by weather or soil conditions. Hybrids don’t make good hay because of their coarse stems, so they are best utilized for grazing, silage or baleage.”
Pure sudangrass is a rapidly growing warm-season annual grass in the sorghum family. It is well-suited for grazing and is classified as a medium-yielding grass. It is possible to graze sudangrass several times during summer and early fall. Because of its fine stems, it makes excellent hay.
“Sudangrass and sorghum-sudangrass hybrids are best adapted to well-drained soils and require pH of 6 to 6.5 for maximum production. Recommendations are to plant these grasses after the last frost either by conventional or no-till seeding,” Teutsch said. “Initiate grazing when these grasses are at least 20 inches tall and cut for hay or silage when stands reach 30 to 40 inches. Multiple grazings are possible if the grasses are not defoliated shorter than 6 to 10 inches.”
Toxic levels of nitrates can accumulate in sudangrass and sorghum sudangrass hybrids under certain conditions, such as high-nitrogen fertilization, excessive manure/poultry litter applications, drought or sudden weather changes. Ensiling reduces nitrate levels by 40% to 60%, but always test suspect forages before feeding.
“Sudangrass and the hybrids may cause prussic acid poisoning when immature or frosted forage is consumed,” said Teutsch. “Chances of poisoning is reduced by delaying grazing until plants reach a height of 20 inches or more and avoiding grazing during frost until the forage is completely dry and brown. Ensiling neutralizes prussic acid and it volatilizes during proper hay curing.”
Crabgrass
“Crabgrass is generally considered a summer weed in pastures from Texas around the Gulf of Mexico to southern Florida. Some producers, however, have discovered great forage attributes in crabgrass,” said Dr. Rocky Lemus, Mississippi State University. “Crabgrass is used as an annual warm-season forage crop that has prolific seed production and good reseeding capabilities. It is adapted to a wide range of environmental conditions from drought to soils with good water-holding capacity. The forage grows well at soil pH ranging from 5.5 to 7.5.”
“Across the South, crabgrass is planted from mid-April to early June, when rainfall is more dependable. It is possible to graze the forage within 30 to 40 days after planting, if proper fertilization is provided. Crabgrass can provide two hay cuttings per season, when harvested at the boot stage and leaving 3- to 4-inch stubble height. It is slower to dry than some hays due to hair on the leaves and stems,” said Lemus. “Grazing should begin when the grass is 6 to 8 inches tall.”
Before choosing a warm-season annual grass, consider the planting site environment, how the crop fits the current management style and expected economic benefits.
