Warm season annual grasses

Warm season annual grasses are established from seed and are productive during spring and summer. These plants are frequently used as temporary forage for stocker cattle and mature cows. They can be used for grazing, hay, and silage. Under stress conditions, these species may contain nitrate and/or prussic acid concentrations that are toxic to livestock. Careful management is necessary to ensure that these forage species are well-utilized and free of toxins.
New varieties of warm-season annual grasses are released periodically, so it is important to examine the yield comparison trials in UGA’s Statewide Variety Testing Program. The data are published annually in the “Soybean, Sorghum Grain & Silage, Summer Annual Forages, and Sunflower Performance Tests.” These reports are available on the Variety Testing program’s website or through your county Extension office.

Pearl Millet

Pearl millet originated in Africa and is the most widely-planted summer annual grass in Georgia. This tall growing, erect annual grass produces several stems from a central plant. As a result, it requires at least six to eight inches of stubble to regrow. Improved varieties can produce more than six tons per acre. Even under moderate drought conditions, these varieties will rarely yield less than four tons per acre.

Pearl Millet (Pennisetum glaucum)

Pearl millet can be grazed or harvested as hay or silage. Researchers at Tifton have found that pearl millet grazing should begin when plants reach 20 to 24 inches, but regrowth rate and animal performance is best if a nine to 12 inches stubble height is maintained. Pearl millet can make good quality hay if cut when plants reach two to three feet tall. This prevents the forage from maturing beyond the boot stage and therefore being too mature to provide high quality. The drying rate of millet hay can be sped up by the use of a roller/crimper-style conditioner.
If harvested prior to advanced maturity stages, the range of total digestible nutrients (TDN) can be expected to be 52 to 58 percent, while crude protein (CP) will range from eight to 11 percent. There is some evidence to suggest that seeding rates at the high end of the recommended ranges will promote a higher leaf:stem ratio. This may improve forage quality, but these gains may not compensate for the expense of the higher seeding rate.
Since pearl millet does not produce prussic acid, this species has a distinct advantage over sorghum, sudangrass, and sorghum x sudangrass hybrids. This allows pearl millets to be grazed or harvested at any growth stage and during droughts without the risks associated with prussic acid poisoning. However, pearl millets can have high nitrate levels.

Figure 10. Cows grazing a pearl millet pasture in Tift Co.

Millet produces good quality forage and supplies grazing from June through August (Figure 10). Millet planted in spring (April) should be ready for grazing 30 to 40 days after planting and should be productive for 80 to 110 days. The crop is most productive during the first 60 days of the life of the stand. Well-fertilized millet should carry three to four stocker cattle or two to three mature cows per acre during the first 60 days. Lower the stocking rates during the last part of the grazing season. Even out the grazing supply over the summer by making multiple plantings.
Plantings made in early June will be in peak production when April plantings are starting to decline.
Several hybrid millets are marketed in Georgia. Some hybrids are tall-growing and produce high yields. Other hybrids are dwarf, low-growing millets that are excellent for grazing. Dwarf millets produce leafy forage and will provide higher average daily gains than the taller hybrids. Newer dwarf millet varieties are also resistant to Pyricularia leafspot, which can reduce millet yields in late summer. Currently, no millet varieties are available with the brown mid-rib (BMR) trait.
Information about recommended varieties of pearl millet may be found on the “Forage Species and Varieties Recommended for Use in Georgia” web page.

Sorghums

Members of the Sorghum family are often used for forage in Georgia. These warm season grasses are also of African origin and may contain toxic levels of nitrates and prussic acid under stress conditions (drought, frost/freeze, etc.). As such, they are not preferred choices for grazing or hay production (unless irrigated). Sorghums are generally more difficult to cure for hay than pearl millet or other summer annual forage crops. Therefore, they are best adapted to use as a silage crop. The ensiling process results in the dissipation or breakdown of prussic acid and high nitrate levels after two to three weeks, reducing the toxicity problem for livestock.
In addition to the potential for prussic acid toxicity, the presence of an unidentified toxin in sorghum, particularly in sorghum x sudangrass hybrids has been reported. This toxin or factor appears to cause spinal cord degeneration and, in extreme cases, paralysis in horses (sorghum cystitis ataxia syndrome). The potential for this problem and the lack of an effective treatment or cure for this syndrome has led to a general recommendation that horses should NOT be fed forage from the sorghum family.

Sorghum x sudangrass hybrids:

Hybrids of forage sorghum and sudangrass are commonly grown as a warm season annual crop in Georgia. These hybrids have the highest yield potential of any of the summer annuals, if adequate rainfall is received or irrigation is provided. However, sorghum x sudan yields are more severely affected by drought than pearl millet, and are less tolerant of poor soil conditions and soil pH values less than 5.8. Sorghum x sudans can be used for grazing or silage, but like other annual sorghums, their forage is difficult to dry to moistures suitable for hay production.
Sorghum x sudan hybrids should be rotationally grazed, allowing the forage to reach 24 inches before grazing (i.e., managed like sudangrass). At this stage, sorghum x sudans will generally have TDN values in excess of 53 to 60 percent and CP concentrations of nine to 15 percent. Brown midrib (BMR) varieties are usually preferred varieties for grazing since they have less lignin and higher digestibility than other varieties. Research in Texas has indicated that BMR varieties may improve animal gains by as much as five to eight percent relative to non-BMR varieties.

Forage sorghums:

Forage sorghums are high yield producers that may have from zero to 50 percent grain in the forage, depending upon the hybrid and stage of maturity at harvest. As plants mature, lignification increases sharply, reducing digestibility and quality. BMR hybrids can improve digestibility, but this trait has increased lodging in some varieties. Careful selection of BMR hybrids and timing of harvest are necessary to get the highest total digestible nutrients (TDN). Highest crude protein and digestibility will usually be obtained by harvesting in the vegetative growth stage, while dry matter production will be increased from more mature plants. Harvesting in the late grain dough stage will result in a lower average TDN value, but will maximize the amount of TDN harvested per acre.
Grain sorghums, as the name suggests, are prolific grain producers and are best used for that purpose. Grain sorghums are dwarfed cultivars of the same species as forage sorghums. Though they may also produce substantial forage yields, the quality of the forage from grain sorghum varieties is generally much lower than forage sorghums.

Sorghum x Sudangrass hybrid

Forage Sorghum (Sorghum bicolor)

Sudangrass (S. Bicolor ssp. drummondii)

Sudangrass:

Sudangrass has finer stems, tillers more profusely, and is leafier than forage sorghums. It produces very little seed. Its rate of regrowth after cutting or grazing is superior to that of sorghums. For this reason, it is sometimes used for temporary rotational grazing. However, sudangrass usually produces lower yields than pearl millet or the sorghum x sudan hybrids and requires fertile soils with a pH of 5.8 or higher. Sudangrass tends to have less prussic acid accumulation than forage sorghums, and the levels decrease with maturity. For this reason, care should be taken to let sudangrass accumulate to at least 24 inches of regrowth before grazing.
Information about varieties of sorghum x sudan hybrids, forage sorghum, and sudangrass that are currently recommended may be found on the “Forage Species and Varieties Recommended for Use in Georgia” web page.

Planting Warm Season Annual Grasses

Plantings of warm season annual grasses can be made in the spring as soon as the soil temperature (at a two-inch depth) warms to 65º F and can be planted as late as July without a yield penalty (Table 3). Seed can be broadcast or drilled in narrow (more than 15 inches) or wide (up to 36 inches) rows. Seed should be planted at a soil depth of 1/2 to one inch. Ideally, summer annual grasses should be established on well-drained, fertile soils with good water-holding capacity.
Higher seeding rates may help to decrease stem size, but it is unlikely that this will be valuable enough to compensate for the expense of the higher seeding rate. Further, sorghums that have been seeded at higher rates will often have more lodging problems, especially relative to dwarf pearl millet stands.

Table 3. Planting dates and seeding rates for selected warm season annual grasses.
Species		Planting Dates*	Seeding Rate
			Drilled	Broadcast
			—— lbs. of PLS/acre ——
Pearl Millet	LV:	May 1 – July 1	10–15	25–30
	P:	April 15 – July 15
	C:	April 1 – August 1
Sorghum x Sudan Hybrids	LV:	May 1 – July 15	15–20	20–25
	P:	April 15 – August 1
	C:	April 1 – August 15
Sudangrass	LV:	May 1 – July 1	20–25	30–40
	P:	April 15 – July 15
	C:	April 1 – August 1
Forage Sorghum	LV:	April 25 – May 15	15–20	20–25
	P:	April 15 – May 15
	C:	April 15 – June 1
* LV = Limestone Valley/Mountains Region; P = Piedmont Region; C = Coastal Plain Region.

Even out the grazing supply over the summer by making multiple plantings. Two plantings made four to six weeks apart provide good quality forage throughout the summer. Plantings made in early June will be in peak production when April plantings are starting to decline in productivity. If plants become stemmy from selective grazing, mechanically clip them to a height of 10 to 12 inches and fertilize with N. With good grazing management, clipping may not be necessary.
Photoperiod-sensitive sorghum x sudan and forage sorghum cultivars are available. These varieties are capable of sustaining more consistent growth over a longer growing season because they remain in a vegetative stage late into September (until daylength is less than about 12 hours and 20 minutes). This trait may negate or lessen the need for staggered plantings. Otherwise, these varieties are generally managed (planted, fertilized, etc.) in the same way as conventional cultivars. However, some research indicates that the quality of photoperiod-sensitive varieties is lower than the conventional cultivars. Reports from other states indicate that some companies are claiming their cultivars are photosensitive when they merely mature later.

Pest Management for Warm Season Annual Grasses

It is important to prevent yield damage from pests. Though options are limited for weed control in summer annual grasses, pearl millet and the sorghums are typically fast growing and competitive with weeds. However, insect damage to summer annual grasses can be very problematic. For example, chinch bugs often limit yields during a drought year and reportedly are more severe when the summer annuals are planted into small grain stubble. Additional details on weed control options and treatment thresholds for insect pressure are presented in the Georgia Pest Management Handbook’s section on “Temporary Grazing.”

Fertilization of Warm Season Annual Grasses

Summer annual forages must be fertilized to reach their yield potential. As with all crops, fertilization and lime application should be done according to recommendations based on the results of a soil test. Warm season annual grasses generally perform well on sites with soil pH values around 6.0 or higher. However, pearl millet is less sensitive to soil acidity than the sorghums. Nitrogen is needed in large quantities and is most often the least-limiting nutrient. When used for grazing, apply 40 to 60 lbs. of N per acre for establishment and 50 to 60 lbs. of N per acre each month during the grazing season. When harvested for hay or silage, apply 40 lbs. of N per acre at planting and 40 to 60 lbs. of N per acre after the first two cuttings. Reduce N rates after the crop growth rate slows down. Summer annuals grown under irrigation should receive N rates on the upper end of these ranges.

Other Summer Annual Forages

Corn:

Corn (or maize) originated in southern Mexico and is notable for its tremendous forage yields (eight to 12 tons of DM per acre) and high energy content. In Georgia, corn is primarily grown for grain (225,000 acres), but a substantial acreage (40,000 acres) is planted for silage each year, as well. Management of corn for silage is practically identical to corn grown for grain, with the exception of plant populations (for some hybrids) that are up to 10 percent higher than corn grown for grain. For more information on corn management, visit the “Georgia Grains” web site.