UGA Forage Extension Team

Comparing summer annual forages

By Jeremy Kichler

Colquitt County CEC

Summer annual forages can provide high yields of good quality forage during late spring and summer for both beef and dairy producers. Most of the warm season annual grasses emerge and establish quickly and are very drought tolerant. They can be used for grazing, hay or silage. Producers need to manage these species carefully in stressful conditions because they can accumulate levels of prussic acid and nitrates that can be toxic to livestock. There are many choices when it comes to summer annual forages, let’s compare a few of them.

New varieties of warm-season annual grasses are released periodically, so one should frequently evaluate yield data from UGA’s Statewide Variety Testing Program. This information can be obtained from the following link (http://www.caes.uga.edu/commodities/swvt/).

Below are seeding rates, planting dates for summer annual forages.

Pearl millet can be grazed or harvested as hay or silage. It is a medium to high yielding summer annual forage and is more productive in drought conditions.   Planting can begin when the 2 inch soil temperature reaches 65 degrees F. Seed can be broadcasted (25-30 lbs of PLS/Acre) or drilled (10-15 lbs of PLS/Acre). The seeding depth needs to be around ½ to 1 inch deep. Pearl millet can tolerate lower soil pH than sorghums and is very responsive to nitrogen.

Growers can begin to graze pearl millet once the plants reach 20 to 24 inches, but regrowth rate and animal performance is best if a 9 to 12 inch stubble height is maintained. Pearl millet tillers well, making it very suitable for grazing.

Pearl millet can make good quality hay if cut when plants reach 2 to 3 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 if a roller/crimper-style conditioner is used.

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 8 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.

Pearl millet has one major advantage over sorghum, sudangrass, and sorghum x sudangrass hybrids in that it does not produce prussic acid. This advantage allows pearl millet 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 similar to other warm season sorghums.   Horses may suffer from subclinical and acute prussic acid poisoning, so species in the sorghum family should not be fed to them.

Sorghum x sudan hybrids have the highest yield potential of the summer annual forages if adequate rainfall or irrigation is received. 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. Seed can be broadcasted (20-25 lbs of PLS/Acre) or drilled (15-20 lbs of PLS/Acre). 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). This species can be harder to manage in a grazing situation due to the fact it does not tiller as well as other summer annual species. This property can impact recovery time if sorghum X sudan is grazed too hard. Sorghum x sudans will generally have TDN values in excess of 53 to 60 percent and CP concentrations of 9 to 15 percent. Brown midrib (BMR) varieties are usually preferred varieties for grazing or conserved forage since they have less lignin and higher digestibility than other varieties.

Sudangrass has finer stems, tillers profusely and is leafier than forage sorghums. They produce very few seed. When compared to other sorghums, the growth rate is better after a cutting or a grazing event. This growth characteristic makes it a great candidate for rotational grazing. They tend to have less prussic acid accumulation than forage sorghums, and these levels tend to decrease with maturity. Sudangrass seed can be broadcasted (30-40 lbs of PLS/Acre) or drilled (20-25 lbs of PLS/Acre).

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.

Forage sorghum is a high yielding summer annual forage. They may contain 0 to 50 percent grain in the forage depending on the hybrid and stage of maturity at harvest. As plants mature, lignification can increase which results in reduction in forage quality. Forage sorghum have thick stems that make hay production difficult but makes excellent silage. Nutritive value is often times 85 to 90 percent of corn silage. Highest crude protein and digestibility will usually be obtained when harvested in a vegetative stage of growth but dry matter production can be increased as plants mature. Harvesting in the late grain dough stage will maximize TDN. Forage sorghum seed can be broadcasted (20-25 lbs of PLS/Acre) or drilled (15-20 lbs of PLS/Acre).

There are a variety of options for summer annual forages that can provide excellent grazing or harvested forages for livestock producers. If you need assistance selecting a variety or comparing options, contact your local extension office.