Sorghums and Sudangrass - Management for Supplemental and Emergency Forage

Dave Koch, Extension Agronomy Specialist, University of Wyoming

Warm-season annuals are useful in complementing the cool-season forages on which Wyoming producers rely. Growing forage sorghum, sudangrass and sorghum x sudangrass hybrids can add flexibility to the forage program. These forages are in peak production when growth of cool-season grasses has slowed down due to summer heat. These sorghum crops can produce a significant amount of forage in a short time (i.e., 5 to 6 weeks after planting).

Uses. The supplemental forage provided by the forage sorghums, hybrids and sudangrass allow rest of pastures or range, extend the grazing season and/or provide additional winter hay. They provide emergency feed when range and pasture forage is diminished due to drought, fire, severe grasshopper invasion or other causes. In general, summer annuals provide more feed in the year of seeding than perennials. They are more competitive than oats or barley and, therefore, should not be used as companion crops. Summer annuals are desirable in rotation with winter annual cereals in that they help control problem weeds such as downy brome and other annual weeds.

Crop selection. Forage sorghum (also known as sorgo or cane) has largely been replaced by hybrid crosses (forage sorghum x sudangrass). Hybrids are equally as vigorous, but not as stemmy as the forage sorghums. Also, they are lower in prussic acid (hydrogen cyanide) potential. Grain sorghum can be used as forage in case of crop failure, however, like forage sorghum can be high in prussic acid potential. Most currently available sudangrass varieties and hybrids are low in prussic acid hazard. Sudangrasses are finer stemmed than forage sorghums or hybrids, regrow more readily and, therefore, are more flexible in use (Table 1). Recently, they have been windrowed prior to or just after light frost and left for fall and winter grazing. This preserves the nutrient value and alleviates the cost of baling, hauling, stacking and feeding.

Adaptation. All sorghums (including sudangrass) have the ability to go dormant during periods of drought, then resume growth as soon as sufficient rainfall occurs. Under irrigation, sorghum x sudan hybrids can be equal in yield, however, the energy content of corn will be greater. Irrigated corn will tend to be more consistently high yielding year-to-year in Wyoming. Sorghums have a higher heat tolerance and requirement than corn and in cool years yield suffers. Yields in Wyoming are shown in Table 2. Yields at Archer are lower than at other sites due to the elevation (6,000 feet), shorter growing season and cooler temperatures. Sorghum crops and corn are among the most water use efficient crops, producing a ton of forage with 2 1/2 to 3 inches of water (see 'Crop selection for supplemental and emergency forage'). These crops make good 'catch crops' in that they can successfully produce a forage crop after the full-season grain crop is hailed out or otherwise devastated. Above 5500 feet elevation sorghum-sudan hybrid and sudangrass production is marginal.

Seeding methods. The seedbed should be warm and mellow. Minimum tillage methods may be used as a way of conserving soil moisture, however, a disadvantage is that the soil warms more slowly due to the surface mulch. Sorghum-sudan hybrids and sudangrass require a soil temperature of 55-60⁰F, compared to a soil temperature of 50 ⁰F for corn. The sorghums should be planted about two weeks after the normal planting time for corn. For silage, sorghum-sudan hybrids should be planted in rows and cultivation or herbicides used to control weeds. For pasture or hay, sudangrass should be seeded with a drill.

Weed control. If planting is delayed until the soil warms, many weeds can be destroyed with cultivation during seedbed preparation. If planted in 30-inch or wider rows, weeds can be controlled with cultivation. There are relatively few herbicides available for weed control in sorghum-sudan hybrids and sudangrass. Check the herbicide label or call your county extension office for herbicide use and precautions. Particular attention should be paid to the minimum delay period following application before harvesting or grazing. Also, if a sensitive crop is to follow sorghums, a residual herbicide should be avoided or used with caution.

Rotation with other crops. On dryland, the summer annual forages generally follow winter wheat or fallow, but could follow barley or oats. The sorghums and sudangrasses are noted for their ability to extract soil moisture to a low level. Therefore, in many locations, they will need to be followed by fallow to insure adequate moisture for the following crop.

Fertilization. Response to nitrogen fertilizer will depend mainly on moisture availability. Nitrogen may not be needed under dryland conditions if soil nitrate level and organic matter are high. Nitrogen will more likely be needed following small grains than after fallow. Under irrigated conditions, nitrogen will be needed in order to achieve maximum yields. Soil tests should be utilized to determine amounts of nitrogen and phosphorus to apply. Toxic levels of nitrates may accumulate in summer annual grasses if high levels of nitrogen fertilizer are applied, followed by drought. Nitrate accumulation is more likely with inadequate phosphorus (see bulletin, 'Managing Forages to Reduce Nitrate Poisoning of Livestock').

Grazing management. Livestock will better utilize sudangrass because it has finer stems than sorghum-sudangrass hybrids. Neither, however, should be used for horse pasture. To minimize prussic acid toxicity, sudangrass should not be grazed until it reaches at least 18 to 24 inches (see bulletin 'Managing Forages to Minimize Prussic Acid Poisoning'). There will be less wastage and better utilization with rotational or strip grazing. Confine animals to small enough areas so that forage is removed to a 4- to 6-inch stubble in 10 days or less. Two to three grazings can be expected if moisture is available. Sorghum-sudan hybrids can be grazed in a similar manner, but regrowth will be less. Regrowth of these crops should not be grazed until reaching at least 18 inches or until after a frost that kills the whole plant. Sequential plantings will help even out the supply of pasture.

Under irrigation, an acre will provide 180-250 grazing days for mature cattle (i.e., 2 ½ to 3 ½ head for 70 days). On dryland, carrying capacity will vary more, but will probably be about 1/3 to 1/2 that under irrigation. In a grazing study at Archer Research Extension Center (dryland) over a 5-year period sudangrass provided an average 94 ewe days and 138 lamb days per acre (Rauzi, F. 1969. Grazing Annual Crops. Res. J. 30). Sorghum hybrids and sudangrass can be stockpiled (left standing) and grazed in the fall. Stalks maintain sweetness and are, therefore, very palatable; however, leaves can be lost after a killing frost. An alternative for preserving nutrients for fall grazing is to swath and windrow prior to frost.

Mechanical harvesting. As with grazing, at least 18 inches of growth should be present before cutting sudangrass for hay. Conditioning at time of swathing will enhance curing. Sorghum-sudan hybrids are difficult to cure as hay and are better suited to ensiling, particularly if planted in rows. Sudangrass can be ensiled as well. Greater feeding value per acre can be attained with ensiling, compared with grazing or storing as hay. This is because the crop can be allowed to reach a later stage of maturity and, because the forage is chopped and fermented, the crop will be more fully utilized.

In order to avoid seepage and loss of soluble nutrients, ensiled forage should contain not more than 70% water. If seepage occurs, soluble nutrients are lost and fermentation may not be adequate. Water content of the crop in the field can be 80% or more. Water content is reduced by allowing the crop to more fully mature or by wilting, a process of allowing swathed or windrowed forage to naturally dry. On a good drying day, 10-20 units of moisture may be lost in 2-4 hours.

Sudangrass and sorghum hybrids can be direct-cut and ensiled safely at the late-dough stage. At this maturity, the water content will be less than 70%. A frost will help dry down the crop and allow the ensiling of less mature sudangrass and sorghum-sudan. With less mature forage or with succulent regrowth, it may be more practical to feed green chop.

Table 1. Culture of forage sorghum, sudangrass and hybrids.

¹Use lower rate for dryland and higher rate for irrigated.

Table 2. Yields of hybrid forage sorghums grown at five locations in Wyoming.¹

¹Adapted from Kail and Hoff. 1970. Wyo. Agr. Exp. Sta. Bul. 468R and from Sackett. 1969. Wyo. Agr. Exp. Sta. Bul. 468R.

²Adjusted to 70% moisture. Multiply by .30 for tons of dry matter per acre.