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Meeting Late Season Grazing Gaps for Sheep with Sudangrass and Sorghum×Sudangrass Hybrids

by Wyoming Livestock Roundup

By Whit Stewart and Anowar Islam  

Market timing of feeder lambs is largely determined by forage resources or lack thereof for many sheep enterprises in the Intermountain West and Northern Plains. 

With record lamb prices and widespread drought, contingency planning to meet forage gaps in the late summer forage production should be considered now. Warm season annuals such as sorghum-sudan hybrids and sudangrass might be considered, especially with the hot and dry weather forecast for this summer. 

Agronomic considerations 

Sorghums [Sorghum bicolor (L.) Moench] are annual and important cereal crops in semiarid regions of the world. They are also important components in Great Plains cropping systems as they are often grown in dry areas too difficult for corn to grow.  

There are several types of sorghum commonly grown for forage. These include forage sorghum, sudangrass and sorghum×sudangrass hybrids. They are erect with upright growth, can grow four to eight feet tall, leafy and have an open panicle.  

Sudangrass and sorghum×sudangrass hybrids have more tillers, narrower leaves, and finer stems compared to other sorghums. They have extensive fibrous root systems, including many fine roots which make them more drought tolerant. 

Sorghums are well-adapted to a range of soils. In general, sorghums prefer well-drained soils, but can tolerate poorly drained soils provided there is enough surface drainage. They have moderate tolerance to acidic and low fertility soils; however, they respond well to fertilization. Sorghums have low tolerance to flooding and salinity. They avoid drought by going through semidormant conditions. 

Sudangrass and sorghum×sudangrass hybrids are relatively easy to establish. They can be planted in spring or early summer when soil temperature is above 60°F.  

Seeds may be drilled, broadcast or planted in rows with a spacing of 15 to 40 inches apart. Seeding depth varies between 0.5 to 2.0 inches, depending on soil moisture. Seeding rate is 15 to 25 pounds per acre when drilled and 30 to 40 pounds per acre when broadcast. They are very responsive to nitrogen fertility. Liming is needed to increase soil pH if the soil is highly acidic. 

Grazing management considerations 

Sudangrass and sorghum×sudangrass hybrids are used for pasture, hay and silage but grazing conditions will be noted for the purposes of this article. Prussic acid poisoning or nitrate accumulation is cause for some producer hesitancy of utilizing sudangrass and sorghum×sudangrass hybrids.  

It is important to understand young plants less than 18 to 24 inches tall contain cyanogenetic glycosides, or dhurrin, which, when grazed at this younger stage, can cause prussic acid poisoning. It is critical stands be taller than 18 to 24 inches before grazing.  

Similarly, if grazing later in the fall, it is important to let the plant dry completely, roughly seven to 10 days, after a killing frost as the prussic acid concentrations are highest immediately after a killing frost. Generally, sorghum-sudangrass hybrids contain more prussic acid than sudangrass, and newer varieties that are “low prussic” acid can further mitigate the risk of prussic acid poisoning.  

Like any drought-stressed annual, nitrate accumulation should be considered, and a quick nitrate test can determine if nitrates are at a risk. A good management practice when abruptly changing sheep diets on pasture is to make sure sheep have some gut fill to avoid engorging and resultant digestive or toxin issues.  

The common saying when dealing with dietary toxins in sheep is, “dilution is the solution with pollution,” which is especially true with nitrates in sheep where mixing of high-nitrate forages with low-nitrate forages can help dilute toxins. 

Economic benefits 

Total forage production will vary, especially across a myriad of production environments, but with three to five tons per acre production potential with an approximate cost of $10 to $15 per acre in seed cost, these may be very cost-effective alternatives. With rangeland conditions deteriorating and potentially higher feed costs, sudangrass and sorghum×sudangrass hybrids can not only provide high-quality feed in July and August, but have also been effectively stockpiled as late-fall and early-winter feed resources.  

Using some back-of-the-napkin math, consider a 20-acre sudangrass field costs $25 per acre to plant, for a total of $500, and will conservatively produce two tons per acre or 40 tons of dry matter. An equivalent 40 tons of $150 per ton hay would cost $6,000. An 80-pound feeder lamb consuming 2.5 pound per day would have an approximate feed cost of $0.20 per day consuming hay versus $0.02 per day with the sudangrass grazing scenario.  

This is a simplistic comparison as additional costs associated with each feeding strategy will vary. Still, in drought years where inflated feed costs and destocking can limit profitability, planting a warm-season annual might be advantageous. 

Whit Stewart is a professor and the University of Wyoming Extension sheep specialist. He can be reached at Anowar Islam is a professor and the University of Wyoming Extension forage specialist in the Department of Plant Sciences in the College of Agriculture and Natural Resources. He can be reached at 307-766-4151 or 

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