Sorghums, sudangrasses, and sorghum-sudangrass hybrids
For Forage

Dan Undersander - Dept. of Agronomy
Woody Lane - Dept. Meat and Animal Science

Sorghums are warm weather crops that originally came from Africa. They have done very well throughout Wisconsin during the 1987 and 1988 seasons because of their drought tolerance and the above average temperatures for the growing seasons. Sorghums are either annuals or perennials but all act as annuals in Wisconsin.

Sorghums are diverse but generally fall into the following categories:

Grain Sorghum - also called milo, used for grain production in arid regions. This type grows 3.5 to 5 feet tall depending on variety and conditions. It is usually not considered for forage production because of low dry matter yield.

Forage Sorghums - including sorgo, sweet sorghum, dual purpose (grain and forage) varieties, and hybrids. They usually grow 8 to 13 feet tall. Major use is for silage. Stems and leaves are similar in size to corn. Yields in central Wisconsin have ranges from 4 tons/A in cool years to 11 tons/A dry matter in years with above average temperatures. Feeding value of sorghum silage is 80 -90% that of comparable corn silage. Some long season and/or non-flowering types will need to be killed by frost to dry down enough for ensiling.

Sudangrass - grows from 4 - 7 feet tall, has leaves about 1/2 inch wide and stems about 1/4 inch in diameter. It can be harvested as pasture, green chop, hay, or silage. Yields have ranges from 3 - 5 tons/A dry matter. It can be ready for harvest about 45 days after planting. The smaller stems give it better drying characteristics than other sorghums for hay making. Hybrids are available that are slightly larger and higher yielding.

Sorghum-sudangrass Hybrids - are intermediate in plant size between sorghum and sudangrass. Yield is generally less than for forage sorghums but similar to slightly higher than sudangrass. It can be used for hay, haylage, green-chop, and pasture. Larger stems make drying for hay more difficult than for sudangrasses.

Establishment

Sorghum can be established either by conventional or reduced tillage methods. Soil pH should be between 6 and 7.5 with 6.5 being considered optimum. Soil temperature at 2 - 4 inches should be 65 - 70 F. Recommended seeding depth for all sorghums is 3/4 to 1-1/4 inches in heavy soils and up to 2 inches in sands. Seed at the rate of 20-30 lbs/A.

Prussic Acid (Cyanide) Poisoning

Young plants, including roots, and leaves of older plants contain a compound called dhurrin which can break down to release a substance called prussic acid or hydrogen cyanide (HCN). Sudangrass has low levels of this compound and rarely kills animals. Sorghum has the highest levels and sorghum-sudangrasses are intermediate. There is also considerable varietal difference in prussic acid content for all types of sorghums.

This compound is highest in young plants. Therefore the recommendation is not to graze or cut for green chop until the plant is 18 to 20 inches tall. It is important to watch for young regrowth in pastures. After a drought, new shoots may appear and the cattle will switch from the taller forage to the new tender shoots. Also do not graze or green chop for 10 days after a killing frost.

High levels of nitrogen fertilizer will increase the likelihood of prussic acid poisoning as well as nitrate poisoning. Very dark green plant growth often contains higher levels of prussic acid.

Most prussic acid is lost during the curing process. Therefore hay and silage are seldom toxic even if the original forage was. Do not leave green chop in a wagon over night and then feed. The heat that occurs will cause a release of prussic and make the feed more likely to be toxic.

Individual animals vary in susceptibility to prussic acid poisoning. Cattle are more susceptible than sheep. Animals receiving grain with the pasture are less likely to be affected.

Harvest

Silage - Forage sorghums should be harvest at the mid dough stage for ensiling. At this point quality is still good and most types have dried down enough for ensiling. Non-heading types usually require a killing frost for the plant to get dry enough to ensile. This can be a problem in that logding and leaf loss (therefore quality) may occur during the drying period after frost.

Hay - Highest yields are obtained when sudangrass and sorghum-sudangrass hybrids are harvested at the soft-dough stage, if a heading type. Curing is difficult and quality is low when harvested so late. So the general recommendation is to harvest either type for hay whenever forage is about 30 inches high. sorghum-sudangrass hybrids are generally more difficult to make hay out of because of the larger stems.

Green chop - Sudangrass and sorghum-sudangrass hybrids can be used to provide green chopped forage over summer. Begin chopping after the plant is 18 inches tall or cut at least 10 days after a killing frost. Forage at either time will have elevated levels of prussic acid. First cutting should be taken by heading.

Pasture - Sudangrass or sudangrass hybrids can be grazed any time after the plant has reached a height of 18 inches, which is usually 5 to 6 weeks after planting. For best results, it should be grazed rotationally with a sufficiently heavy stocking rate to remove forage down to a 6 - 8 inch height in a few days. The pasture will grow rapidly when the cattle are removed for more total tonnage. Additionally, if the grazing period is short, cattle will be less likely to be grazing regrowth that is high in prussic acid.

Feeding Value

Sorghums and sudangrasses grown for forage are most appropriately compared with corn silage in feed value. The table below lists representative feed values for the various classes of sorghum and sudangrass forages. Corn silage is also included in this table for reference.

Although these forages are generally similar to corn silage in feed value for beef cattle and sheep, there are some differences. Sudangrass grazed in its early vegetative stage contains as much available energy as corn silage and considerably more protein. However, mature sudangrasses and most sorghum and sudangrass silages are 15-20% lower in available energy than corn silage. This is because of the lower grain-to-forage ratios of the sorghums, and also because the seed coat is harder than corn and far more grain passes undigested through the animal. Crude protein levels are similar to corn silage, but they are rather variable and depend in part on the amount of nitrogen fertilization.

Calcium and phosphorus levels are somewhat higher than corn silage, and the calcium-phosphorus ratio is better. Sorghum and sudangrass contain relatively high levels of potassium. Sheep producers should be aware that these forages evidently accumulate copper more than corn and often show copper levels of more than 30 ppm. This is usually not a problem for cattle, but sheep grazing or being fed sorghum and sudangrass forage should have access to a mineral mixture containing molybdenum and no additional copper.

There are no reports of prussic acid (Cyanide) poisoning from feeding hay or silage made from these forages. Only the fresh forage is dangerous. The drying and ensiling processes greatly reduce the level of the cyanide-containing compound. However, never enter a silo during the first 2-3 weeks after making sorghum or sudangrass silage, because traces of the escaping cyanide gas may still be in the atmosphere. Cyanide has the distinctive odor of bitter almonds.

As of this writing, wet chemistry methods should be used to analyze for the feed value of sorghum and sudangrass forages. Infrared analysis (NIR) will not give accurate results because the essential calibration curves for these types of forages have not yet been developed.

Table 1. Forage Composition of Sorghum Types

 

100% Dry Matter Basis

 

DM

TDN

NEg

NEm

CP

EE

Ca

P

K

NDF

ADF

Grain Sorghum

 

 

 

 

 

 

 

 

 

 

 

Silage

30

60

1.31

0.74

7.5

3.0

0.35

0.21

1.37

n/a

38

Forage Sorghum

 

 

 

 

 

 

 

 

 

 

 

Sorgo

27

58

1.24

0.68

6.2

2.6

0.34

0.17

1.12

n/a

n/a

Sudangrass

 

 

 

 

 

 

 

 

 

 

 

Fresh, Early Vegetative

18

70

1.63

1.03

16.8

3.9

0.43

0.41

2.14

55

29

Fresh, Midbloom

23

63

1.41

0.83

8.8

1.8

0.43

0.36

2.14

65

40

Hay, sun-cured

91

56

1.18

0.61

8.0

1.8

0.55

0.30

1.87

68

42

Silage

28

55

1.14

0.58

10.8

2.8

0.46

0.21

2.25

n/a

42

Corn Silage

 

 

 

 

 

 

 

 

 

 

 

Corn Silage
(well-eared)

33

70

1.63

1.03

8.1

3.1

0.23

0.22

0.96

51

28


Undersander©2001

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