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Oregon Cover Crops: Sudangrass ans Sorghum-Sudangrass Hybrids (Sorghum bicolor L.) EM 8703 January 1998 R. Sattell, R. Dick, R. Ingham, R. Karow, and D. McGrath Do you want to print a copy? We recommend downloading the print version (74 K). First download the free Adobe Reader, if you don't already have it. Contents Quick facts Hardiness zone map Environmental preferences Uses Dry matter and N accumulation Management Pest interactions Varieties/cultivars Sudangrass and sorghum-sudangrass hybrids are frost-sensitive, warm-season, erect annual grasses. They can grow from 6-8 feet tall and produce large amounts of dry matter if planted in the summer well before the first frost. Their root systems are fibrous. Environmental preferences and limitations Sudangrass and sorghum-sudangrass hybrids require warm weather to grow, and winter-kill with the first hard frost. Minimum air temperature for growth is 60°F, and optimum temperatures are from 75 to 90°F. Sudangrass and sorghum- sudangrass hybrids do best in southwestern Oregon, the Columbia River basin, and the Snake River basin. They also have been grown successfully as cover crops in the Willamette Valley, although cool night temperatures may reduce growth. Although maximum growth occurs with ample moisture, sudangrass and sorghum- sudangrass hybrids are drought-tolerant. They also tolerate mowing, high pH, salinity, and partial shade. top Uses Sudangrass and sorghum-sudangrass crosses are used as warm-season cover crops, forage, and silage. When used as a cover crop, their fibrous roots and organic matter contributions improve soil structure; and their rapid, dense growth suppresses weeds. When sudangrass and sorghum-sudangrass crosses winter-kill, they form a dense mat that protects the soil surface and reduces weed emergence until the residues decompose. The partially decomposed residues are incorporated easily in spring. Growers have successfully used no-till and strip-till methods in combination with short-term residual herbicides to plant into the mulch formed by winter-killed sudangrass. Note that although sudangrass and sorghum-sudangrass hybrids produce high- quality forage, at certain times they can poison livestock. Young plants and plants stressed by drought or light frost may contain hydrocyanic (HCN) acid, which is toxic to livestock. Do not graze young plants or regrowth following drought or frost stress. Dry matter and N accumulation Sudangrass and sorghum-sudangrass crosses are capable of producing large amounts of dry matter. However, the amount of dry matter produced depends on how long they grow before being killed or winter-killed, and if the soil is dry, whether or not they are irrigated. N content is low, in the range of 1 to 2 percent of dry matter. Although the total amount of N accumulated in plant residues may be considerable, because of the high C:N ratio, very little or none of the N is available to subsequent crops. top Quick facts: Sudangrass  Common names  Sudangrass Hardiness zone  10, i.e., no frost tolerance (see Figure 1)  pH tolerance  Tolerates high pH, but optimum is near neutral  Flood tolerance  Low  Drought tolerance  High  Shade tolerance  No information  Mowing tolerance  High  Dry matter accumulation  May be very high but depends on kill date  N accumulation  May be high but depends on kill date  N to following crop  Very little or none  Uses Use as summer annual cover crop to suppress weeds and improve soil tilth. May be planted in late summer and allowed to winter-kill.  Cautions   Needs hot weather to grow. Large amounts of dry matter require a long time to decompose sufficiently before planting subsequent crop.   top Management Seeding rates for cover cropping range from 20-60 lb/acre. Best stand establishment is obtained by drilling seed to a depth of approximately 1 inch in a smooth, well-prepared seedbed. Although row spacing generally does not affect yield, narrow rows are better for cover cropping purposes. Alternative seeding methods that can reduce seedbed preparation but require higher seeding rates are: drill into a rough seedbed prepared by disking, or broadcast over a rough or smooth seedbed and then disk lightly to cover the seed. Dry matter accumulation is very sensitive to planting date. During a normal year in western Oregon, sudangrass and sorghum-sudangrass crosses planted in early August (following beans for example) and irrigated will attain heights of 6-8 feet before they winter-kill. But if they are planted in early September without irrigation they will grow to only 2 feet tall, and if planted in late September they will grow to only 6 inches tall before they winter-kill. Sudangrass and sorghum-sudangrass planted in mixtures with legumes in early fall (early September in western Oregon) will winter-kill, forming a mulch that protects the soil surface during the winter but allows the legume to grow without competition in the spring. Planting too early can result in large amounts of residue that may smother the legume, and planting late results in minimal winter soil protection. Sudangrass and sorghum-sudangrass crosses may be planted in the spring after soil temperatures reach 60°F, and killed or incorporated in early summer. If you plan to incorporate residues soon after the plants are killed, chop or flail them first. Succulent residues from young plants decompose quickly. However, residues from older, larger plants decompose slowly due to their high carbon:nitrogen ratio. When large amounts of residue are incorporated, N availability to the succeeding crop may decrease due to microbial competition for plant-available N during decomposition. You can reduce this problem by planting a mixture of sudangrass and legumes in spring, planting a legume following incorporation, delaying planting the succeeding crop until residues have decomposed, or adding fertilizer N. N availability to succeeding crops is not likely to be a problem if you incorporate residues that have been decomposing over the winter, or if you leave residues on the soil surface as a mulch rather than incorporating them. top   Figure 1.--Oregon plant hardiness zone map. Sudangrass normally will survive in Zone 10 or any warmer zone; thus, it is not winter-hardy in Oregon. (Extracted from the USDA's national plant hardiness zone map, based on average annual minimum temperature in °F.) Zone 4 = -30 to -20; Zone 5 = -20 to -10 Zone 6 = -10 to 0; Zone 7 = 0 to 10 Zone 8 = 10 to 20; Zone 9 = 20 to 30 top Pest interactions Sudangrass and sorghum-sudangrass hybrids do not become weeds as long as they are not allowed to go to seed. Vigorous growth by these species smothers summer weeds. However, if air temperature is cool, growth is slow, and competition from weeds may be a problem. Sudangrass cover crops can be used as part of a rotation to lower soil populations of Columbia root-knot nematode (Meloidogyne chitwoodi), which infects potatoes. Sudangrass varieties 'Trudan' and 'Sordan 79' are non-hosts of Columbia root-knot nematode (other varieties are poor hosts), and sudangrass residues release nematicidal compounds as they decompose. Incorporate sudangrass after it has been stressed (e.g., first frost or drought) to increase HCN production and maximize the nematicidal effect. Rotational control before potato often involves planting non-host summer crops for 1 or more years, as well as controlling host weeds and using sudangrass cover crops. Note that the ability of sudangrass to decrease nematode populations is specific to the nematode type. Varieties/cultivars Besides common sudangrass, many sudangrass and sorghum-sudangrass hybrids are available. 'Piper' is a sudangrass variety that is widely available, has been used in Oregon for many years, grows to a height of 8 feet, and is low in HCN. 'Trudan 8' is a sorghum-sudangrass cross that is widely available and has been used in Oregon for many years. top Prepared by Robert Sattell, faculty research assistant in crop and soil science; Richard Dick, professor of soil science; Russ Ingham, associate professor of botany and plant pathology; Russ Karow, Extension cereals specialist; and Dan McGrath, Extension agent, Willamette Valley; Oregon State University. January 1998.

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