D.J. Undersander1, A.R. Kaminski1, E.A. Oelke2, L.H. Smith2, J.D. Doll1. E.E. Schulte1, and E.S. Oplinger1
1Departments of Agronomy and Soil Science, College of Agriculture and Life Sciences and Cooperative Extension Service, University of Wisconsin-Madison, WI 53706.
Turnip (Brassica rapa L.) is a root Brassica crop and has been used as a vegetable for human consumption in Europe since prehistoric times. Turnip root has been a popular livestock fodder for at least 600 years wherever the crop can be grown. For most of that time turnip roots have been managed as forage. Researchers in the United States determined in the early 1900s that turnip roots are valuable energy sources for young ruminant animals. However, livestock farmers at that time were turning away from the Brassica root crops (which also include rutabagas or swedes) for fodder because much hand labor was required for the production and utilization of the large roots. One study showed that the labor requirement on a nutrient basis for these crops was three times that needed for corn silage production.
In the late 1970s, however, researchers began to demonstrate the potential of turnip as pasture. The development of varieties with partially exposed roots rendered the roots more available to grazing animals. Livestock graze turnip tops and roots readily, and the forage is of high quality. Pasturing eliminates the need for manual labor in harvesting and storing. In general, the root Brassicas are fast-growing, high yielding and well adapted to seeding into existing pastures with little or no tillage or to seeding into a conventionally prepared seedbed.
Turnip is a cool-weather crop and well adapted for the northern parts of the United States and Europe and for Great Britain and Canada. However, truck-growing areas of the South also produce turnip roots and greens in all seasons for human consumption.
Turnip produces high-quality forage if harvested before heading. Livestock eat the stems, leaves and roots of turnip plants. Above-ground parts normally contain 20 to 25% crude protein, 65 to 80% in vitro digestible dry matter (IVDDM), about 20% neutral detergent fiber (NDF) and about 23% acid detergent fiber (ADF). The roots contain 10 to 14% crude protein and 80 to 85% IVDDM.
The high levels of glucosinolates (which can cause thyroid enlargement in young growing sheep and cattle) can be a problem if turnip forage is fed for long enough. Glucosinolates are higher in older (90-day) than 60-day forage. Oral or subcutaneous iodine administration can alleviate thyroid problems. Turnip roots usually are higher in glucosinolates than the tops and leaves. Two other anti-quality factors, S-methyl cysteine sulphoxide (SMCO) and free nitrates can also be present. SMCO is the main problem and can cause anemia. To minimize the potential for animal health problems from these factors, forage from turnips should be fed in combination with other forages.
Turnip and other Brassicas can provide grazing at any time during the summer and fall depending on the seeding date. A promising use may be for late fall grazing. These crops maintain their forage quality, if not headed, well into the fall even after freezing temperatures and may be grazed in the Upper Midwest into November. Many turnips can be grazed twice to permit utilization of top growth and roots.
Turnip is a member of the mustard family and is therefore related to cabbage and cauliflower. Turnip is a biennial which generally forms seed the second year or even late in the fall in the first year if planted early in the spring. During the first or seeding year 8 to 12 erect leaves, 12 to 14 in. tall with leaf blades 3 to 5 in. wide are produced per plant. Turnip leaves are usually light green, thin and sparsely pubescent (hairy). In addition, a white-fleshed, large global or tapered root develops at the base of the leaf petioles. The storage root varies in size but usually is 3 to 4 in. wide and 6 to 8 in. long. The storage root consists mainly of the hypocotyl, the plant part that lies between the true root and the first seedling leaves (cotyledons). The storage mot generally has little or no neck and a distinct taproot. The storage root can overwinter in areas of mild winter or with adequate snow cover for insulation and produce 8 to 10 leaves from the crown in a broad, low-spreading growth habit the following spring. Branched flowering stems 12 to 36 in. tall are also produced. The flowers are clustered at the top of the raceme and are usually raised above the terminal buds. Turnip flowers are small and have four light-yellow petals.
Brassicas are both cold-hardy and drought-tolerant. They can be planted late-even as a second crop-and provide high-quality grazing late in the fall. Turnip planted in July will provide grazing from September to November. The most vigorous root growth takes place during periods of low temperature (40 to 60¡F) in the fall. The leaves maintain their nutritional quality even after repeated exposure to frost.
Like other Brassicas, turnip grows best in a moderately deep loam, fertile and slightly acid soil. Turnip does not do well in soils that are of high clay texture, wet or poorly drained. For good root growth turnip needs a loose, well aerated soil.
Turnip seed is small and it is essential that it be seeded into a fine, firm seedbed with adequate moisture for germination. Plow and disk or harrow to produce a seedbed that is fine, firm and free of weeds and clods.
Turnip, like other Brassicas, can also be seeded into a sod or into stubble of another crop with minimum tillage. When seeding into sod, it should be suppressed or killed, as the young Brassica seedlings cannot compete with established grasses. To kill sod, apply 2 qt/acre of Roundup at least three days prior to seeding. A 0.5 qt/acre rate of Roundup can be used in 3 to 10 gal water/acre to suppress sod or to prepare a field of wheat stubble for seeding with turnip. Once established, turnip will compete with most weeds.
The advantages of direct drilling turnip into sod include fewer crop losses due to insect pests, such as the flea beetle, and less soil erosion on sloping sites where pastures are often located. A field of turnip established in sod gives animals a firm footing in all kinds of weather. It also allows the original sod to grow again the following spring if it has only been suppressed.
Turnip seed does not germinate well in cold soil. Turnip should not be planted until the soil temperature is at least 50¡F or at corn planting time. The crop can be planted any time during the summer until about 70 days before a killing frost (August I in the southern half of Wisconsin. earlier elsewhere in Wisconsin and in Minnesota). Plantings after these dates may not have sufficient time to produce good forage growth.
Turnip seed can be planted in 6 to 8 in. rows at a rate of 1.5 to 2.5 lb/acre with a minimum-till drill when sod seeding. In conventionally prepared seedbeds, the crop can be seeded with a forage crop seeder or broadcast followed by cultipacking. The seed should not be covered with more than 1/2 in. of soil. A plant population of 5 to 6 per sq. ft. is desirable.
Good soil fertility is very important for good yields. Soil tests should be taken to assure proper fertilization. Lime acid soils to pH 6.0. Fertilizers should be applied at the time of seeding or within 3 days of seeding to give the crop a competitive edge on weeds. Apply 100 lb/acre nitrogen to soils containing 2 to 5% organic matter, 120 lb/acre if less than 2% organic matter and 60 to 80 lb/acre if more than 5% organic matter. Requirements for phosphorus and potassium are similar to those of a small grain. In Wisconsin and Minnesota, when soil tests are in the medium range, about 20 to 30 lb/acre of P2O5 and 120 lb/acre of K2O should be applied. Fertilizer applications should be banded at least 2 in. to the side and below the seed or broadcast. Boron and sulfur may also be needed. If the soil tests "low" in boron, apply 1 lb boron/acre on sandy soils, and twice this amount on other soils. Apply 10 to 15 lb of S/acre if a soil sulfur test indicates a need for this element.
Three forage turnip varieties are recommended for use in the Upper Midwest: Green Globe, and York Globe from New Zealand and Sirius turnip from Sweden. In Pennsylvania, Green Globe and York Globe yielded more than Sirius at 60 days after planting, but Green Globe reached its peak yield later than the other two. Sirius yields were more variable from year to year than Green Globe or York Globe. The tops and leaves of Sirius have less glucosinolate than the other two varieties.
Weeds are generally not a problem once the turnip crop is established. However, sod and annual weeds should be controlled chemically and/or culturally before planting. Sod can be suppressed or killed with Roundup, as described under Seedbed Preparation. If annual weeds are present at planting time, eliminate them with a burndown herbicide such as Gramoxone. Tillage before planting can be used for weed control on a conventional seedbed.
Turnip crops may suffer from clubroot, root knot, leaf spot, white rust, scab, anthracnose, turnip mosaic virus and rhizoctonia rot. in some cases, diseases can lead to crop failure if rotation or other control measures are not used. Resistant varieties are available for some diseases. To prevent problems with diseases, Brassicas should not be grown on the same site more than two years in a row. If clubroot is a problem, rotation should be six years.
Turnip crops are attacked by two different flea beetles, which eat holes in the cotyledons and first leaves, chew stems and cause extensive plant loss. The cabbage flea beetle and the striped flea beetle feed exclusively on Brassicas, including related weeds such as yellow rocket. Problems with these flea beetles are much greater when Brassicas are grown under conventional tillage. Both flea beetles can be controlled with insecticides applied to the soil at planting.
Turnip crops can also be damaged by infestations of the common turnip louse or aphid. This insect feeds on the undersides of the leaves and may be so close to the ground that it is difficult to reach with a dust or a spray. In cases of severe infestation, the outer leaves curl and turn yellow. Aphid-tolerant varieties such as 'Forage Star' can give some protection against this insect.
Turnip plants are ready for grazing or green-chop when the forage is about 12 in. tall (70 to 90 days after planting). It is best not to wait too long because fungal diseases may begin to cut yields approximately 110 days after planting. The pasture should be grazed for a short time and the livestock removed to allow the plants to regrow. If grazed to a 5 in. stubble, 1 to 4 grazing periods may occur, depending on planting date and growing conditions. Strip or block-grazing is desirable to insure complete grazing.
The forage quality of turnip is sufficiently high, especially in protein, that it should be considered similar to concentrate feeds, and precautions should be taken to prevent animal health problems. Livestock should not be hungry when put on pasture the first time so they do not gorge themselves. If the livestock are moving from a feed low in nutritional value, feed a high-quality diet for two to three weeks prior to grazing turnip, or feed turnip for 30 min/day for one week prior to heavier grazing. This will allow for the development of a rumen microbial population that is adequate to digest the high levels of protein in forage turnips. A lower quality hay should be made available (2 to 3 lb of dry roughage/head/day for sheep and 10 to 15 lb for cattle) to provide some fiber in the animals' diet.
Livestock should not feed on turnip during the breeding season or after the plants have begun to flower. Nitrate nitrogen toxicity can be a problem, especially if ruminants are allowed to graze on immature crops or if soil nitrogen levels are high. The risk may remain for a longer period of time in autumn than in summer. Dairy cows should not be fed more than 50 lb turnip/head/day and should not be milked immediately after feeding on turnip to avoid milk tainting.
Yields of forage turnip range between 3 and 4 tons of dry matter/acre when harvested or grazed about 90 days after planting. Up to 1,000 grazing days/acre for 900 lb steers and 2,300 grazing days/acre for 90 lb lambs have been obtained for Forage Star turnip.
Turnip is a highly nutritious forage crop that has a short growing season and can provide late fall grazing after other forage crops are finished for the year. Seed prices range from $1.50/lb for garden-variety turnips to about $8/lb for some new hybrid varieties.
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