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Radishes: A new cover crop option lanting forage or oilseed radishes began to take root several years ago, and their P use as cover crops is grow- ing. Both are members of the mus- tard family (Brassicaceae), which has a long history of being used for cover crops and animal forages. Forage rad- ish (Raphanus sativus L. var. niger J. Kern.), also known as the daikon rad- ish or Japanese radish, has a very large taproot. Originally developed for oil production, oilseed radish (R. sativus L. var. oleiformis Pers.) is similar to the forage radish, but its taproot is stubbier and more branched, and it tends to be somewhat more winter hardy than the forage radish. Forage and oilseed radishes can be helpful in no-till operations where their large roots can help retain soil moisture and reduce erosion. They are excellent at breaking up shallow layers Forage radish. Photo courtesy of Sjoerd Duiker, Penn State. of compacted soils, earning them the nickname “biodrills” or “tillage rad- Sudangrass, according to Gruver. For net nitrogen mineralization. They also ishes.” Once planted in late summer, nitrogen fixation, legumes such as tolerate cold temperatures very well. the radishes are not harvested but die clovers, vetches, lentils, cowpeas, soy- An additional special feature of in the winter, decay, and contribute a bean, and field peas are best. For nu- most brassicas is that they produce nitrogen store for spring planting. Dy- trient cycling, Gruver recommended compounds, called glucosinolates, ing off in the winter, the radishes leave sunflower, sugarbeets, brassicas, and which are toxic to soil-borne pests root channels so that soil dries and small grains. and pathogens. Mustards usually warms up faster in the spring. Brassica cover crops have a number have higher concentrations of these As part of a recent Illinois extension of beneficial attributes, including rapid chemicals. More than 100 different telnet series on utilizing cover crops in fall growth, high biomass production, glucosinolates are found in brassicas. conventional cropping systems, Joel a well-developed taproot, excellent Breakdown products from glucosino- Gruver, assistant professor of soil sci- nutrient-scavenging ability, high re- lates are volatile and similar to the ac- ence and sustainable agriculture at sponsiveness to nitrogen, competitive- tive chemical in the fumigant Vapam. Western Illinois University, spoke on ness with other plants, and special pest Glucosinolate concentrations differ the benefits and management of bras- resistance capabilities. according to plant part, age, health, sicas and legumes as cover crops. According to Gruver, the large and nutrition. Despite this complex- “Cover crops are multi-functional,” taproot of radishes and other brassi- ity, Gruver said there is evidence that Gruver said. “It is important to remem- cas gives the crops an above-average brassica cover crops can be used to ber that capturing multiple benefits ability to penetrate compacted layers; reduce pests, pathogens, and weeds if takes more management. Cover crops this promotes deeper rooting by subse- the right species/cultivar is planted and are not idiot proof, but there are few quent crops and increases water infil- managed strategically. profits in idiot-proof systems!” tration. The residue from brassicas de- If you want to reduce soil com- composes very quickly, and this means paction, good cover crop choices are that they immobilize less nitrogen than Benefits of radishes Various research groups have been radish, canola, turnip (and hybrids), cereal cover crops and often result in growing different types of radishes in sugarbeet, sunflower, and sorghum- 14 Crops & Soils | November–December 2009 American Society of Agronomy different areas to determine their effi- Data suggests that biodrilling with Oilseed radish can scavenge nitrate ciency as a cover crop. cover crops like forage radish can sub- from deeper soil layers after harvest of Ray Weil, Charlie White, and stitute for expensive and energy-inten- the cash crop. Upon decomposition, Yvonne Lawley at the University of sive deep ripping and other mechani- the nitrogen uptake becomes available Maryland have studied the use of for- cal methods to reverse soil compac- to the next cash crop. In the Michigan age radish. Although it is fairly new to tion. Some farmers plant forage radish State tests, a cultivar called Renova, the Mid-Atlantic region, the use of for- in 24- or 30-inch-wide rows (with for example, was shown to recycle age radish as a cover crop has some another cover crop species broadcast more than 140 lb of nitrogen/acre in advantages over other cover crops in in between rows—see “Cover Crop a growing season. In muck soil, the the region. Mathieu Ngouajio and Cocktails” section on page 17) to common cultivar recycled more than Dale Mutch at Michigan State Univer- maximize its root-to-shoot ratio. They 60 lb of nitrogen/acre in two months. sity have experimented with the use of then plant the following summer crop The Maryland group found that, un- oilseed radish. Because of its quick es- in these same wide rows to alleviate like rye and other cereal cover crops tablishment and rapid growth in cool restriction of root growth into the sub- whose residues decompose slowly and weather, it has been used successfully soil. immobilize nitrogen in the spring, for- in Michigan as a cover crop in diverse In a similar manner, oilseed radish age radish residue decomposes rapidly production systems. produces large taproots. Upon decom- and releases its nitrogen early. In fact, According to the Michigan re- position, these roots leave large holes on sandy soils, it is important to plant searchers, the classification of these and other types of radishes is not well defined because they can easily cross- One of the great features of forage radish cover crops pollinate, and therefore distinctions is that they can be used as a biological tool to reduce among subspecies are often blurred. Most of the traits and management the effects of soil compaction, hence the term “tillage recommendations described in this ar- radish”.... After the cover crop dies in the winter and ticle apply to both forage and oilseed its roots decompose, the remaining root channels are radishes. One of the great features of forage used by the growing roots of following crops to pen- radish cover crops is that they can be etrate compacted deep soil layers. used as a biological tool to reduce the effects of soil compaction, hence the term “tillage radish.” The roots of all in the ground that improve water in- as early as possible, following forage cover crops can penetrate compacted filtration and possibly soil microbial radish cover crops, to take advantage soils in fall to some extent because activity. of this flush of nitrogen before it leach- they are growing when soils are rela- Oilseed radish emerges shortly after es out of the rooting zone. Forage rad- tively wet and soft. But the Maryland planting and provides quick ground ish recycles large amounts of N taken researchers found that forage radish cover that smothers weeds. When up from the soil profile in fall and can roots can penetrate plow pans or other planted in fall, it prevents weed ger- reduce the need for nitrogen fertilizer layers of compacted soil better than mination and, consequently, seed pro- in spring. most other cover crops. The thin lower duction. Early planted forage radish part of the taproot can grow to a depth can also produce a dense canopy that of 6 ft or more during the fall. The all but eliminates weed emergence in Growing radishes as a cover thick, fleshy upper part of the taproot the fall and winter. To obtain this near- crop grows 12 to 20 inches long and creates complete weed suppression, forage Oilseed radish cultivars used as vertical holes and zones of weakness radish should be planted by September cover crops include the common vari- that tend to break up surface soil com- 15 (in Maryland) with a stand of 5 to ety, Adagio, Arena, Colonel, Remonta, paction and improve soil tilth. After 8 plants/ft2. The near-complete weed Revena, Rimbo, and Ultimo. Accord- the cover crop dies in the winter and suppression can be expected to last ing to the Michigan researchers, most its roots decompose, the remaining until early April but does not extend of these cultivars are imported from root channels are used by the growing into the summer cropping season. Europe. The common cultivar is the roots of following crops to penetrate most readily available in Michigan. u compacted deep soil layers. In Maryland research, four times as many corn roots penetrated compact subsoil after a forage radish cover crop than after winter fallow, and twice as many as after a rye cover crop. agronomy.org November–December 2009 | Crops & Soils 15 u Left: Oilseed radish taproot compared with 1-ft ruler. Photo by Alan Sundermeier. Right: “Tillage radishes” can be used as a biological tool to reduce the effects of soil compaction. Photo by Steve Groff (www. tillageradish.com). Oilseed radish seed is generally more seedbed. To improve weed and pest gust or early September, but signifi- expensive than seed of other cover management, planting oilseed radish cant amounts of N can be captured by crops commonly grown in Michigan. on the same field more than two years this cover crop when planted as late Whether planted in spring, late sum- in a row is not recommended. as October 1. Forage radish planted in mer, or early fall, oilseed radish grows The Maryland researchers recom- late September may be less susceptible quickly and produces a large amount mend seeding at 8 to 10 lb/acre us- to frost and more likely to overwin- of biomass in a relatively short time.

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