By Trenton Roberts, Research Assistant Professor, Crops, Soil & Environmental Sciences
The tillage radish or daikon radish has been bred/developed to produce a large taproot and penetrate compacted soil layers in an effort to increases soil aeration, water infiltration, decrease compaction and provide increased rooting depth opportunities to successive crops. Tillage radish are often promoted to help alleviate compaction, but do not do well in poorly-drained soils that are prone to water-logging or extended wet periods that are typically seen in our “rice” soils in eastern Arkansas. Although tillage radish may not penetrate and grow as deeply in our soils as we might hope, they can serve another useful purpose that can be of great value to producers; nutrient retention. The large taproot that is developed by tillage radish can absorb and retain a significant amount of macro- and micronutrients that might otherwise be prone to leaching or other loss mechanisms during the 5-7 month window that fields are fallow in the winter. Think of the tillage radish taproot as a giant sponge that will absorb residual nutrients from the soil and hold them until termination in the spring. The other nice thing is that the nutrients which are absorbed by the taproot are readily available to the following cash crop as the taproot is mostly water and desiccates and decays quickly releasing those nutrients almost immediately (2-4 weeks) for uptake and utilization by the following cash crop. Tillage radish seeds are larger (20,000-25,000 seeds/lb) than one might think and are almost as large as grain sorghum and work well as a single-seeded cover crop or part of a cover crop blend.
Crop Rotations for Tillage Radish
Considering the cash crop that will be planted the following spring is the first step in developing an effective winter cover crop management plan. Tillage radish are best suited to proceed summer crops such as soybean, corn, grain sorghum and cotton. Ideally the following cash crop will be no-tilled into the terminated tillage radish.
In order to use these radishes to their full potential producers should do all of their land preparations in the fall (pulling beds etc) with the intention of planting the following cash crop in the spring with no tillage. One of the primary benefits of the radish are the “hole” or cavity that is left when the root dies and decays, which allows the soybean or following crop’s roots an easy channel to bore deeper in the soil profile. If you plant radishes and then do your normal tillage operations in the spring prior to planting you are potentially wasting one of the benefits of the radishes. If you are flat planting or are happy with the beds that were used during the previous summer, tillage radish can be effectively no-till planted into crop residues, but close attention should be paid to planting depth to ensure adequate seed to soil contact for optimum radish establishment. When flat planting tillage radish into a stale seedbed or previous crop residue considering pulling drainage ditches to aid in the removal of standing water from the field during the wet winter months. Tillage radish are similar to winter wheat and do not like saturated soil conditions or ponded water.
Tillage radish performance is highly influenced by planting date and the earlier you can get them established the better the results you will see the following year. Tuber growth occurs primarily in the fall prior to winter dormancy or winter-kill. There is minimal tuber growth in the spring after the radishes break dormancy as they switch into reproductive growth. Therefore the majority of your benefits associated with tillage radish occur in the fall so the early planting dates will perform better. Minimum soil temperatures for tillage radish germination is 45̊ F, so they can be planted late in the fall and still germinate. However, late planted radishes do not have adequate time to grow and develop the large taproot or aboveground biomass that is critical to their success as a cover crop, prior to the cold temperatures that restrict growth during December- February. Ideal planting dates for tillage radish in Arkansas are as follows:
Northern Arkansas (north of Hwy. 64) Optimum planting window is August 15th – Oct. 1st.
Central Arkansas (south of Hwy 64 to north of Pine Bluff) August 15th –Oct 15th
South Arkansas (south of PineBluff) August 15th – Nov. 1st.
Early plantings following corn or grain sorghum can be done as early as August and often times leads to the best performance of the tillage radish due to the early establishment date and the residual nutrients that are found following these particular crops. Crops that tend to be harvested later in the year such as rice and soybean limit the window of opportunity to establish tillage radish, but can be done. Planting date should also take into consideration any post emergence herbicide applications that were made to the preceding crop. Residual herbicide activity that can be problematic for tillage radish include the following flumetsulam (Python), chlorimuron (Classic, Canopy, Cloak, etc.), imazethapyr (Pursuit, Newpath), and fomesafen (Reflex etc.). Herbicide activity is a function of soil texture, moisture and microbial activity. If you are concerned about residual herbicide activity influencing your radish establishment you can easily gather soil from the fields of interest (both on beds and in-furrow) and do a simple germination test with your tillage radish seed to determine if there is a potential problem.
Picture 2. Influence of Tillage Radish planting date on establishment and winter-kill, photo taken February 9th, 2015 near Colt, AR. Planting dates are (L to R) Sept. 15th, Oct. 1st and Nov. 15th. Note that early planted radishes were able to emerge and establish prior to cold temperatures. Also notice how Sept. planted radishes tended to winter-kill due to above ground tuber exposure, whereas Oct. planted radishes did not.
Precision Planter- 4 lbs seed/acre
Drilled on 7.5- 15” row spacings- 4-6lbs seed/acre
Broadcast or aerial seeding- 8-10 lbs seed/acre
Planting depth 1/4” -1”
Most tillage radish seeds are similar in size to grain sorghum or milo and can be planted with similar equipment.
In most of the soils in eastern Arkansas tillage radish that is planted prior to Oct. 1 will not require N fertilizer in order for producers to get the full benefits of the radish. Tillage radish planted after Oct. 1 will benefit from 30-60 units of N to grow properly and achieve maximum rooting depth. For radish following rice on silt loam soils somewhere between 50-80 units of N are needed to maximize rooting depth. We have not conducted any research on clay soils, but 50-60 units of N will probably be needed for late plantings. There is no need to fertilizer tillage radish with P or K for optimal results on the majority of our soils. Land that has been recently leveled will often times benefit from the application of 1-2 tons of poultry litter per acre. Applying the litter prior to tillage radish plating will help with tillage radish performance and also capture nutrients released by the poultry throughout the fall and winter months. Research has shown that the majority of nutrients N, P, and K that are taken up by the radish are made available to the following crop within a month after the radishes are killed.
Burn down/Killing Radishes
In the spring prior to flowering the radishes should be killed to prevent seed dispersal and the potential for volunteer radishes in successive crops. Like other brassica (mustard) species the plants will send up a tall flowering structure prior to maturity. In our experience, it is best to apply your herbicide prior to this flowering structure reaching more than 4-5 inches in height. Current recommendations are to apply 1 qt/A glyphosate with 1 pt/A 2,4-D and apply before flowering for adequate burndown. If tillage radish is in the reproductive stage, it will difficult to kill with any herbicide. Tillage radish that are planted early will often times grow large enough to winter kill, which is another benefit of early planting. Typically if the radish tuber is 3-4 inches above the soil surface an average Arkansas winter will kill these radishes. However, if tubers are close to the ground or there is no tuber exposed then it will rarely get cold enough to winter-kill the radish.
Picture 3. Above ground tuber growth due to early planting date aids in winter-kill of tillage radish. Notice smaller tillage radish to lower left of picture did not winter-kill due to small tuber and less exposure to cold temperatures (insulated by soil).
We are continuing to work with these and other cover crops to improve row-crop performance in Arkansas. This research was supported by the Arkansas Soybean Promotion Board and special thanks to Dr. Jason Norsworthy for contributions to this article.