The tillage radish or daikon radish has been bred and developed to produce a large taproot and penetrate compacted soil layers to increase soil aeration and water infiltration, to decrease compaction and to increase rooting depth opportunities for successive crops.
Tillage radishes are often promoted to help alleviate compaction, but they do not do well in poorly-drained soils prone to water-logging or during extended wet periods that are typically seen in our “rice” soils in eastern Arkansas.
Although tillage radishes may not penetrate and grow as deeply in our soils as we might want, they serve another useful purpose that can be of great value to producers — nutrient retention. The large taproot that is developed by a 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 five- to seven-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 nutrients absorbed by the taproot are readily available to the following cash crop because the taproot is mostly water and desiccates and decays quickly, releasing those nutrients almost immediately (two to four weeks) for uptake and utilization by the following cash crop.
Tillage radish seeds are larger (20,000 to 25,000 seeds per pound) than one might think and are almost as large as grain sorghum. They 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 radishes are best-suited to precede summer crops such as soybean, corn, grain sorghum and cotton.
Ideally the following cash crop will be no-tilled into the terminated tillage radish.
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 radishes are the “holes” or cavities left when the root dies and decays, which allows the soybeans or other following crop 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, consider pulling drainage ditches to aid in the removal of standing water 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. 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 winterkill. 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 — the early planting dates will perform better.
Minimum soil temperatures for tillage radish germination is 45 degrees 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 above-ground biomass critical to their success as a cover crop prior to the cold temperatures that restrict growth during December to February.
Ideal planting dates for tillage radish in Arkansas are as follows:
• Northern Arkansas (north of Hwy. 64) — Aug. 15 to Oct. 1.
• Central Arkansas (south of Hwy 64 to north of Pine Bluff) — Aug. 15 to Oct. 15.
• South Arkansas (south of Pine Bluff) — Aug. 15 to – Nov. 1.
Plantings following corn or grain sorghum can be done as early as August and often lead 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 it can be done.
Planting date should also take into consideration any postemergence herbicide applications made to the preceding crop. Residual herbicide activity that can be problematic for tillage radishes includes 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.
• Precision planter — 4 pounds of seed per acre.
• Drilled on 7.5- to 15-inch row spacings — 4 to 6 pounds of seed per acre.
• Broadcast or aerial seeding — 8 to 10 pounds of seed per acre.
• Planting depth — 0.25 inch to 1 inch.
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 radishes that are planted prior to Oct. 1 will not require N fertilizer for producers to get the full benefits of the radishes. Tillage radishes planted after Oct. 1 will benefit from 30 to 60 units of N to grow properly and achieve maximum rooting depth.
For radishes following rice on silt loam soils, somewhere between 50 and 80 units of N are needed to maximize rooting depth.
We have not conducted any research on clay soils, but 50 to 60 units of N will probably be needed for late plantings.
There is no need to fertilize tillage radishes with P or K for optimal results on the majority of our soils. Land that has been recently leveled will often benefit from the application of 1 to 2 tons of poultry litter per acre. Applying the litter prior to planting tillage radishes will help with performance and will capture nutrients released by the poultry litter throughout the fall and winter months.
Research has shown that the majority of N, P, and K taken up by the radishes is available to the following crop within a month after the radishes are killed.
The radishes should be killed in the spring prior to flowering 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 to 5 inches in height.
Current recommendations are to apply 1 quart per acre of glyphosate with 1 pint per acre of 2,4-D and to apply before flowering for adequate burndown. If tillage radishes are in the reproductive stage, it will difficult to kill with any herbicide.
Tillage radish planted early will often grow large enough to winterkill, which is another benefit of early planting. Typically, if the radish tuber is 3 to 4 inches above the soil surface, an average Arkansas winter will kill these radishes. If tubers are close to the ground or there is no tuber exposed, it will rarely get cold enough to winterkill the radishes.
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 I give special thanks to Jason Norsworthy for contributions to this article.
Trenton Roberts is a Research Assistant Professor, Crop, Soil and Environmental Sciences, University of Arkansas