Producers have been pushed to accomplish as much planting possible in the little time given as calendar dates roll by in a late spring start. The goal has been to get the seed in the ground when fields are ready for equipment and between rain events. It has been common across the state in previous years for producers to ground roll fields within hours of planting soybeans.
However, the push to plant between rain events and other delays this year has left many fields unrolled. The question is, "Can I still roll without causing significant damage to the plants or stand?"
To address the question, "When is the best time to roll soybeans?" University of Minnesota Extension conducted a three-year research project (2008-2010) funded by the Minnesota Soybean Growers Association.
Understanding that the first two ground rolling windows have passed (pre-plant and immediate post plant), producers are now wondering about a new timing at pre-emergence. This option will be addressed following a summary of the research.
The research took place at 11 locations across western Minnesota over a span of three growing seasons and included multiple styles of land rollers. The rolling treatments consisted of 1.) Pre-plant, 2.) Post-plant, 3.) 50% emergence, 4.) First trifoliate stage (V1), 5.) Third trifoliate (V3), and 6.) No rolling.
We collected data on the initial impacts that follow treatment, including plant population, percent plant damage, and residue decomposition. Yield, test weight, seed protein, oil, and moisture were determined at harvest.
Surprisingly, we did not find significant yield differences between treatment timings for each year when combined over locations. Also, no significant differences for plant populations, seed oil, protein, moisture, or test weight were found. We did document significantly more plant damage with the V3 treatment in two of the four sites in 2010 but did not see significant, negative consequences on yield.
The research project concluded that, with good conditions, rolling can be performed out to the V3 stage. We do not have data on the impacts of rolling after the V3 stage and are reluctant to recommend it, but we recognize that for some fields it may be the only option.
Iowa State University researchers conducted a similar study where at one site in 2010 they evaluated rolling soybeans with 6 leaves. They reported no significant differences between V3 or earlier rolling, like the Minnesota studies. However, they did find a significant yield reduction when rolling at 6 leaves compared to all other treatments and a reduction of almost 9 bu/ac compared to not rolling at all (http://www.extension.iastate.edu/CropNews/2011/0103alkaisi.htm).
Producers will need to decide if rolling the field is necessary based on rock pressure, corn root balls, and improved field harvestability. Is the risk worth the benefits of this optional operation? Rolling pre-emergence, but post-germination/sprouting, has the risk to seriously impact stand establishment; post emergence rolling has a chance of damaging plants.
Rolling after the seed has been in the ground for more than a few days appears dangerous. The germination process will be quick with warm soils and adequate moisture. A germinated seed with pre-emergent growth could be subjected to the following problems due to ground rolling: increasing seeding depth due to soil leveling; breaking of the hypocotyl growth below ground; increasing compaction of the planting layer; and, soil crusting. Rolling after seedling emergence was demonstrated to be a viable option and still accomplished targeted goals.
There are a couple of considerations when rolling emerged soybeans. The most important factor is to avoid rolling in cool or cold conditions common to morning and evening. Plants are more rigid and brittle at those times and rolling can snap stems instead of bending them. It has also been observed that tractor tracks cause more significant damage than the roller, emphasizing the importance of matching the tractor to the roller width. Ground speed should not make a difference, provided the roller is not "rock hopping" and the driver is paying close attention when turning to avoid pushing up soil berms.
Again, it is important to weigh risks with benefits before proceeding. If a decision to roll a soybean field is made, take the time to evaluate the operation's initial results quickly in the field. If the damage appears to be too excessive, rethink your decision to avoid making the 60 acre mistake and allow for peaceful sleeping that night.
-ByDoug Holen and Phil Glogoza, University of Minnesota Crop Extension Educators.