Soybean replant decisions
Tuesday, June 17, 2014 12:01 AM
BY CURTIS E. YOUNG
Agriculture and Natural Resource Extension Educator
Van Wert County OSU Extension
VAN WERT -- As the planting season continues, many have finished and others are getting closer and closer to being finished with their planting of corn and soybeans. Unfortunately, Mother Nature has not been totally kind to some of these newly planted fields resulting in thin stands to no stand. Now soybean producers are going back and assessing the condition of their soybean stands and discovering these less than desirable areas of some fields. The question then arises, "What does one do with these fields…replant, patch-in, or leave it alone?"
Before any type of replant decision is made, it is essential to conduct a stand count to determine what the soybean plant population is left in the field. Research data from various land grant universities indicates that a relatively uniform stand of 100,000 to 120,000 plants per acre in drilled 7.5" rows and 15" rows, and 80,000 to 90,000 plants per acre in 30" rows in fields that were planted by mid-May, will yield 100% of maximum for that field. Furthermore, research also indicates that uniform plant stands of 50,000 to 55,000 soybean plants per acre of a May planted field, remaining after a stand reduction event yields only 13-15 percent less than maximum.
There are several techniques for estimating soybean plant populations in fields. One method is to measure a set length of row that represents 1/1000th of an acre, count the number of plants in that row length, then multiple that number of plants by 1000. The row length varies based on the spacing between rows: 69' 9" for 7 1/2-inch rows (drilled soybeans); 34' 10" for 15-inch rows; and 17' 5" for 30-inch rows. For example, if 90 plants were counted in 34' 10" of row of a 15-inch row planting, then this would estimate a soybean plant population of 90,000 soybean plants per acre. This procedure should be repeated in several areas within a field and averaged to produce a better estimate for the field. Additionally, one should not selectively pick the worst or the best looking spots in a field within which to conduct the estimates. Evaluation spots should be randomly picked (e.g. walk to an area of a field, toss a hat away from your body, wherever the hat lands that is the starting point). A second method is the hula-hoop method. A hula-hoop of a known diameter can be used to rapidly count plants, especially in narrow row widths. Toss or roll the hoop into the area to be counted and allow it to fall at random, then count plants inside the circle. Average at least 10 samples areas within a field for a reliable estimate of plant population. The following are multiplication factors for several hula-hoop diameters, multiple the number of soybean plants within the hula-hoop by: 10,200 for a 28" diameter; 8,900 for a 30-inch diameter; 7,800 for a 32-inch diameter; 6,900 for a 34-inch diameter; and 6,200 for a 36-inch diameter.
For those of you who would like visual instruction on plant population estimation, Purdue University Extension Soybean Specialist Shaun Casteel has short YouTube video clips that can be watched for your education and can be found at the following URLs: http://www.youtube.com/watch?v=CA7teyzb20w (drilled) ; http://www.youtube.com/watch?v=c8oMiqobvE0 (15-inch rows); and http://www.youtube.com/watch?v=6JxVCxHiemw (30-inch rows).
If one determines that the stand is below acceptable population levels and elects to replant the field or replant drowned out areas of the field, remember that June planted soybeans are recommended to be planted at a higher seeding rate (10 to 20 percent higher) than May planted soybeans.
If soybean stands are only thin in sections of a field, one might consider patching-in additional soybeans to beef up the stand counts in those areas. However, research conducted a Purdue University suggests that there is little or no positive impact in this practice.
Things to consider before attempting a patching-in replanted of soybeans:
• We are beyond the optimal window for planting soybeans (i.e. mid-May). Every day after this time, it is estimated that there will be a 1/2 bushel/acre drop in yield potential from the maximum yield.
• What are the costs of patching-in extra soybeans?
• How much damage might be done to the already established stand when patching-in spots? Will one cause more damage to yield than what might be recovered from the replant?
• By the time conditions are good enough to attempt patching-in, how much difference in maturity will there be between the replanted soybeans and the original crop?
• If the separation in maturity is great enough to cause one to harvest around the immature soybeans, will there be enough left over soybeans in the field to justify spending the time and money (labor and machinery expenses) to return to harvest the patches left behind after the first harvest?
• What type of soybean was planted originally? A bush soybean? A semi-bush soybean? Or a linear soybean? Bush and semi-bush soybeans have a great potential to fill in gaps in rows while linear soybeans have limited potential.
If one has a thin yet savable stand of soybeans, weed control must become the greatest priority. Reduced soybean stands allow additional light to reach the soil surface to stimulate weed seed germination bringing on more weeds to compete with the soybean plants. Monitor fields closely and use appropriate weed management practices to minimize the impact of weed competition on yield. In the long run, the cost of an additional pass over the field for weed control will be less expensive and more cost effective than patching-in more soybean seed.