Replants And Supplementing Poor Soybean Stands

DR. ANGELA MCCLURE

JACKSON, TENN.
   Wet weather has caused some cotton failures and enough damage to soybean stands in a few areas where replanting is being considered.
Since we are in June, I would consider keeping a low but uniform soybean stand.  Low meaning 70-90 thousand plants per acre – especially if beans are planted on narrow rows. Fields with extremely poor stands or complete bare areas should be replanted or spot replanted to fill in the holes in stand. Where only spot replanting is needed, do it early while the existing crop is small. Use the same variety or at least a similar maturity bean for a more uniform crop later in the season. Timely weed control will be important in fields with thinner stands that tend to canopy late. Supplementing thin stands is something folks have tried with varying degrees of success. This involves planting a low population of the same variety back into a field with a too-low stand. If you split row middles, some of the original rows will be run down in the process leaving more skips for weeds to grow. Driving diagonally across the old rows (think of an angled checker board) to increase the plants per square foot in the field can work fairly well and entire rows are not flattened in the process. Limited data suggests that these gaps fill in more quickly than missing rows.
   When replanting entire fields, the best maturity group choice for June planted soybeans is a MG 4.  My next choice would be a MG 5. While we are in June, put away the Group 3’s and work with fuller season options. TN collaborated with other states in a USB funded planting date/maturity group study and found that MG 4 varieties tend to have the best yield return as a late plant option over MG 3 and even MG 5 varieties. I would strongly urge planting anything now on narrow rows to speed up canopy closure. Since we are working with single crop beans, I would not plant much thicker than normal to ‘force’ height yet, but if we are still planting into July that would certainly be needed.
   Nitrogen fertilizer carried in from a failed cotton or corn crop will reduce and delay nodulation on a bean crop but may not impact yield. Figure that at least half or more of the nitrogen applied to cotton fields is still available unless the field went under water for several days. In Jackson in 2010, we showed reduced soybean nodulation with 50 lbs N at planting and in a dry year we had some yield loss with those treatments (probably because microbial activity is further hindered in droughty soils). Data from other states suggest that although nodulation is delayed by soil N, we may not see yield reductions at all with N amounts up to 100 lbs at planting where soil moisture is adequate during the growing season. ∆
   DR. ANGELA MCCLURE: Extension Corn and Soybean Specialist, University of Tennessee