Stand Uniformity and Soybean Yield

Dennis B. Egli

LEXINGTON, KENTUCKY

   Are precise plant-to-plant spacings in the row and uniform seedling emergence necessary for high soybean yield? The importance of precise uniform stands is well known in corn, but is it important for soybean? The short answer is NO!    

  Plant-to-plant spacing in the row (spatial uniformity) and the timing of emergence of individual seedlings (temporal uniformity) are determined by the characteristics of the planter, the planting process and the seed bed. The proportion of the seeds that germinate and produce emerged seedlings and the planting date also influence stand uniformity.  

  Spatial and temporal non-uniformity have the same effect on plant growth. Plants with wider in-row spacings or plants that emerge first (dominant plants) have access to more sunlight and grow faster than plants that are closely spaced or emerge later (dominated plants). The faster growing plants set more seeds than the slow growing plants. The effect on yield depends on the ability of the fast-growing dominant plants to produce enough ‘extra’ seeds to compensate for the reduced seed number on the dominated plants.

    The key for soybean is that it can compensate - the early emerging (or widely spaced) plants produce enough ‘extra’ seeds to make up for the reduction in seeds on the late emerging (or closely spaced) plants so that the total seeds per acre and yield are not affected (compared to a perfectly uniform stand). The dominant plants have enough plasticity to make up for the loss of seeds on the dominated plants. Corn, on the other hand, usually cannot produce enough ‘extra’ seeds to make up for the loss on the dominated plants, so the seeds per acre and yield may be reduced. 

   The difference between corn and soybean lies in their reproductive plasticity. Plasticity (or flexibility) refers to the ability of the plant to increase the number of seeds it produces when grown in more productive environments. Soybean plants are very plastic; they can easily increase the number of pods and seeds they produce by branching to increase the number of nodes per plant, by increasing the number of flowers per node, and by decreasing flower and pod abortion when grown in favorable environments. 

   Most modern corn hybrids are not very plastic; they seldom produce multiple tillers and they often have only one ear per plant. When all of the florets on that ear produce seeds, the plant can no longer increase seed number which limits the capacity of the dominate plants to produce the necessary ‘extra’ seeds. When the dominant plants in non-uniform stands cannot compensate, seed number per acre and yield are reduced. Corn hybrids that are more flexible (i.e., produce multiple ears or ears on tillers) would do a better job of maintaining seed number and yield in non-uniform stands.

   We tested these relationships in a soybean field experiment where we planted every other seed in the row either 4- or 7-days after the original planting and compared it to a control where all seeds were planted at the same time. The early emerging plants in the 4-day delay treatment produced 86 seeds per plant vs. 52 seeds per plant on the delayed plants. Similar numbers for the 7-day delay treatment were 96 and 39 seeds per plant. The total combined yield of early and late emerging plants was the same as the uniform planted control in both years of the study. The soybean plant had the flexibility to adjust and maintain a constant yield in non-uniform stands.

   It is important to note that soybean yield will be reduced if the variation of in-row spacing is so large that the plants cannot fill in the gap (creating a skip). If the gap is so large that you can see the soil when the soybean plants start flowering, the interception of sunlight by the plant community and yield will be reduced. Even a flexible plant like soybean can’t compensate for gaps that reduce sunlight interception.

   Interest in ultra-early plantings of both corn and soybean to increase yield have ballooned     in recent years. Unfortunately, ultra-early plantings may reduce the uniformity of emergence and reduce corn yield. Low soil temperatures, often associated with these early plantings, delay emergence which increases non-uniformity of emergence. The longer the delay, the greater the non-uniformity. Low soil temperatures can also reduce the percentage emergence which would decrease spatial uniformity. Reducing average soil temperature from 68 to 58°F increased the time to 10% of final emergence of corn seedlings from 6 to 12 days in greenhouse and growth chamber experiments. This delay roughly doubled the non-uniformity of the resulting stand. Experiments with soybean produced similar results. Since soybean yield is not influenced by non-uniformity, planting soybean before corn in the early spring (as others have proposed) would reduce the effects of this temperature induced non-uniformity on overall-all yield. Planting in warm, moist soil and avoiding heavy rainfall before seedling emergence provides the best opportunity for rapid uniform emergence of both corn and soybean seedlings.  ∆