With corn planters beginning to roll soon, it is a good time to think about the importance of uniformity of stands and emergence. These are two different things. Stand uniformity has to do with how consistent plant spacing is within the row. Uniformity of emergence deals with timing. Do most plants come up at the same time, or are some delayed by several days?
Stand uniformity: Although uniform stands are desirable, how important is it that the distance between plants be the same from one plant to the next? Past research has indicated the potential for a 1 to 3.4 bushel/acre decrease in yield for every 1-inch deviation in plant spacing. Stu Duncan, Northeast Area Crops and Soils Specialist, has been looking at corn plant populations and stand uniformity the past few years. His results indicate little yield reduction from non-uniform stands as long as the final population is within 15 percent of the target population.
This agrees with work done by other researchers who have concluded that reduced population and non-uniform emergence have more potential to negatively influence yields than does non-uniform plant spacing. In fact, one study indicated that "doubles" (two plants where one was intended) can increase yield in favorable environments because the effective plant population was increased. Individual corn plants have enough flexibility in yield components (primarily ear size) to make use of the small changes in available resources resulting from non-uniform plant spacing.
Try to obtain plant spacings that are as consistent as possible, but don’t become overly anxious about it as long as the typical spacing between plants is within two to three inches of the desired plant spacing and the final population is not substantially lower than what was desired.
Uniform emergence: Emergence can be delayed by non-uniform moisture in the seed zone, crusting, non-uniform planting depth, or non-uniform crop residue. Uniform emergence can be important for maximizing yield. Research has shown that if one out of six plants is delayed by two leaf stages, yields can be reduced by 4 percent. If one out of six plants is delayed by four leaf stages, yields can be reduced by up to 8 percent. Other research has indicated that if plants emerged within a period of two weeks, yield reductions were minimal (<3 percent). A 3 percent yield reduction may not be enough to justify replanting but it is enough to justify efforts to minimize variability in emergence when it could affect gross receipts by as much as $36/acre at 200 bushel/acre yields and $6/bushel corn.
Planter speed can affect both stand and emergence uniformity. Research conducted in northeast Kansas supports the conclusion that final plant population, which was reduced with higher planting speeds, had a greater impact on yield than did accompanying reductions in uniformity of plant spacing. Be sure to follow manufacturer guidelines for recommended planter speeds.
Adjust planter units to optimize seed placement and depth. Seed firmers may help place seeds more uniformly. Emergence might be delayed slightly with deeper planting, but it will likely emerge more uniformly than if it were planted too shallowly (e.g. one inch or less). This is especially true with early planting and cool soil temperatures. With warm soil temperatures and adequate, uniform moisture, corn seedlings emerge more quickly and uniformly from any depth. Regardless of soil temperature, stand and emergence uniformity generally is maximized when seed is placed between 1.5 and 2.5 inches in depth. See the January 20, 2012 e-Update for more information on how planting depth can influence emergence and stand uniformity.