Effects of Missing and Two-Plant Hills on Corn Grain Yield

Missing plants (“skips”) in corn (Zea mays L.) rows cause yield loss by decreasing plant population. The effect on yield of two plants growing in the space normally occupied by one (“doubles”) is not known. A 2-yr study at Monmouth and Urbana, IL, measured the effects of skips and doubles on grain yields of individual corn plants grown at different target populations. As plant popunation increased from 18 000 to 30 000 plants/acre, grain yield per plant of two corn hybrids decreased from 8.40 to 6.32 oz/plant, while yield increased from 169 to 211 bu/acre. The net effect of doubles was an increase in yield at all populations. Yield of each plant in a double was 10 to 17% less than that of plants spaced uniformly, but together the two plants in the double yielded 81% more than the controls at the lowest population, and 67% more at the highest population. Plants next to doubles yielded from 2% less than the controls at the lowest population to 12% less at the highest population. The two plants on either side of a skip compensated for only 47% and 19% of the yield lost due to the missing plant at the lowest and highest populations, respectively, so skips always decreased yield. Skips and doubles affected grain yields almost entirely through their effects on plant population. While both contribute to plant spacing variability, their effects on yield are in opposite directions. Yield decreases due to skips can be minimized by increasing plant population. Research QuestionTwo-plant hills (doubles) and missing plants (skips) in corn contribute to uneven spacing, and may affect yield. Plant population affects the amount of variability in plant spacing as measured by standard deviation, and probably affects the extent to which skips and doubles influence yield. This study measured the effects of skips and doubles on yield of two corn hybrids grown at different plant populations. Literature SummaryTwo studies have resulted in predictions that corn yield might be reduced by about 2.5 to 3.4 bdacre for each 1-in. increase in the standard deviation of plant spacing. At least one other study showed much less yield loss when corn rows had gaps, as long as there were compensating higher populations in the rest of the row. Study DescriptionThe corn hybrids Pioneer Brand 3343 and Pioneer Brand 3379 were planted late April to early May on a Muscatine si1 soil at Monmouth, IL, and on a Drummer sic1 soil at Urbana, IL, in 1991 and 1992. High levels of fertility were maintained throughout the study. Target plant populations ranging from 18 000 to 30 000 plants/acre were established by hand-planting, with skips and doubles included within each population. Ears from uniformly-spaced plants (controls), doubles, plants next to doubles, and plants next to skips were harvested separately by hand, with yields calculated for individual plants and for averages of five hills with skips or doubles in the middle. Applied QuestionsDid skips and doubles affect yield? Plants next to skips yielded more than the controls, while plants next to doubles yielded less than controls. Averaged across hybrids and environments, five-hill groups of plants containing a skip yielded 11% less than controls at 18 000 plants/acre and 16% less at 30 000 plants/acre. Doubles, however, always yielded more than controls, from 15% more at the lowest plant population to 8% more at 30 000 plants/acre. Yield loss predicted in populations with 10% skips was 8.9 bu/acre at 18 000 plants/acre and 17.1 bu/acre at 30 000 plants/acre compared with yields of the same populations of uniformly spaced plants (Fig.1). With 10% doubles, predicted yield was 12.9 bdacre higher in populations of 18 000 planWacre and 10.8 bu/acre higher at 30 000 plantdacre than yields of uniformly spaced plants at the same populations. Did hybrids differ in their response to skips and doubles? The two hybrids in this study showed no difference in response to plant population or to skips or doubles. We do not consider it likely that responses would vary greatly with hybrid. Can we use plant spacing variability to predict yield loss? Skips contribute to standard deviation slightly more than doubles, but both increase plant spacing variability. Skips decrease yield in fields where the intended population is at or below the optimum, while doubles increase yield if (as in this study) the intended population is below the optimum. Thus, standard deviation alone may not always adequately predict yield losses. While it is important to maintain planters to minimize variability in plant spacing, skips in the row should cause more concern than doubles. With 5 to more than 15% of the seeds failing to establish plants in the field, however, we can never get rid of skips completely. We may, however, by increasing seeding rates if plant populations are not adequate for optimum yields, partly overcome the effects of skips. Fig. 1Grain yield responses to target plant population for plants grown at uniform spacings or with 10% of the hills consisting of skips or doubles. Data are averages over two hybrids and four environments.