Corn Development and Variable Stands

June 4, 1999
Much of the corn crop is off to a good start. Corn planted the first week of May in central Illinois is now in the V5 to V6 stage (five to six leaf collars visible) and is just beginning the rapid growth phase. At about V6, the growing point (the tip of the stem) is pushed above the soil level as the stem of the plant begins to grow. This is also the time that the tassel is initiated at the growing point, marking the end of leaf initiation, and thus fixing the number of leaves and of nodes on the stem, usually at about 18 to 20. Rapid upright growth will occur during most of June as the stem elongates, pushing leaves up and out of the whorl. During this time, new leaf collars will appear about every 65 growing degree-days, or about every 3 to 4 days at normal temperatures. Vegetative development culminates with emergence of the tassel and is followed shortly by pollen shed and silking. If temperatures are about average in June, we would project silking for mid-season hybrids in central Illinois to begin by the end of the first week or the beginning of the second week in July. This would be several days ahead of normal, as a result of warm weather so far in the season.

Though most fields have good stands, we have had a few reports of uneven distribution of plants in the row due to mechanical problems with planters. One agronomist sent some plant-spacing data from such fields, from
which I calculated standard deviation of plant spacing (SDPS) values in the range of 4 to 5 inches, with plant populations of 27,000 to 28,000. Normal SDPS values are in the range of 2 to 3. SDPS values above 4 are unusual, especially with high plant populations, which tend inherently to have lower SDPS values than do low plant populations.

Dr. Bob Nielsen of Purdue did some initial work on plant-spacing variability in which he showed that each 1-inch increase in SDPS decreased yield about 2.5 bushels per acre. Work that we did later showed that the nature of the variability may affect yield loss; for example, skips and doubles affect SDPS similarly, but skips usually decrease yield and doubles may increase yield by increasing plant population. We also did some work with planting speed and found that yields did not suffer even when faster planting increased SDPS, unless plant population was decreased as well. In general, we concluded that if plant populations are adequate and plant spacing variability is not due to large skips in the row followed by clusters of plants, then we expect little effect on yield.

We rarely encountered SDPS values above 4 inches in our work in farm fields, so we do not have a good way to predict if or how yield might be affected in such cases. I would expect some yield loss in such fields, but the only way to know the extent of such loss might be to find portions of rows with uniform spacing and compare yields from those portions with yields from more variable sections of rows found nearby. At least 100 ears from each type of row section would be needed to obtain such a measurement, and even then it may not be very precise.--Emerson D. Nafziger

Author: Emerson Nafziger