Research Answers Planting Questions

April 2, 1999
Q: It's early April and the soil is fit to plant. Should I start planting?

A: Based on research in Illinois, yields of corn are generally highest when the crop is planted in the last week of April. Yields are close to the maximum if planting is the week before or the week after the last week of April. Yields declined, though, both when planting earlier than April 20 and when planting was later than May 7. Our work showed an average of 3 to 5 bushels per acre lower yields for corn planted in mid–April. Even with slightly lower yields with early planting, most farmers will start to plant in the first two weeks of April in order to finish on time. Delays in late April due to weather push planting later than optimum, and also delays soybean planting. Risk management, in other words, favors early planting. If soils are fit to plant, planting can probably start the first week of April in Southern and Central Illinois (and we have reports of some corn planted in 1999 before April 1). In the northern part of the state, expectations for lower soil temperature probably mean that planting should not start much before April 10.

Q: But shouldn't we wait until soil temperatures are at least 50 degrees before we plant?

A: When it's April, we expect soil temperatures to start to increase, so we don't normally pay a lot of attention to soil temperature if the calendar suggests starting and if the soil is in good shape for planting. Drier soils warm more quickly that wet soils, so by the time there has been enough drying weather to bring soils into good shape for planting, soil temperatures usually have at least started to rise. They can fall again, as we saw happen in 1997, but unless there is rainfall along with the lower temperatures, the seed and seedling are probably not badly injured by soil temperatures as low as the low 40s. The main concern would be corn seeds planted in soils that are both cold and wet. Planting into such conditions is never a good idea in April, and if such conditions occur after planting, emergence may be low.

Q: Besides "getting it done," is there any other advantage to early planting that would offset slightly lower yields?

A: One advantage that is often not given proper value is that if replanting is necessary, it can usually be done in a very timely manner when the first planting date is in mid–April or earlier. Especially if seed for replanting is provided without additional charge, returns to replanting can be increased by planting early the first time. Although we don't have data as to how often early–planted corn needs to be replanted compared with later–planted corn, replanting early is much preferred to replanting late.

Q: Should I operate the planter at lower speeds than I'd like in order to produce more uniform plant spacing?

A: It is certainly possible to plant too fast for some planters in some conditions, but our evidence would suggest that most farmers probably aren't planting too fast. A study we conducted recently involved 11 field trials in which farmers operated their planters at 3, 5, and 7 miles per hour in replicated strips. Averaged over the 11 trials, driving faster increased the variability of spacing between plants from a standard deviation of 2.87 inches at 3 mph to 3.22 inches at 7 mph. That's not a large increase--it would not be noticeable by looking at the rows--but in a few fields, especially when older planters were used, the planting got pretty sloppy at the faster speed. There was, however, no effect of planting speed on plant population or on yield, which was 152.5 bushels per acre at 3 mph and 153.1 bushels per acre at 7 mph. Although we don't have data yet, it may well be that effects of planting speed on uniformity of seed depth might be at least as important as on uniformity of spacing between plants. While it is difficult to do, digging up 20 or more seeds going down a row is the only way to tell whether your planter is placing seed at reasonably uniform depth.

Q: Is there a best depth for planting?

A: In a study we conducted in 1998, corn planted 1.5 inches deep emerged at 97 percent in about 12 days and yielded 181 bushels per acre, whereas seed planted 3 inches deep emerged at 90 percent, took two days longer to emerge, and yielded 18 bushels per acre less than that planted 1.5 inches deep. The lower yields were probably due both to the lower plant population and to the delay in emergence, which delayed pollination by a day or so, perhaps exposing the pollination process to slightly drier weather. We think that 1.5 inches is an ideal depth regardless of other factors, such as planting date and soil moisture. With depth somewhat variable, it may be necessary to set the planter slightly deeper to prevent placement of seeds very near the soil surface, where establishment of the nodal root system may suffer. Just setting the planter "by the book" is not enough: planting depth should be checked as you start to plant each filed, and ideally checked again as soil stypes or conditions change within the field.

A final note: Even if it turns wet and planting corn cannot be completed until mid–May or even late May, very good yields are still possible: in 1998, corn planted in mid– to late May produced some very high yields in Illinois, especially in the northern part of the state (DeKalb County, where little planting took place before mid–May, produced 172 bushels per acre). Statewide, it took until about May 15 to reach 50 percent planted, and the final yield was 141 bushel per acre. On the other hand, record early planting in 1997 (77 percent of the crop was planted by May 4) produced a statewide average yield of only 129 bushels per acre. While planting on time is one thing that we (rightly) spend a lot of money and effort on, the weather during the growing season is still the main thing that determines yield.--EN
Author: Emerson Nafziger