Emerson Nafziger Professor of Crop Production Extension Phone: (217) 333-4424 E-mail: ednaf@illinois.edu

Corn plant populations in Illinois have increased over the past decade or more, up about 10 percent from the early 1990s to a harvest population of close to 26,000 in 2000. While some producers are still establishing stands that are lower than optimum levels, others have raised populations above 30,000, raising questions about whether some plant populations are too high.

We know that the "optimum" plant population varies considerably from year to year, with more plants per acre needed to take advantage of more favorable growing conditions. Optimum plant populations for a given field are best estimated by running extensive sets of plant population response trials with different hybrids over a wide range of years and locations with similar productive potential, then averaging results over all site-years in order to produce a response curve. This curve is then used to calculate an optimum economic population, where the last investment in extra seed is just paid for by the yield increase from that seed. We can also use response data to look at the penalty for having plant population too high. With the increases in plant population, this question has become an important one. If yields only level off at above-optimum populations, then all we have lost is the cost of the additional seed. If yields decline at populations above the optimum, then we lose both the cost of the seed and the bushels of lost yield. In the latter case, planting high populations represents a real risk, especially when the weather turns out to be unfavorable.

Work in Illinois during the late 1980s indicated that optimum plant populations are in the upper 20,000s, but we generally did not have enough data to assess how "dangerous" it might be to have plant populations above 30,000. Data collected during the 1990s showed that raising the plant population above the optimum level usually either increased yield (often in better weather), or, in more average weather, produced no additional yield increase. About 10 to 15 percent of the time, raising plant population above the optimum resulted in yield decreases.

Pioneer Seed Co. recently provided a large amount of data from plant population studies that they have conducted in the Corn Belt with a large number of their hybrids. Averaged over more than 50 locations with some 20 to 30 hybrids per location over 1999 and 2000, their studies produced the response to population shown in. As shown, the optimum population (based on a seed cost of $1.60 per thousand seeds and a corn price of $2.00 per bushel, or a ratio of 1.60/2.00 = 0.8) is about 29,700 harvested plants per acre.

One of the questions of enduring interest is whether or not optimum plant population changes with yield level. Breaking the Pioneer data from 56 site-years into four groups of equal size and different yield levels showed that optimum population increased slightly as yield level increased. The optimum population for the highest-yielding group of environments was less than 2,000 plants per acre-more than the optimum population for the lowest-yielding group of environments, however. This modest return suggests that the cost of getting the information needed to fine-tune population according to yield level may exceed the value of being able to predict and apply optimum plant populations by expected yield level, either within a field or even among fields.

The large amount of data in the Pioneer dataset also allows us to look again at the risk of yield loss when plant population is higher than the overall optimum. Of course, good weather conditions and productive soils often result in yield increases between, say, 30,000 and 35,000 plants per acre. Averaged over the 20 to 30 hybrids per location and 56 locations in 1999 and 2000, the average yield change from raising population from 30,000 (close to the overall optimum) to 36,000 was 2.1 bu/acre, but yield decreased with this increase in population at about one-fourth of the locations. In 1999, the better of the two years, yields were more likely to increase with population, and we have statistical confidence that yield increased on average as the population increased. In 2000, more of the locations showed yield decreases as population increased, and we can be less confident that the increase in population paid its cost.

We are also able to look at individual hybrid responses to population in order to see whether or not different hybrids should have different target populations. The responses of 12 hybrids to plant population in the Pioneer trials are shown in. Note that two or three of these showed yield decreases as population was increased above 30,000. The optimum populations for individual hybrids ranged from about 27,000 to just over 32,000. Six hybrids had optimum populations above the overall optimum of 29,700, and the yield penalty from using the overall optimum for these hybrids averaged just under one bushel. The other six hybrids had optimum populations less than the overall optimum, and using the overall optimum for these produced an extra one-third of a bushel of yield. In other words, trying to fine-tune population by hybrid doesn't offer much return over using an optimum population based on averages over hybrids.

Soybean

We have less data on soybean plant population responses than we do on those for corn, but results from a study that we initiated in 2001 confirm some earlier findings on the response to dropped seeding rates. We used seeding rates of 75, 125, 175, and 225 thousand live seeds per acre, with and without seed treatment, planted in 30-inch rows on four different dates at three Illinois locations. This study was partially funded by Pioneer (A Dupont Company.)

shows the yield response to dropped population for the different planting dates and seed treatments at Monmouth. We found that seed treatments did not consistently increase percent emergence at any of the locations. At both Monmouth and Urbana, the earliest planting date was in the first week of April, and plants were emerging when temperatures dropped below freezing on April 18. At Monmouth, about 68 percent of the seeds established plants on the first planting date, while about 75 percent established plants on the later dates. This loss of plants at the earliest date contributed to the response to seeding rate at the earliest planting date. Averaged over seed treatments and planting dates, the optimum seeding rate at Monmouth was just under 175,000 live seeds per acre, which on average produced about 130,000 plants per acre.

The response of yield to seeding rate at Urbana was greater than that at Monmouth, and was somewhat inconsistent among planting dates. Frost damage was worse than at Monmouth, and the lowest seeding rate on the first planting date produced only some 30,000 plants per acre. Averaged over planting dates, the optimum seeding rate at Urbana was about 125,000 seeds per acre, which at the later planting dates resulted in about 110,000 plants per acre. The interaction between planting date and seeding rate was large, though, and we will need more data from additional trials before we can draw recommendations from this work.

At the Dixon Springs location, where planting took place from mid-April to late June, there was less response to plant population than at the northern locations. Unlike the other locations, planting early at Dixon Springs did not decrease yield, though "early" at this location was a week or two later than at Monmouth and Urbana. It does appear that higher seeding rates might be useful when planting is very late.

Overall, we found that soybeans tend to produce consistent yields over a fairly broad range of plant populations, from 100 to 120 thousand to 200 thousand plants per acre. Percentage of seeds that produce plants varied considerably with planting date and location, however. These results suggest that relatively high seeding rates may help to provide some insurance against stand loss, and that they do not, in most cases, tend to cause yield decreases. Under most conditions, dropping about 175,000 germinable seeds per acre appears to be a sound strategy for soybean.

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