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Issue No. 1, Article 12/March 23, 2007

Should We Increase Corn Plant Populations?

Some recent articles in the farm press have suggested that raising corn plant populations will pay off in many fields. In some cases, suggestions to increase populations have been accompanied by the suggestion that higher plant populations in turn need higher nitrogen rates, or perhaps that using more N increases the optimum plant population. Our research has not shown much interaction between N rate and plant population. Instead, we think that decisions on both need to be reached independently of one another.

We have been conducting studies of planting dates and plant populations for the past two years at five or six locations in Illinois, with planting dates running from early April through late May and final plant populations ranging from 20,000 to 40,000 plants per acre. Planting date and population responses are complex, and they are very much affected by weather, making solid conclusions difficult. This means that we need to continue such work for a few more years. In the meantime I can share the following:

  • When seasonal growing conditions are favorable in areas with productive soils, 35,000 plants per acre is a reasonable harvest population. Averaged over four high-yield locations in 2006, 35,000 plants yielded 2 to 11 bushels more than 30,000 plants, with an average yield increase of 6.5 bushels. Using a corn price of $4 and a seed cost of $2 per thousand, a population of 35,000 plants was more profitable than 30,000 plants. Raising the population from 35,000 to 40,000 meant lower yields at two of these four locations, but the average was a 2-bushel yield increase. The additional cost of seed exceeded the gross return, meaning that 40,000 plants provided less profit than did 35,000 plants.
  • In locations with less-productive soils or with a fairly dry growing season, the best population has generally been between 25,000 and 30,000, with less overall response to plant population. In extremely dry weather, such as what we had at Brownstown in 2005, the highest yield (100 bushels per acre) was at the lowest population (20,000), and yield decreased as population increased, to about 60 bushels at 40,000 plants. At the same location in 2006, where it was again quite dry, there wasn't much response to population between 20,000 and 30,000 plants, but yields dropped again at higher populations. These were similar to responses we saw in 2005 at Urbana, at similar yield levels (125 to 130 bushels per acre). These results confirm that today's hybrids don't crash and burn when populations are higher than the water supply can support. Rather, they are able to maintain yields up to populations in the range of 25,000 to 30,000 plants per acre. This doesn't provide much return to seed costs within that range, but at least the crop is better protected from large yield losses. From a risk management standpoint, it makes more sense to have higher populations to take advantage of good weather than lower populations to protect against unfavorable weather.
  • While 35,000 plants might be reasonable if we're fairly sure that yields will be high, a target population of about 30,000 plants is still reasonable for many producers. Yield increases usually taper off as population rises, so going from 25,000 to 30,000 increases yield more than increasing from 30,000 to 35,000. So if we include the possibility of less-than-ideal conditions, under which yields stay level or even decrease as population increases above 30,000, the yield penalty for having 30,000 instead of 32,000 or 35,000 plants is not very great. By the same token, our recent results are showing that some producers who target populations less than 30,000 might want to consider raising their targets modestly.
  • While we have not done very many direct comparisons between corn following soybean and corn following corn, we do not think that raising population will benefit corn following corn more than it does corn following soybean. In one ongoing study, raising the population from 32,000 to 40,000 in continuous corn has often decreased yield, and it seldom increased it. Because we think that corn following corn tends to be in a more stressful environment than corn following soybean, it is reasonable to expect slightly less response to increasing plant population in corn following corn. With a slightly lower expected stand establishment for corn following corn, it makes sense for most producers to leave the planter setting the same for both corn following corn and corn following soybean.
  • Planting date responses are notoriously variable depending on seasonal weather, but with a few exceptions, planting from mid-April through early May has produced the highest yields. In 2006, highest yields were produced by the earliest planting date in four of six locations. At the other two locations (Urbana and Monmouth), the second and third planting dates yielded more than the first date. This difference was 12 bushels at Monmouth, where the first date was April 5. At Urbana, however, the March 30 planting yielded 40 bushels less than the April 24 planting. We think it's likely that the first planting at this location was large enough to be damaged in some way by the cool, wet conditions in mid-May, while later plantings did not have plants large enough to be similarly damaged. Excluding that instance, results over the past two years indicate that planting in the last half of April is on average the best time to plant. When planting is delayed past late April, yield responses have ranged from slight increases to about 1 bushel less per day of delay. On average, planting into May costs about one-third to one-half bushel per day of delay, but the range is quite wide.
  • Should we change population depending on planting date? In most cases, the answer is no, at least if planting can be done by May 10 or so. Planting later than that has sometimes flattened the population response curve, meaning that population could be dialed back by a few thousand. However, it is safe to decide on a best population and try to plant to achieve that population, taking planting date into account only to the extent that soil conditions on a given date might affect how many seeds will establish healthy plants. Under good planting conditions, planting 5% more seeds than the desired population is reasonable, while using up to 10% extra seeds might be appropriate if planting into more stressful conditions.

--Emerson Nafziger

Author:
Emerson Nafziger

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