2001 Corn Planting Considerations
by Greg Stewart, OMAFRA Corn Specialist


Plant Populations: Have We Broken the 30,000 Ceiling?

Corn plant population has been an intensely studied aspect of corn production. The seed buyer has always been interested in maximizing yields while spending the least amount possible on seed. However, over the years the trend has been that higher yields, with modern hybrids, were generally produced with higher populations. In fact, the most significant change in modern hybrids has been their ability to cope with the stress of increased density.

Figure 1 outlines research done where hybrids from various decades were all grown to compare their yields under two extreme populations (4,000 and 32,000 plants per acre). Note how old and new hybrids yield almost identically when grown under low-density situations. You may question whether plant breeding has made any progress at all. But look at the massive yield response for the newer hybrids when they are grown at the high density. The trend is very clear. Breeding programs within seed companies realize that much of the potential yield gain has come and will yet come from developing hybrids that excel under higher densities. So while over the past decade we may have set 28-30,000 plants per acre as all you would ever need to maximize corn yield, it wonít be true in the future. In fact, some hybrids now on the market need over 30,000 plants per acre to maximize returns to the grower. Work with your seed representative to fine-tune hybrid management and planting density.

Varying Population with Yield Potential

When considering optimum plant densities, many growers get concerned that they donít have the yield potential to warrant bumping up seeding rates. This is a valid concern, particularly on drought-susceptible fields where water availability, not light interception, is almost always the yield-limiting factor. Some savings can be realized by adjusting populations downward in these types of situations. A recent research report published in the Agronomy Journal (D. Bullock et. al., 1998) indicated that the optimum final plant population in the Midwest Corn Belt on average is 27,160 plants per acre. These researchers also predicted, based on their work, that for every 15 bu/acre increase in a fieldís yield potential (or parts of fields), economically optimal populations increased by 450 plants per acre. So in Ontario, while we may generally shoot for higher average final stands than in the Midwest, you will also want to ensure that your productive fields are near the upper end of the plant population range for the hybrids you are planting.

Also note that in the shorter season areas of the province, where smaller stature hybrids are naturally grown, populations may be under even more pressure to remain high to maximize light interception and optimize yields. Donít let the fact that your yield goal is 105 bu/acre sway you into thinking that 24,000 plants per acre will do the job.
Table 1 Number of bushels per acre required to pay for a 4,000-seed/acre increase in planting rate for seed corn of various prices

Cost per bag of seed

Cost per 4000 seeds

Additional bushels per acre require to break even. (Assume $ 3.00/ bu)

$ 120

$ 6.00

2

$ 140

$ 7.00

2.3

$ 160

$ 8.00

2.7

$ 180

$ 9.00

3.0

Plant Population and Stalk Problems

Having higher plant populations in the early part of the growing season generally results in increased light interception and creates the potential for greater yields. The advantages afforded by high densities in the early going, however, need to be weighed against the additional stress that occurs in the post-silking period. Additional stress caused by high densities in this period can have a negative effect on stalk strength and standability. In looking at population and stalk lodging data from Pioneer this winter (Pioneer 2000 Northern Agronomy Research Summary), it seems that the Ďtake homeí message is fairly clear. Increased population does cause some increased stalk breakage. However, stalk strength and plant health were not so seriously impacted as to cause a reduction in yield with the higher populations, even though lodging increased. Hybrids with a good reputation for stalk strength generally handle the shift to higher populationswith more ease. Dropping populations to try and take stress off hybrids with lower stalk strength may reduce lodging, but cost you too much in yield. A better approach is to consult your seed company representative, plant for optimum yield, and monitor fields so that harvesting is done in a timely fashion to reduce losses in the case of lodging.

Economics

The economic return to planting more seed is a necessary consideration, especially in the face of tight margins. Table 1 outlines the costs involved and the additional bushels required to make it a break-even proposition. Adjusting planting rates on your most expensive hybrid so that final stands are 4,000 plants per acre higher may be a profitable venture, but you will need to net three bushels to break even. Adjusting populations upwards should be based more on evidence that the particular hybrid will respond to higher densities than on the cost of the seed.

Corn Planter Performance

All sorts of recommendations and targets in regard to final plant populations wonít be realized unless your planter will co-operate. Having a well-maintained planter is critical to achieving the big three of corn planting: 1) uniform emergence, 2) correct population, and 3) uniform spacing. (I am a bit bruised and confused in the discussion on the importance of uniform spacing, so I will place it as #3 and deal with it in an upcoming issue!) 2001 will be a year when seed size will be small due to last seasonís growing conditions. If you are going to hit your target populations, you will need a planter that can handle the smaller seed. Be sure finger tension is correct and brushes are in good shape. In air units, you may need to get a smaller seed disc and adjust your air pressure. Also, if you are using a seedbox treatment (see boxed item by Dr. Art Schaafsma), ensure that the residue doesnít gum up the planter works. Finally, make sure that your planter is dropping the correct amount of seed by checking it out before you plant.

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