To say that the answer to this question requires on-site judgment is an understatement. In 1994, many producers were able to plant corn into a very good seedbed in mid-April, while in 1993, late-May plantings were mudded in. In the ideal spring of 1994, two-thirds of Iowa corn was planted in April a complete reversal from 1993. Because there is no way to know what the 1995 spring planting season will be like, you must plan for a normal spring if there is such a thing.
You can always find individual year and site exceptions, but long-term Central Corn Belt studies indicate yields are best for plantings made during the April 20 to May 5 period. Yield loss accelerates after May 10 to 15. But should you start before April 20 or 25 if conditions permit?
If I had to select the best date for a typical Iowa corn field, I would select the last week of April. But what if it rains then, or if you need two weeks of planting days? On average, yields for plantings made a week or so earlier or later than the last week of April should not differ greatly, if soil conditions are desirable. The main issue is soil moisture and the resulting seedbed. Is it too wet to have equipment in the field? This determination is especially important for no-tillers, because a dry soil surface may mask a too wet and cold seed placement zone.
Once it is late April, and certainly by May, Im not a fan of using soil temperature as a planting guide. If it isnt warm enough, it soon will be, and you cant pass up good planting days in late April or early May. In early to mid-April, consider soil temperature, the weather forecast for the next few days, and even the 6 to 10 day outlook, in addition to soil conditions. Remember, if you are forced to replant, it will cost more and may yield less.
But what if soil moisture and temperature conditions are ideal? In this case, consider the number of planting days you need. If your entire acreage can be planted in two or three days, why not wait until the last week of April? If it takes five to seven days, it may make sense to start around April 20. In a normal spring, you can be in the field about half of the days. If you need even more planting days and if soil conditions permit, you may choose to start as early as April 15 to 20 in northern Iowa and April 10 to 15 in southern Iowa. Very early planting often works, but the risk of uneven stands with no yield advantage seems of little advantage, except for those who may need about two weeks of planting days. Very early planting may be advantageous in years when soils are too wet to plant during the late Aprilearly May period.
No matter how early you plant, I suggest starting on well-drained upland soils where the prior crop was soybeans. Regardless of planting date, your goal is to establish an even stand of the intended plant population. This requires skill and some good luck. Plant full-season hybrids first so they can express their yield potential, and you wont need to be concerned about hybrid maturity if serious planting delays develop. Ultimately, your planting completion date and the quality of stands you produce are of greatest importance.