It's all about location.
According to Dr. Joe Lauer, University of Wisconsin-Madison corn agronomist, the optimal date to plant corn varies with the field's latitude.
In documents released May 1 and 2, Lauer reviewed research data showing the optimum corn planting dates for Wisconsin are often later "up north" than in the south and that yield loss accelerates more quickly. That means a shorter planting window.
Growers in the northern corn belt deal with cool, wet soils in spring that delay planting and early frosts in the fall that kill plants prematurely. The combination limits heat units during the growing season.
From 1991-94, Lauer headed a statewide corn planting dates experiment. Full and shorter season corn hybrids were planted at six locations on five-eight planting dates between April 19-June 22.
For the northern sites of Ashland, Spooner and Marshfield, the planting date for maximum corn yield averaged May 12. For the southern sites of Hancock, Arlington and Lancaster, the optimum yield date was one week earlier averaging May 5.
Lauer said corn yields were still at 95 percent of maximum yield on May 17 in the north and May 12 in the south.
By June 1, corn yield was decreasing at an average rate of 2.3 bushels per acre per day in the north. The rate slowed moving south to Lancaster.
The likelihood that the farm, hybrid, field, tillage system and other management factors will affect the optimum date to plant corn also has affects northern Wisconsin growers.
"These factors, along with latitude, result in lower yield potential in northern Wisconsin", Lauer said. "When combined with grain moisture and drying cost, they often make corn production marginal as we move north."
As the weather continues to delay planting dates, the likelihood grows that fields intended for grain will be harvested for silage, Lauer noted, especially if the year remains cool. That prompts the question of how planting date affects corn silage yield and quality.
The short answer is starch content decreases with later planting dates.
Lauer used data collected over the last 10 years on the grain response to full season hybrids to planting date to illustrate the issue. Eight-row plots were established at Arlington from 2003 through 2012, with four rows harvested for silage and four rows harvested later for grain.
The day when maximum forage yields occurs is April 24, Lauer said, nearly four days earlier than the date of maximum grain yield of April 28 for those same plots.
"The relationship is more 'broad shouldered' than what is measured for grain," he summarized. "In other words, planting date window is longer than it is for grain with forage yields still within 95 percent of maximum yield on May 15."
Lauer explained that the size of the plant "factory" is not affected by planting date. "The number of leaves, the size of the stalk, shank and husk are largely genetically controlled," he pointed out.
However, the planting date does affect starch content so the digestibility of the stover and grain pools is different. The digestibility of stover (ivNDFD) is close to a flat line across the range of planting dates tested, Lauer observed.
"When yield and quality are combined using the Milk2006 performance index, we find that milk per ton, or quality, is not affected as much as 'Milk per Acre' due to the forage yield impact," Lauer said.
When measured using 'Milk per Acre', the optimum planting dates for corn silage is the same as it is for grain yield, Lauer said. "The difference is that the planting date window is slightly longer for silage than it is for grain", he concluded.