I found this article, that was written for Iowa producers to be interesting.

How Late Can Soybeans be Planted?

https://crops.extension.iastate.edu/cropnews/2008/06/how-late-can-soybeans-be-planted

Here are some of the key points:

"The yield benefit we see from early planting is a result of increased seasonal canopy photosynthesis, greater number of main-stem nodes, potential for earlier flowering, increased crop growth rate during pod set, and greater seed filling rate. Late-planted soybean goes through the vegetative growth stages much faster than early-planted soybean. The reason is that in addition to temperature, soybean development is influenced by day length.

When soybean planting is delayed, vegetative growth is reduced since flowering can start as soon as the plants have one or two nodes. Thus, soybean planted later does not develop the same canopy biomass as soybean of the same variety planted earlier. Late-planted soybeans are therefore shorter. "

metmike:

The angle of the sun in the sky will peak in 2 days, when we have the Summer Solstice,  along with the power of the sun, which plants use to produce sugars and energy. Corn and beans planted in May have been using this powerful sunshine from the high angled sun to develop and increase vegetative growth(which in turn is able to absorb more sunshine). This, along with establishing deep roots, enables the plants to have more access to minerals and nutrients from the soil. 

A plant that is just emerging now or, even worse, in early July has already missed out on 45 of the best days of powerful sunshine in the sky before the Summer Solstice(from early May to June 21st).

Those tiny plants in the next couple of weeks are not as efficient at absorbing the sun as would be plants that are fully developed because they were planted 6 weeks ago. 

As they do develop, during July with the sun angle already well passed its peak, the sun will not be able to contribute as much to photosynthesis as it would have, if that same plant was in that same stage............but a month earlier when the sun was higher in the sky and still climbing(getting more powerful). 

The best/strongest sun is 30 days before and after June 21, so May 21-July 21.

What surprises some is that the sun's strength is the same on April 21 as it is on August 21, each being 60 days away from the peak on June 21. So a late crop that has not matured yet on August 21 is using the equivalent of the sun in April to generate sugars/energy. 

Corn needs growing degree days and heat units to mature but all things being equal, the higher sun angle of the earlier planted crop, even with perfect weather will add many bushels.

Beans will actually take its cue from the sun/length of day. So they won't wait until fully developed like an early May planting, they can start flowering in August, regardless of how far developed/late the plant is, as long as the plants have a couple of nodes. This is the recipe for low yields. You want the biggest plant with the most vegetative growth possible to soak up more sunshine with the highest angled sun left before the plant goes into the reproductive phase.