Plenty of data points in the Long term trend.  

Irwin on farmdoc has broken down significant parameters in some models that affect/estimate National yield and summer weather is big, as I recall.  Temps and rainfall.  Planting date relatively small.

And why wouldn’t National yield keep increasing.  Predictive genomics bring better yield potential, traits are ubiquitous to keep yield potential, tile and fertility expense has been huge,  super planters put the crop in fast, sprayers, chemistry, biologicals, and micronutrient packs are everywhere.