We have cooler soils than most anyone one here, and grow cereals, pulses, oilseeds. I have never observed anything like what is being described, and have never heard anyone mention it before. We often are planting into soils under 5 degrees C, because we have no choice. No one waits for a specific soil temp, because we simply have no time to waste. 

IMO, maybe it is normal alleopathic effects of certain weeds? Because several weeds do have an alleopathic effect, and upon dying off, release chemicals that can harm crops.

But I have taken over some pretty darn neglected, weed infested land, sprayed with a couple litres of glyph. to clean it up in the spring, and had excellent stands and crops.