okay, I have heard that wheat will lose test weight when it gets rained on after being ripe, and I do not dispute that.  I'm also thinking about the explanation above of test weight being a reflection of how kernals will pack into a certain area. So, here's my question:  why does wheat lose test weight when it gets rained on?  is it losing mass?  or it a reflection that those kernals don't pack as well, so the same number of kernals will fill a larger volume - in essense, it takes less kernals to fill that "1-unit volume called a bushel" ? 

Okay, I just read PaKettle's post above, and his post confirms that the second case is what is happening - the wheat does not lose weight.  It just does not fill the volume as tightly.

P.S.  I would still disagree with you that "test weight effects yield".  (I think you really meant "affects", not "effects".)   Enviromental conditions affect yield; weather affects yield; genetics affect yield.   Test weight is a qualitative measure, an end result of yield.  In your example, it was the environmental conditions that affected your yield, not the test weight.  Test weight was simply a qualitative measure that showed it happened.