As Bill points out -- don't get confused with unique situations.

The more and more I learn about haying from reading posts from producers around North America here, the more I realize that hay making is very highly regional and that nice, tidy academic studies have not a whole lot to do with the most important job of a hay producer: getting green, good hay into a bale. And to get hay into a bale, you have lots of factors to consider; drying the hay down is only one of these.

For example, here, we rarely get enough rain to soak a windrow to the bottom. But starting in late June, we get the monsoonal moisture coming up out of Baja, which sometimes can make it clear up to Mike's area. This can inject enough mid and high-level moisture to create spotty thunderstorms that come over and might drop only 0.05" of rain. These storms are so tight that often only two farms will get rained on, but everyone else will be watching the rain from a distance. I've actually called neighbors to tell them that there were storms headed for their specific farm. That's how tightly isolated some of these monsoon t-storms can be in Nevada. But that little bit of rain will get only the top inch or so of a high windrow wet, not through to the bottom -- IF you had your hay in a windrow instead of a swath. It is just enough rain to screw things up. As others point out, when the top layer gets rained on and dries out, it bleaches. The rule of thumb I've learned is that green sells better than brown hay. Always. Even if I have a stack of rained-on hay that makes California dairy test, the green stuff that makes the same test will sell first. Always. And in hay that doens't make test, or is targeted for the retail market, green is paramount. Tan hay here too frequently ends up going to ranch cattle, and you don't get much for it.

But you guys back east -- and I grew up back east -- you don't get little tinkle-showers out of a thunderstorm. You get stuff like soaking rain, hail, tornados, etc. You need that hay to dry down ASAP to get out from underneath the possibility of getting caught in one of those situations - and you don't always have the optimal drying conditions we have in the west. Already in June, we've had days with relative humidity as low as 5%, 95F temps and 20 MPH winds. It doesn't come much better than that for hay dry-down. I could just about lay my hay down in a big wadded-up ball and see it dry in those conditions. 

The other thing that is regional here is our wind issue. You can't bale hay real easy out of a fence. I noticed when laying hay down in wide swathes that our southwest winds that blow very regularly all summer long can get a good laminar flow going across the field and move a bunch of hay. High windrows tend to prevent that laminar flow from getting set up as the wind crosses a field, and the result is that high windrows don't move. Counter-intuitive, but there it is. The net:net result is that wide, flat swaths get torn up and moved downwind by 30+ MPH winds and I've now seen high windrows withstand 40 MPH winds with no movement. I learned what was going on in windstorms by grabbing a beer (to instigate the thought process, mind you), going down the field in a pickup and stirring up a little bit of dust upwind from the hay field and watching what the wind was doing as it whipped across the field, rather like putting something in a wind tunnel and putting smoke on it. You can see that high windrows break up that smooth flow of wind across the field, thereby preventing the hay from moving so easily. That works here, where this farm is, which is at the south end of the valley, close to some mountains no more than eight miles southwest of here.

Now, further out in Diamond Valley (about 10 miles northeast of where we are), the character of the wind changes and it doesn't seem to matter much whether the hay is in windrows or swaths -- it ends up in fencelines at 40 MPH.

The type and width of your conditioner matters too. I can cut hay side-by-side with a NH 1116 windrower with rubber-on-rubber and a Hesston 1360 mo-co, with steel-on-steel. The 1360 has a wider conditioner and we have it set very aggressively. It can dry down a full day faster than the NH 1116 rubber-on-rubber. But the 1360 takes gobs of horsepower -- if you don't have the horsepower (say, only about 130 HP), you get lumpy windrows. If you feed the 1360 great greasy gobs of horsepower (like 160 to 180HP), the hay comes flying out the back of that machine and lands in a nice, poofy, stacked-up windrow - and oh, does it dry down nicely. Hesston likes to say that you should use at least 125 HP in that machine. I would recommend no less than 140 HP, from my experience in my hay. In the midwest or back east, well, 125 HP might work just fine. I have no idea how that machine works in other hay areas.

As you like to say Bill, there ain't no absolutes and it is a matter of "what works here." I recommend folks experiment in a structured way, side-by-side, to determine what works for them in their situation.