Ride along in the combine cab pictured here. You're driving through flat, level, productive soil. There is just one problem. Even though this field is well-tiled, there is a spot ahead of you that is so low that it can't be drained well. There simply isn't a good outlet to send the water. It's what is known as a wet swag.
You can recognize it here running across the field because it is lighter in color. If you were actually in the cab as the combine moved through that spot, you would see that the stalks are two feet tall, and the ears, while present, aren't much more than nubbins. The lighter color in the landscape as you approach this low swag is due to the fact that it contains a fair amount of dead foxtail instead of just corn plants.
So what, you say? The swag appears to be very narrow. How much could that hurt corn Yields? The maps haven't been analyzed yet to get down to actual numbers, but a simple answer is possible. The wet swag hurts more than you might think.
Actually, it's not the swag itself but the sections leading up to and away from the swag that caused the damage. Yield falls off long before you reach the heart of the swag and continues for many feet after it. That's taking into account the delay in the yield monitor from actual point of harvest to when the results show on the screen.
The explanation is simple enough. Soils not quite as wet as the swag were still wetter than desired. Corn didn't grow as well there as it did where drainage was better higher up on the landscape. The take-away message is that while a wet hole or swag may not look that big, it's the damage that is harder to see in the areas around it that helps cut yields. Yield in those areas is much better than the swag, but not nearly as good as they could be in a normal year.