This water filled rut is at the top of the field, normally the driest part. I think that it has been there since the crop was sown in autumn, when the ground was already saturated. I presume that the variety of seed selected by the farmer did not allow him to wait until spring to sow the rape that is growing in this field, and the result is a lot of deep ruts from the tractor.
I have had my full share of mud today on a long walk with my daughter and her family, was secretly pleased to note the mud on my walks isnt as bad as hers!