Glyphosate's mode of action is to inhibit an enzyme involved in the synthesis of the aromatic amino acids tyrosine, tryptophan and phenylalanine. It is absorbed through foliage and translocated to growing points. Because of this mode of action, it is only effective on actively growing plants; it is not effective as a pre-emergence herbicide.
Glyphosate kills plants by interfering with the synthesis of the aromatic amino acids phenylalanine, tyrosine and tryptophan. It does this by inhibiting the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), which catalyzes the reaction of shikimate-3-phosphate (S3P) and phosphoenolpyruvate to form 5-enolpyruvyl-shikimate-3-phosphate (ESP).
ESP is subsequently dephosphorylated to chorismate, an essential precursor for the amino acids mentioned above. These amino acids are used in protein synthesis and to produce secondary metabolites such as folates, ubiquinones and naphthoquinone.
if it were that persistent in the root zone there would be no need of a new application on each new years crop,
A couple years back I went to a hazelnut field day (badgersett research farm) and was surprised to hear the owner say that the system they are developing isn't for marginal land which too often is the default choice for any alternative to agriculture. They want their system (which would be totally compatible to permaculture by the way) not to be just another 3rd crop or side project but to compete with and replace an annual crop-tillage-pesticide regime.
One more idea I'm going to suggest may get me kicked off this forum but here goes: Pick out half or a third of this field and for at least the next several years keep it corn or beans or whatever is grown where you are. Make sure that it's some of the best land for this and section it off in a way that fits whatever machinery is being used. Enlist the help of a local organic or at least more ecologically knowledgeable farmer to cut back on the pesticides, grow conventionally bred crops instead of GMO, and use the least harmful fertilizers.