Maybe someone closer to science can correct me on my calculations
After some research it appears that the 200 and 700 should actually be 2000ppm and 7000ppm and it should be under 1000ppm or 0.1%DM before cutting.
http://www.yara.ie/news/94536/nitrates-and-silage-fermentation/
One hectare of grass has 5000kg DM, from 25000kg fresh weight.
https://www.agriland.ie/farming-news/fertilising-rules-for-this-years-grass-silage/ the article mentions 10tonne per acre or 25000kg per hectare.
Thus 0.1% is 5kg of Nitrogen, thus we conclude I have 10kg of N in Field 1 and 35kg of N in Field 2.
Therefore Field 1 has to dissipate via growth 5kg of Nitrogen and field 2 has to dissipate 30kg of Nitrogen.
https://www.agriland.ie/farming-news/fertilising-rules-for-this-years-grass-silage/ This article says growing grass consumes 2.5kg/Ha per day, thus we conclude the Nitrates in Field 1 should be at the 0.1% threshold in 2 days (5/2.5) and in Field 2 in 12days (30/2.5).
Anyone able to confirm my understanding ?