The exact same thing you have just pointed out was brought up at discussion group meeting this week. Alongside this one which was also quite interestingcan't claim credit for thinking of this myself I saw it on a webinar slide but thought it was such a blindingly obvious push to sell more I had to go and find the source and copy it for here. yara (fertiliser manufacturers and sales) reckon your optimum rate of nitrogen is 100kg per ha as in those black lines. But what they really mean is that's when the line stops going up it starts to plateau way before then at roughly where my red arrow is around the 50kg mark. Do they think farmers are stupid
The webinar hosts did a much better more professional job than I did and had other graphs showing how much the extra dry matter would cost and they came up with 40kg of nitrogen being the optimum for growth/cost of nitrogen.