Interpreting grass sugar and nitrate levels for silage

There is not a defined relationship between grass sugars and nitrate. In fact, any more than really high nitrate concentrations can be synonymous with grass that will be difficult to preserve, there is German data showing that negligible nitrate concentrations can also be problematic for preservation (low/moderate concentrations of nitrate can be inhibitory to Clostridia). 

In most cases once grass sugars are >3% of the juice (30 g/L) then preservation should be OK assuming anaerobic conditions are quickly achieved and thereafter maintained and that there is no contamination with soil, manure/faeces, etc. In some cases if buffering capacity is very high (e.g. autumn grass) this 3% threshold may need to be increased to 3.5 or even 4%. Similarly, if buffering capacity is very low (e.g. very stemmy grass) then 2.5 or even 2.0% sugar may be OK. Buffering capacity measures the resistance to a drop in pH. 

Crops with high buffering capacity may have a poor initial fermentation. In the absence of knowing the buffering capacity it is usually OK to use the 3% threshold. As mentioned, main times for concern for this 3% value is if there is grass that has a very high buffering capacity due to high white clover, very high or late application of N fertilizer, or lush leafy grass. If sending a grass sample for sugar or nitrate analysis, it is extremely important to cool the sample quickly. If not the grass sample will remain respiring at ambient temperatures using up sugars.