What is the relationship of nitrogen and grass

Nitrogen is the major nutrient required by grass and is used to stimulate high growth rates and is key to achieving high yields, however if too much nitrogen is applied this can have a negative effects on quality.

High rates of nitrogen can also cause problems with silage fermentation due to excess nitrate having a negative impact on the fermentation process and will produce silage that is less palatable and the animals will be less keen to eat it.

Nitrate is generally taken up by grasses quicker than it is incorporated into proteins and until used this excess known as luxury uptake is stored in the leaves. Excess nitrate will be present if insufficient time is allowed between application and mowing and can also occur under conditions of poor growth eg low light levels, cool temperatures. It is also a problem if there is a dry spell after application, when nitrate cannot be taken up by the roots, followed by a period of wet weather that results in luxury uptake. The plant cannot convert it to protein quickly enough so it accumulates in the plant.

High rates of nitrogen may cause a reduction in crop sugars as they are used to provide energy for the increased rate of plant growth and for the manufacture of plant proteins and this increased growth rate may in turn lead to lower crop dry matter content although in practice this is often not significant.

Excess nitrate can be avoided by following the Grassland Rule to apply nitrogen at the rate of no more than 2.5 kg N/ha/day.

This is the most closely resembles the model. 

The answer to this will be from your lab notes/procedure. it would be best to ask another student who participated in the lab as we do not know what was done, what enzyme or if the temperature is celsius or fahrenheit. if i were to guess though i'd say the rate stopped due to enzyme denturing at high temperature