The Figures below present the conversion of N intake into fecal N, Urine N and milk (protein) N as published in Kebreab et al., (2001) (from PhD thesis of A. Castillo) and from a compilation of approximately 22 studies (70 experimental diets) conducted at the University of Wisconsin (USDFRC and Arlington Research Station). 

The equations presented in the paper of Kebreab et al., 2001 were as follows:

• Fecal N = 76.7 + 0.16 N intake (R² = 0.3)
• Urine N = 0.03 N Intake^1.8 (R² = 0.7)
• Milk N = 38.2 + 0.19 N intake (R² = 0.3)

Instructions:

1. Compare the two data sets in term of the range in N intake, fecal N, urine N and milk N.  
The European work of analyzed by Kebreab et al (2001) has a narrower upper N intake limit compared to the upper N intake limit of the data of Wisconsin. Fecal N, Urinary N and Milk N values appear to be higher in the Wisconsin data set compared with the Kebreab data set. Finally note that the Wisconsin data report Urinary N for N intake in the range of 500 to 750 gr per cow per day, but the Kebrab dataset reports Urinary N for N intake in the range of 300 to 600 gr per cow per day.



2.  Compare the Urine N responses to N intake from both graphs. Are they (dis)similar? what might be the source of differences in these relationships, if any?  
Kebreab reported and exponential increase in Urinary N with N intake, but the Wisconsin data suggested a linear type of relationship.



3. Compare the Milk N responses to N intake from both graphs. Are they (dis)similar? what might be the source of differences in these relationships, if any? 
Although the modeling of Kebreak suggested a positive linear relationship (within the 300 - 600 g of NI per day), the Wisconsin data suggested a quadratic response (due primarily to the two lowest NI data points) indicating no response when NI is above 550 - 600 gr per cow per day.