Profitable milk production depends on efficient use of dietary protein. The target should be for 30% of Total N ingested to be converted to Productive N and for the Productive N/Urinary N ratio to be greater than 100%.
Typically only 25% of N intake is incorporated into milk protein - in other words 75% is excreted in the urine and feces. Fecal N is relatively constant, thus the extra N consumed by the cow is secreted in the urine which is the most volatile form. Olmos, Colmenero and Broderick found that as cows consume more N, the N secreted in the milk as well as the N secreted in the feces was fairly constant. On the other hand, the N excreted in the urine increased linearly in relation to the intake. The net effect was that the efficiency of N utilization decreased from 36 to 25%. Maximizing N efficiency is not only good for the environment; it is also good for the cow's energy status. It takes 7.2 Kcal of Metabolizable Energy per gram of excess N to convert to urea.
Accurately balancing the rations for Amino Acids will further improve the efficiency of N utilization by lowering N inputs while maximizing milk protein synthesis.
The new CNCPS v6 (incorporated in the AMTS, NDS and other commercial software programs) includes reports that help us monitor how efficient N utilization is in a particular situation.
| Excretion | |||
| Fecal (lbs) | 107 | ||
| Urine (lbs) | 47 | ||
| Total Manure (lbs) | 154 | ||
| Fecal N (g) | 248 | ||
| Urine N (g) | 184 | ||
| Total Manure N (g) | 431 | ||
| Productive N:Total N | 0.36:1 | Productive N/Total N > 30% | |
| Productive N:Urinary N | 1.29:1 | Productive N/Urinary N > 100% | |
| Manure N:Total N | 0.64:1 | ||
