Modern dairy production must handle multiple objectives like reduction of costs, higher feed efficiency and lowering the environmental impact of the production. Low efficiency utilization of nutrients, mainly protein, has been one of the main points to improve. For a better utilization of nitrogen, prediction of protein requirements and production of milk protein are critical. Hence the nutrition model used should predict requirements and formulate diets accurate enough in order to make improvements in this area.

Lapierre et al. (2018) compared the performance of different nutrient models used worldwide in the prediction of Milk protein yield (MPY). The models were compared in two published studies which represented different situations: Study 1 involved protein variation in the diet and Study 2 protein and energy variation.

Results from Study 1 are shown in Figure 1a. The authors concluded that nutrient models with fixed efficiency of protein utilization tend to under-predict at low protein values, like NRC, or overpredict at high protein values, as CNCPS and NRC. In contrast, models with variable efficiency, like NorFor, perform well at low protein levels, and perform better than fixed-efficiency models at high levels of protein. The efficiencies used by the different nutrient models at each level are shown in Figure 1b.

Results of the second study, comparing models in a protein and energy shifting scenario, are shown in Figure 2a and 2b. NorFor also performed better than the fixed-efficiency models at predicting MPY and was the best at estimating MPY at low energy and high protein case, which is very common situation in practice regarding the N excess issue.

The authors concluded that variable efficiencies of protein utilization should be included in a nutrient model to better predict MPY, like in NorFor. In the Norfor model, the efficiency of protein from the diet for MPY prediction is determined by equations that relates amounts of metabolizable protein to the amount of energy available for milk synthesis. 

References

LAPIERRE, H., LARSEN, M., SAUVANT, D., VAN AMBURGH, M. E. & VAN DUINKERKEN, G. 2018. Review: Converting nutritional knowledge into feeding practices: a case study comparing different protein feeding systems for dairy cows. Animal, 1-10.