While unique at converting fiber into protein, the rumen remains a low efficiency nitrogen converter when measuring the ratio of nitrogen found in milk and meat to nitrogen intake. In the rumen, this number is around 25 percent, with a wide range of variation between animals as shown in Table 1.
On average, ruminant efficiency is much lower than other animals in intensive systems such as poultry or swine. On these farms, the protein needs of the animals can be more closely met and efficiency may average 30 to 40 percent. Using data from peer-reviewed papers, European researchers calculated the efficiency of nitrogen utilization of typical European Union (EU) diets based on grass or grass silage and United States diets based on corn silage. Within the EU diets, the authors found that feeding higher nitrogen efficiency diets (32 percent) resulted in cows with higher dry matter intake, more milk, and a lower proportion of forage in the ration compared to lower nitrogen efficiency diets (21 percent). In contrast, the U.S.-type diets with high nitrogen efficiencies (32.8 percent) resulted in cows that produced more milk on diets that had lower protein content and higher nonfiber carbohydrates.
Protein intake the biggest factor
A meta-analysis evaluated the effects of dietary protein intake on milk nitrogen efficiency using two large data sets based on North American and North European feeding trials. The average nitrogen efficiencies were 24.7 and 27.7 percent, respectively.
This analysis demonstrated that protein concentration of the diets is the most important dietary factor influencing nitrogen efficiency. The authors also indicated that upping milk yield would boost milk nitrogen efficiency, but the effect is considerably smaller than the effect of reducing protein intake.
Continue reading this article published in Hoard Dairyman.
