Estimating the energetic cost of feeding excess dietary nitrogen to dairy cows

Authors: Reed, Kristan; BONFA, HUGO - Universidade Federal De Viçosa; DIJKSTRA, JAN - Wageningen University; CASPER, DAVID - Furst-Mcness Company; KEBREAB, ERMIAS - University Of California

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/22/2017
Publication Date: 7/12/2017
Citation: Reed, K., Bonfa, H.C., Dijkstra, J., Casper, D.P., Kebreab, E. 2017. Estimating the energetic cost of feeding excess dietary nitrogen to dairy cows. Journal of Dairy Science. 100: 7116-7126.

Interpretive Summary: The interactions between dietary protein and energy (carbohydrates) are complex in ruminant animals, such as dairy cows. Prior to the 2001 National Research Council Nutrient Requirements of Dairy Cattle, an energetic cost was associated with feeding protein in excess of the animal’s requirement. This correction was removed in the 2001 publication, but field experiences demonstrated that feeding too much protein resulted in body weight loss. This re-analysis of energy metabolism data has demonstrated that feeding excess protein -- more specifically, protein that is degraded in the rumen before being passed on to the hindgut -- resulted in an energetic cost that must be accounted for in calculating the nutritional requirement for energy. This information will help other researchers and dairy cattle nutritionists balance protein and energy for more sustainable milk production.

Technical Abstract: Feeding N in excess of requirements could require the use of additional energy to synthesize and excrete urea, however, the amount energy required is unclear. Little progress has been made on this topic in recent decades so an extension of work published in 1970 was conducted to quantify the effect of excess N on ruminant energetics. In Part 1 of this study, the results of previous work were replicated using a simple linear regression to estimate the effect of excess N on energy balance. In Part 2, mixed model methodology and a larger dataset were used to improve upon the previously reported linear regression methods. In Part 3, heat production, retained energy, and milk energy replaced the composite energy balance variable previously proposed as the dependent variable to narrow the impact of excess N. In addition, rumen degradable and undegradable protein intakes were estimated using NRC table values and included as covariates in Part 3. Excess N had opposite and approximately equal effects on heat production (+4 to +7 kcal/g excess N) and retained energy (-4 to -6.5 kcal/g excess N) but had a larger negative effect on milk gross energy (-52 to -68 kcal/g excess N). The results suggest that feeding excess N increases heat production but more investigation is required to determine why excess N has such a large effect on milk production.