Measures of nitrogen use efficiency and nitrogen loss from dairy production systems

Authors: Powell, Joseph; Rotz, Clarence - Al

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/9/2015
Publication Date: 3/11/2015
Publication URL: http://handle.nal.usda.gov/10113/62909
Citation: Powell, J.M., Rotz, C.A. 2015. Measures of nitrogen use efficiency and nitrogen loss from dairy production systems. Journal of Environmental Quality. 44:336–344.

Interpretive Summary: When nitrogen is used excessively on farms, it can contribute to water and air quality impairment at local, regional and global scales. The purpose of this paper is to examine how stocking rate (cows per unit land area), fertilizer nitrogen use, and different terms and calculations of nitrogen use efficiency (NUE) affect interpretations of nitrogen use and nitrogen loss from dairy production systems. As cows are added to a herd, more feed is imported and the resultant manure exceeds the nitrogen recycling capacity of land base, which increases nitrogen loss per unit of land area. Current fertilizer recommendations based solely on economic returns often do not increase crop yields but certainly increase nitrogen loss. The use of terms such as N surplus and N balance, and calculations of NUE, need to be standardized; and tradeoffs in feed, fertilizer and manure use need to be recognized to provide a better understanding of the potential to enhance NUE and reduce environmental nitrogen loss from confinement dairy production systems.

Technical Abstract: Excessive N use in agriculture can impair air and water quality. The purpose of this paper is to examine how stocking rate, feed imports, fertilizer N use and different measures of N use and N loss impact nitrogen use efficiency (NUE) in dairy production systems. First, determinations of NUE and N loss were evaluated for a dairy farm (FG-farm) with a stocking rate of 2.0 animal units ha-1 which grew forage and grain and imported protein supplements (23% of feed N intake) and a farm (F-farm) with a stocking rate of 3.3 animal units ha-1 which grew forage only and imported grain and protein supplements (38% of feed N intake). The FG-farm managed 356 kg N ha-1 of corn silage and grain: 17, 60, and 23% from fertilizer, manure and biologically-fixed N, respectively. The F-farm managed 631 kg N ha-1 of corn silage: 72 and 28% from manure and biologically-fixed N. Ammonia loss was 64% of total N loss for both farms. Nitrogen losses via leaching and denitrification were 15 and 19% of total N loss from the FG-farm and 11 and 13% of total N loss from the F-farm. Each 1% reduction in dietary crude protein did not affect milk production but enhanced dietary NUE by 2% and decreased excretion of N in manure and as urinary urea by 32 and 28 g N cow-1 per day, which reduced N loss. Reductions in fertilizer N applications of 20 kg N ha-1 did not jeopardize corn yield but reduced the reactive N footprint by 9.2%. The use of the terms N surplus and N balance, and calculations of NUE need to be standardized, and tradeoffs in N use and N loss need to be recognized to provide a better understanding of potentials to enhance NUE and reduce environmental N loss.