Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/6/2013
Publication Date: 9/1/2013
Citation: Waldrip, H., Todd, R.W., Cole, N.A. 2013. Prediction of nitrogen excretion by beef cattle: a meta-analysis. Journal of Animal Science. 91:4290-4302.
Interpretive Summary: Ammonia loss from beef cattle feedyards can cause environmental problems and represents a loss of income to the farmer. Most of the ammonia from beef feedyards comes from urine, with only a small part coming from feces. How much nitrogen that goes into urine or feces depends on different animal and feed characteristics; therefore, we need more information on what influences nitrogen excretion, and how. Good estimates of nitrogen output from beef cattle can help farmers develop plans to manage nitrogen in a way that is economically sound and environmentally friendly. From a wide variety of published nitrogen balance studies on beef cattle, a large set of data was created that included the daily dry matter and nitrogen intakes, protein content of the feed, and daily nitrogen output in urine and feces. Statistics were used to find the strongest relationships between the animal and feed characteristics and nitrogen output. Mathematical models were then created that predict nitrogen output in the urine and feces that were based on daily nitrogen intake and how much protein is in the animal feed. Similar models were also created that predict how much of the total nitrogen output will be in urine, and how much will be in feces. The models were tested for their ability to predict nitrogen output, and there was a very good fit between model predictions and actual nitrogen output measurements. These models are simple tool that can predict nitrogen output characteristics for a wide range of feed and animal characteristics, and could help improve ammonia loss estimates.
Technical Abstract: Reliable estimates of nitrogen (N) excretion in the urine and feces of beef cattle are essential for developing cost-effective and environmentally sound nutrient management plans. A meta-analysis dataset was compiled that included data for starting live BW, DMI, N intake, dietary CP concentration, urine N excretion, and feces N excretion. The data were taken from 13 individual feeding trials that included N balance data, and represented a total of 50 different dietary treatments and 243 animals. Correlation analysis was used to determine the animal and dietary parameters that were most closely related to N excretion in urine and feces by beef cattle. Regression equations were then developed to predict excretion of urine N, feces N, and the partitioning of total N excretion between urine and feces, as a function of N intake and the concentration of dietary CP. Univariate, regression, and mean difference comparisons indicated 78 to 97% agreement between observed and predicted values for the developed models. Furthermore, an independent, internal evaluation with two random data subsets showed very good agreement (P < 0.01) for urine N excretion and the partitioning of total N excretion into urine and feces. The empirical models provide a simple tool to predict N excretion characteristics for a wide range of dietary and animal characteristics and could improve ammonia (NH3) emissions estimates by process-based models.