Interpretive Summary: Nitrogen excretion from beef cattle fed a wide range of diets compiled in an intercontinental dataset: a meta-analysis
By: Adeline Bougouin, Alexander Hristov, Diego Zanetti, Sebastiao C V Filho, Lucianna N Rennó, Ana C B Menezes, Jarbas M Silva, Jr, Herlon M Alhadas, Lays D S Mariz, Laura F Prados, Karen A Beauchemin, Tim McAllister, WenZhu Z Yang, Karen M Koenig, Karen Goossens, Tianhai Yan, Pierre Noziere, Arjan Jonker, Ermias Kebreab
Ruminants play a key role in the food system because they can convert fiber-rich plants into highly nutritious food for humans. However, there is a growing concern about livestock production because of its negative environmental impact, mainly due to enteric methane (CH4) emissions but also as a result of N excretion leading to nitrate (NO3−) leaching and ammonia (NH3) and nitrous oxide (N2O) emissions (FAO, 2002). Nitrous oxide is an important greenhouse gas with 265 times greater global warming potential than CO2 over a 100-yr period (IPCC, 2007). Volatilization of NH3 is of particular concern because it contributes to the formation of fine particulate matter that is linked to human respiratory and cardiovascular problems (Fu et al., 1999). In the United States, NH3 emitted from livestock operations was estimated to contribute on average from 5% to 11% (up to as much as 20%) of the total PM2.5 atmospheric concentrations (Hristov, 2011).
The efficiency of feed N conversion into meat protein in cattle varies widely and beef cattle are relatively inefficient, utilizing approximately 20% of dietary N for growth (NRC, 2001). The main driver of N losses from ruminants is excess N intake (Dijkstra et al., 2013b). Variation in dietary N supply affects N excretion in urine and feces, but N in urine is more susceptible to leaching and volatile losses (Hristov et al., 2011). Nitrogen utilization in the rumen is largely dependent on the energy available for microbial protein synthesis, and the large variation in urinary N excretion compared with fecal N excretion presents an opportunity to manipulate diets to reduce N excretion in urine (Dijkstra et al., 2013b).
Predictive equations for estimating N excretion are useful for evaluating potential dietary strategies for N mitigation because measurement of excretion or emissions are costly and difficult to apply on commercial farms. Several models to predict N excretion from beef cattle have been developed (Yan et al., 2007; Waldrip et al., 2013; Dong et al., 2014) using data from various locations around the world. These models used dietary nutrient composition or intake variables and were developed from a limited number of studies based on datasets of treatment means. Models developed using treatment means do not take into account individual animal variability and are less robust than meta-analysis based on individual observations. Therefore, we carried a meta-analysis based on individual beef data from different experiments, and the objectives of the present study were: 1) collate a global dataset of individual observations of fecal and urinary N excretion in beef cattle; 2) determine the suitability of key variables for fecal, urinary, and total manure N excretion; and 3) develop simplified, but robust and reliable, N excretion prediction models based on individual animal data of N excretion from beef cattle consuming different diets.
Read the full article in the Journal of Animal Science.