The 8th revised edition of the Nutrient Requirements of Beef Cattle: Environmental issues

Abstract Text: Since publication of the of 7^th Revised Edition of the Nutrient Requirements of Beef Cattle, (1996/2000) there has been growing concern among producers, regulators, and the general public about the impacts of livestock operations on the environment.  Beef cattle typically retain less than 20% of the nutrients they consume. The remainder is lost via feces, urine or respiration.  The effects of these excreted nutrients, as well as pharmacologically active compounds (PAC) and pathogens on ground waters, surface waters, air quality, global climate change, environmental sustainability, land use, biodiversity, and quality of life are potentially affected by nutritional and management programs used by producers.  Although environmental concerns normally revolve around concentrated animal feeding operations, some effects can also be a concern in extensive systems such as pasture-based cow-calf and stocker operations.  This new chapter in the 8^th Revised Edition summarizes the environmental concerns associated with beef production in North America and reviews the latest scientific approaches to mitigation.  Possible dietary effects on surface and ground water and air quality are discussed.  Water quality concerns include the loss of nutrients such as nitrates and phosphorus and PAC to ground and surface waters.  Air quality issues of greatest concern include emissions of ammonia and the greenhouse gases methane and nitrous oxide.  Empirical equations are provided to estimate excretion of organic matter, nitrogen, and phosphorus and for emissions of ammonia and enteric methane.  A non-linear equation is recommended to calculate the proportion of total nitrogen excreted that is excreted in urine. Enteric methane production of cattle is highly dependent upon factors such as forage quality, forage concentration, DMI, dietary fat, ionophores, and grain processing therefore, multiple empirical equations are proposed to estimate enteric methane production from cattle fed high-forage, medium-forage, and low-forage diets.  The effects of many co-products, such as distiller’s grain, on enteric methane are variable and dependent upon the control diet composition.  By more precisely feeding and supplementing livestock to meet their nutrient requirements, excess nutrient losses can be decreased. Under practical conditions, however, the use of precision feeding systems to manage environmental impacts is limited and challenged by factors such as: 1) inherent biological inefficiencies in the animal; 2) variability in animal performance and/or nutrient requirements; 3) variability in composition of feed ingredients; 4) high nutrient concentrations in many co-products, and 5) other factors.

Keywords: Environment, Beef cattle, Nutrients, Greenhouse gas, Ammonia.