Abstract
Reducing crude protein (CP) in livestock diets may lower ammonia emissions. A feeding trial was conducted with crossbred steers at the Southeast Colorado Research Center in Lamar, Colorado from December 2009 to March 2010. Three diet treatments were investigated: Reduced (11.6% CP), Oscillating (13.5% crude protein 4 days/week and 11.6% CP 3 days/week) and Control (13.5% CP). Intact soil core samples (n = 36 per sampling date) were collected from the pen surfaces on three dates corresponding to 45, 92, and 148 days into the feeding cycle. Four pens from each diet treatment were sampled. Cores were placed into flow-through laboratory chambers for seven days and ammonia fluxes were trapped in acid bubblers that were refreshed every 24 h. Average daily ammonia emissions for the Control diet ranged from 6.6 to 9.4 g NH3 m−2·day−1; average daily emission for the Oscillating diet ranged from 6.3 to 8.8 g NH3 m−2·day−1; and average daily flux for the Reduced diet ranged from 4.1 to 5.8 g NH3 m−2·day−1. Ammonia fluxes from the reduced N treatment were significantly lower (21% to 40%) than from the control diet on the first two sample dates. There was no significant difference between the Oscillating and Control treatments. Reducing CP in cattle feedlot diets may be an effective method for reducing ammonia emissions from pen surfaces. More research is needed to validate these results at commercial scales in different environments to determine if reductions in ammonia can be sustained with lower CP diets without affecting rate of gain, feed efficiency and health.
Highlights
One of the most promising strategies for reducing NH3 emissions to the atmosphere from beef cattle feedlots is reducing the proportion of fed N that is excreted to the pen surface, which involves formulating diets to stimulate physiological N retention mechanisms in cattle
Reducing crude protein (CP) from 13.5% to 11.62% caused a 21% to 40% reduction in NH3 fluxes under the conditions tested
More research is needed at commercial scales to determine if reductions in NH3 emissions can be sustained with lower CP diets without affecting rate of gain and feed conversion in feedlot cattle
Summary
One of the most promising strategies for reducing NH3 emissions to the atmosphere from beef cattle feedlots is reducing the proportion of fed N that is excreted to the pen surface, which involves formulating diets to stimulate physiological N retention mechanisms in cattle. Optimizing animal performance requires determining dietary protein levels above which animal production metrics do not improve. While feeding dietary protein above that level generally does not hurt the animal, it is less efficient—increasing feed costs without increasing profits, and increasing N content in animal excretions. Protein provided in excess of an animal’s needs is broken down into individual amino acids which are deaminated, creating free ammonia, and quickly converted to urea by the liver [1,2]. For animals being fed near the critical point where increasing dietary protein content will not produce additional gains in production, that urea will be recycled to the digestive tract and utilized for ruminal microbial growth, increasing overall N efficiency. As the protein in the diet increases, recycling of urea decreases, and more urea-nitrogen is lost to the environment through excretion in the urine [2,3]
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