Challenges in ruminant nutrition: towards minimal nitrogen losses in cattle

Ruminants play a key role in human food production by converting fiber-rich plant resources that humans cannot (or choose not to) consume into high-quality food that humans can eat. However, this conversion causes unavoidable losses of nitrogen (N) in feces and urine from ruminants that may become an environmental burden, in particular nitrate (NO3 -) leaching, ammonia (NH3) volatilization and nitrous oxide (N2O) emissions. The aim of this paper is to identify the maximal theoretical N efficiency at the animal level, and the challenges and opportunities to achieve this maximal N efficiency. This is done via striving for the lowest possible N excretion in urine and feces, and for purposes here, with a focus on dairy cattle. Inevitable N losses in dairy cattle include losses associated with urinary excretion of urea synthesized from ammonia produced in the rumen; undigested microbial protein excreted in feces; microbial nucleic acids synthesized in the rumen and excreted mainly in urine; fecal and urinary excretion resulting from endogenous secretions; and urinary excretion related to maintenance and milk protein synthesis. The theoretical upper limit of N use efficiency in a dairy cow producing 40 kg fat and protein corrected milk/d is 0.43. Higher efficiencies may be achieved, but these require major inputs of human edible resources. The present analysis demonstrates there is little or no scope to reduce N losses related to microbial nucleic acid synthesis, recycling of N to the rumen, intestinal digestion of microbial protein, and animal maintenance requirements. Strategies to reduce N losses and improve N efficiency should focus on an optimal supply of rumen degradable N and optimal efficiency of utilization of absorbed amino acids for milk protein synthesis. To improve N efficiency, integration between protein and energy metabolism is essential, and energy and protein should be considered together rather than as two distinct entities. A major challenge in strategies to optimize high-fiber diets for high milk N efficiency will be to avoid increases in enteric methane production associated with these dietary strategies.