Nitrogen Recycling in the Ruminant: A Reviewreview
Аннотация: Movements of N across the gut can be more than double N intake and thus have major effects on N metabolism of the ruminant.These movements may be associated with net losses, as occurs with production of ammonia or irreversible loss of secreted proteins, or as a means by which N products can be salvaged and used for anabolic purposes, e.g., use of urea-N as a source for microbial protein synthesis.Understanding the conditions and mechanisms that underlie these movements should enhance our ability to manipulate feed and husbandry to improve production efficiency.Between 40 and 80% of urea-N synthesized by the liver is returned to the gut, and 35 to 55% of this is converted to further anabolic use in both cattle and sheep.While some of this anabolic use may occur in the small intestine, most involves the rumen, a process that is dependent on energy supply under conditions in which the efficiency of conversion of digested N to amino acids is low.Current data would suggest that sheep are more efficient (P < 0.001) than cattle in such conversions.Dairy cattle are also more efficient than steers (P < 0.05), and this may be related to the nature of the rations studied experimentally (type and quality of the forage).The proportion of digestible N lost as ammonia is greater (P < 0.001) for steers fed forages than concentrates.As much as 40% of ammonia absorbed from the gut is derived from endogenous urea-N and forms part of a cycle that conserves N within the body.Loss of N also occurs from intrarumen recycling due to the presence of proteolytic bacteria and protozoa.Their removal, or inhibition, can lead to marked improvements in protein flow to the duodenum.Net inflows due to endogenous protein secretions amount to 30 to 40% of apparent absorption across the small intestine, and 30 to 70% of this may be lost through oxidation.The relative profiles of most amino acids are unchanged during the absorptive process, but disproportional losses can occur for threonine, valine, and cysteine, probably due to poor reabsorption from secretions such as the mucins.Preferred strategies to enhance anabolic performance involves increasing conversion of urea-N to anabolic products and reducing secretory losses due to the presence of environmental or dietary factors.
Год издания: 2001
Авторы: H. Lapierre, G. E. Lobley
Издательство: Elsevier BV
Источник: Journal of Dairy Science
Ключевые слова: Ruminant Nutrition and Digestive Physiology, Reproductive Physiology in Livestock, Soil and Water Nutrient Dynamics
Открытый доступ: hybrid
Том: 84
Страницы: E223–E236