Effect of ammonia concentration on rumen microbial protein production in vitro
British Journal of Nutrition
I . The effect of ammonia concentration on microbial protein production was determined in continuous-culture fermentors charged with ruminal contents obtained from steers fed on either a protein-free purified diet, a maize-based all-concentrate diet, or a forageconcentrate (23 : 77) diet. Urea was infused into the fermentors to maintain various concentrations of ammonia in the incubating mixtures. 2. Under nitrogen-limiting conditions, microbial protein yield measured as tungstic
... stic acidprecipitable N (TAPN) increased linearly with supplementary urea until ammonia started to accumulate in the incubating ingesta. Increasing the ammonia concentration beyond 50 mg NH,-N/l had no effect on microbial protein production. 3. The molar proportions of volatile acids produced were not affected by the level of urea supplementation. Total acid production was decreased slightly under N-limiting conditions, but not to the same extent as microbial protein production. 4. Estimated yield of microbial dry matterlmol ATP produced averaged 15.6 when nonlimiting N as urea was provided with the purified diet. 5 . These results suggest that addition of non-protein N supplements to ruminant rations are warranted only if the prevailing concentration of ruminal ammonia is less than 50 mg NH,-N/l ruminal fluid. It has long been recognized that supplemental non-protein nitrogen is not well utilized in ruminant rations containing relatively large amounts of protein or nonprotein N, or rations which are low in digestible energy. What is not known is the level of dietary protein below which addition of non-protein N to the diet improves animal performance, or above which it has little or no benefit. The dietary protein level at which point further addition of non-protein N to the ration ceases to be of benefit varies with the digestible energy content of the ration and the extent to which dietary protein is degraded to ammonia. The total amount of non-protein N that can be utilized wilI depend upon the amount of fermentable energy available for microbial growth and the amount of ammonia derived from dietary and salivary sources. It can reasonably be assumed that dietary non-protein N will be of little benefit to the ruminant unless it is first converted into ammonia, and then utilized for microbial protein synthesis in the rumen. If this is so, it is then important to know what concentration of ruminal ammonia is necessary for maximal microbial growth. Maintenance of ruminal ammonia concentration in excess of the bacterial requirement would be unnecessarily costly. The purpose of this study was to determine the concentration of ammonia necessary for maximal growth of rumen micro-organisms. This would allow a definitive understanding of when to expect benefit from non-protein N supplementation of ruminant diets varying in protein and digestible-energy content. 53706, USA.