The origin of nitrogen incorporated into compounds in the rumen bacteria of steers given protein- and urea-containing diets

Abstract

I . Two young Friesian steers fitted with rumen cannulas were each given three different isonitrogenous and isoenergetic diets for successive periods of 2-3 weeks. The diets consisted mainly of straw and tapioca, with the nitrogen supplied mainly as decorticated groundnut meal (DCGM; diet A), in approximately equal amounts of DCGM and urea (diet B), or entirely as urea (diet C).2. At the end of each period on a given diet, part of the dietary urea of a morning feed was replaced by a solution of [lSN]urea which was infused into the rumen. Samples of rumen contents were removed just before giving the 15N dose and at I , 3 , s , 7 and 24 h afterwards, concentrations of ammonia and its I5N enrichment were determined and samples of mixed bacteria were prepared. Amino acids, ammonia derived mainly from amide groups, and hexosamines were prepared by ion-exchange chromatography of acid-hydrolysates of the bacteria and analysed for 15N.3. Approximate estimates of net bacterial N synthesis were made from turnover data for rumen fluid and I5N enrichments in rumen fractions. From the determined efficiency of incorporation of urea-N into bacteria recovered at the duodenum, it was calculated that on diets A, B and C respectively 82 x, 37 and o % of the bacterial N was derived from dietary protein or other non-urea sources.4. ['5N]urea was converted rapidly to ammonia and the 15N then incorporated into bacterial amide-N; it appeared at a slower rate in total bacterial non-amide-N. Rates of incorporation into non-amide-N were highest for glutamic acid, aspartic acid and alanine, and generally lowest for proline (pro), histidine (his), phenylalanine (phe), arginine (arg), methionine (met) and galactosamine. A similar ranking was also generally observed for relative 15N abundances (15N atoms "/, excess in N componentt15N atoms % excess in total bacterial N) achieved after several hours. Relative 15N abundances in his, arg and pro increased with decreasing protein @CGM) in the diet but those in the other protein amino acids, including the poorly labelled met, phe (and its derivative tyrosine) did not.5. It was concluded that different extents of labelling of the amino acids (at least those present mainly in protein) indicated that different amounts of preformed units (amino acids or peptides) were used. When an adequate supply of such units was available (particularly on diet A) pro, arg, his, met and phe were derived in this way to a greater extent than the other amino acids, but whereas synthesis of pro, arg and his increased on the low-protein diet C, that of met and phe did not. Thus met and phe may be limiting for bacterial growth on diets low in protein and high in non-protein-N.6. Differences in the extent of labelling of other bacteria1 N components may be due to different turnover rates.

Influence of the pattern of peptide supply on microbial activity in the rumen simulating fermenter (RUSITEC)

“…Salter et al (1979), summarizing data from the literature, found that between 18 and 100 % microbial protein was derived from NH 3 . Hristov & Broderick (1996) in vivo and Carro & Miller (1999) in vitro divided the microbial population into LAM and SAM, and measured the N coming from the NH 3 pool.…”

Ruminal microbial protein synthesis in sheep fed forages of varying nutritive value

“…Numerous studies have been conducted to determine microbial protein synthesis in the rumen under various conditions Ertle et al, 1977;Salter et al, 1979;Salter et al, 1983;Argyle et al, 1989;Cecava et al, 1990; DeVisser et al, 1992;Singh et al, 1994;Ludden et al, 1995; Griswold et al, 1996).…”

“…First, urea is completely degraded in the rumen within two hours of feeding, resulting in peak ruminal ammonia-N concentrations at one to three hours after feeding. Although some of ammonia-N was used for microbial growth.28 much of the ammonia is either absorbed or passed to the duodenum and lost in digestion (Salter et al, 1979). Second, NPN sources did not supply the amino acids and peptides required for optimal microbial growth (Argyle et al, 1989).…”

“…Henning et al (1993) concluded that merely improving the degree of synchronization between energy and N release rates in the rumen did not increase microbial yield. However, in the studies conducted by Salter et al (1979) and Henning et al (1993), urea was used as the nitrogen source, and therefore, performed amino acid and/or peptides might have limited ruminal microbial protein synthesis. In order to increase microbial yield, it seems that manipulation of energy and N fermentation in the rumen should first be aimed at obtaining the most even ruminal energy supply pattern possible with a particular dietary regimen.…”

Effect of feed 15N incorporation into solid-associated bacteria on the in situ nitrogen degradability of 15N labelled Italian ryegrass

“…The proportion of the ammonia nitrogen incorporated into bacterial cells may be highly variable, ranging from 300 to 800 g ammonia N/kg total bacterial nitrogen (Salter et al, 1979;Wallace et al, 1999). Our results indicate even a higher rate of incorporation of ammonia-N into microbial protein, assuming the incorporation rate of 15 N represents the direct incorporation of feed N. However, the incorporation of feed 15 N represents just the rate of direct incorporation of N from feed N contained in the bag, whereas SAB attached to the residue particles may also incorporate N compounds derived from degradation of proteins supplied by the diet ingested by the animals.…”