Digestibility of nitrogen in sheep fed heat damaged protein

LIVESTOCK PRODUCTION SCIENCE ELSEVIER

Livestock Production Science 39 (1994) 93-96

Digestibility of nitrogen in sheep fed heat damaged protein J. Voigt a, K. Krawielitzki a'*, P.-M. Zwierz b, R. Krawielitzki a, R. Prym a, F. Weissbach c "Research Institute for the Biology of Farm Animals, Department of Nutritional Physiology "Oskar Kellner' ', J.-v.-Liebig-Weg 2, D- ! 8059 Rostock, Germany bVDLUFA Rostock, Germany L'Federal Research Centre for Agriculture, Research Institute for Grassland and Forage Plant, Braunschweig, Germany ( Accepted 4 December 1993)

Abstract

Four wethers ( Merino, BW 70 kg) were fed ryegrass normally dried ( 70-100°C, rotary dryer ) or normally dried and overheated ( 140°C, 3 h) to investigate the origin of increased crude protein excretion in faeces after feeding heat damaged protein. The endogenous urea N pool of the animals was labelled by intravenous infusion of ~SN-urea and the appearance of 15N in faeces were studied. In comparison to normal ryegrass the overheated diet caused a higher excretion of 8.2 g total N/kg dry matter (DM) intake consisting of 4.7 g neutral detergent-N ( = 57% of total N), 2.1 g bacterial N ( = 25%) and 1.4 g residual N ( = 18%). The unchanged excretion of endogenous N in faeces (in g/kg DM-intake, calculated from the ratio of 15N excess in faeces and in the blood-NPN fraction) and the smaller incorporation of endogenous N into microbial N in the damaged variant suggest that the microbes of the hindgut apparently prefer dietary residues of the heat damaged proteins as nitrogen source compared with urea nitrogen from the blood. As a result of overheating the increased faecal N is of bacterial origin and predominantly from dietary protein. Key words: Sheep; Nitrogen digestion; Overheated protein

I. Introduction

In ruminants the feeding of overheated feedstuffs increases the faecal excretion of N and organic matter (Prym and Weissbach, 1985). It is not clear whether the increased faecal N is predominantly of dietary origin or originates in the gastro-intestinal tract from endogenous sources (metabolic faecal N). Heat treatment of forage decreases the ruminal degradability of protein and the availability of protein within the small intestine (Beever and Thomson, 1981). Because the ruminal degradation of crude protein is correlated pos*Corresponding author supported within the framework of the scientist integration programme, by KAI e.V., Berlin 0301-6226/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSD10301-6226(93) E0108-5

itively with the degradation of dry matter in the rumen (Voigt and Piatkowski, 1987) the duodenal passage of undegraded feed is expected to be higher when heat treated forage is given. An increased supply of available substrate in the large intestine may be the reason for use of endogenous N for the synthesis of microbial protein excreted in faeces (Orskov et al., 1970, Mason et al., 1981). The objective of our study was to estimate the effect of overheating of forage on the origin of the increased faecal N. For this purpose the endogenous urea nitrogen pool in sheep fed heat damaged and non-damaged grass was labelled by intravenous infusion of ~SN-urea and the appearance of ~SN in faeces were studied,

J. Voigl el al. / Livestock Production Science 39 (1994) 93 96

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2. Materials and m e t h o d s

A metabolism trial consisting of two periods was carried out on 4 adult wethers (Merino, 70 kg BW). The wethers were fed ryegrass (secondary growth) normally dried in a rotary dryer (70-100°C in the vitiated air) (period I) or normally dried followed by overheating (140°C, 3 h) (period II). The ration (Table 1 ) was offered in two equal meals at 09.00 and 16.00 h. In period II the ration contained 10 g urea in order to prevent a NH3 deficiency in the rumen because a low ruminal degradation of damaged grass protein was assumed. Each period consisted of 14 days of adjustment to diets followed by a 7 day collection period during which offered feed, refused feed and faeces were measured and sampled. During the first 3 days of the collection period in 3 sheep 7 g 15N labelled urea (95 atom% ~SN, dissolved in 1000 ml physiological saline solution ) per day was continuously infused into the jugular vein. Blood samples of these animals were obtained from the jugular vein (opposite site of the infusion catheter) at 72, 96 and 120 hours after beginning of infusion. Heparin was used as anticoagulant. In feed and faeces crude nutrients were analyzed by the Weende procedure (Naumann and Bassler, 19881. Protein-N was separated from non-protein-N (NPN) by trichloracetic acid (TCA) precipitation. Neutral detergent fibre N (NDF-N) was isolated by boiling with neutral detergent reagent for 60 rain and hot filtration. N was determined by Kjeldahl technique. The Table I Composition of feed rations and chemical analysis of dried grasses

Grass, dried (normal) g / d Grass, dried (damaged) g / d Urea g / d Mineral-Vitamin-Mix g/d

Period 1 Grass normal

Period 11 Grass damaged

900

90(1 10 15

15

Chemical analysis of the grass (g/kg DM Organic matter 914.7 Crude fibre 314.0 N-free extracts 401.6 Crude protein 179.5 True protein 129.0 Detergent insoluble crude protein 68. I

915.3 317.8 393.0 186.9 137.0 132.2

bacterial fraction from faeces was separated according to Bergner et al. ( 19851. Diaminopimelic acid (DAP) was determined by the procedure of Mason and BechAndersen (1976). Plasma N was subdivided into protein-N and NPN using 20% TCA. The 15N enrichment of total N and bacterial N of the faeces and of NPN of the blood was measured in the titrated Kjeldahl distillate using an emission spectrometer. Faecal bacterial N was estimated from the content of DAP in bacteria and faeces. The endogenous portion of the total N in faeces was calculated from the ratio of ~SN excess in faeces and in the blood-NPN fraction, assuming that the tSN excess in the endogenous N and in the blood NPN is similar. Thus the amount of endogenous N resulted from the following equation:

Endogenous N ( g / d ) = N . . . . .

t

in fa. . . . ( g / d )

X 15Nexces s of faecal N(%) / 15Nexces s of blood NPN(%)

Data were analyzed by the Student's test. In period II (damaged grass) 1 sheep which refused more than 10% of offered feed was excluded from the balance experiment.

3. Results and discussion

The content of organic matter, crude fibre, N-free extracts, true and crude protein (CP) was similar in both grasses (Table 1). The content of NDF-N was distinctly higher in the damaged grass. The apparent digestibility of DM, organic matter, carbohydrates and CP was significantly reduced by heat damage (P < 0.05) (Table 2). Feeding of damaged grass significantly increased the amount of total N, bacterial N and NDF-N excreted in the faeces (P < 0.05) (Table 3). The enhanced excretion of NDF-N, bacterial N and residual N (non-bacterial N and non-NDF-N) suggests that the increased faecal N is of dietary, bacterial and endogenous (e.g. shedded epithelial cells, mucins, enzymes and plasma proteins) origin. In both variants the portion of bacterial N of the non-NDF-N amounted to about 70% (Table 3).

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Table "2 Apparent digestibility of dry matter, organic matter, carbohydrates and crude protein in sheep ( in %, mean ± SD)

Dry matter Organic matter Carbohydrates Crude fat Crude protein a

Grass normal n=4

Grass damaged n=3

Difference

66.0 ± 1.6 66.5 ± 1.7 65.9 ± 1.8 54.3 ± 3.2 69.5 ± 1.3

52.6 4- 2.6 52.9 +_2.5 54.5 ± 2.8 50.7 4- 3.5 43.7 ± 1.4

- 13.4+ - 13.6 + - 11.4+ - 3.6 - 25.8 +

+P < 0.05" awithout urea-N. Table 3 N fractions and calculated amounts of endogenous N in faeces (mean ± SD)

Total N g/kg DMId Bacterial N g/kg DMI % of non-dietary N NDF-N g/kg DMI Residual Na g/kg DMI 15N-excessb % of infusion (n = 3) Endogenous Nc % of total N g/kg DMI

Grass normal n= 4

Grass damaged n=3

Difference

8.79 4- 0.37 5.52 4- 0.45 70.7 ± 2.7 0.98 4- 0.10 2.294-0.11 2.144-0.52 46.1 + 28.1 3.99 ± 2.45

16.97 4- 0.54 7.60 + 0.27 67.2 ± 7.4 5.66 ± 1.46 3.71 ± 1.01 2.90±0.38 23.9 ± 7.0 4.03 4- 1.17

8.22 + 2.08 ÷ 3.5 4.68 + 1.42 0.76 + - 22.2 0.04

"Total N - (bacterial N + NDF-N); bin the whole time of the collection period; CMeasured 24 hours after the end of the JSN infusion; dDMI dry matter intake; + P < 0.05. The faecal microbial N is largely synthesized in the large intestine f r o m h y d r o l y z e d products f r o m bacterial ghost cells, e n d o g e n o u s and dietary residues ( M a s o n , 1984). B l o o d urea, w h i c h crosses the gut wall, is another N - s o u r c e for microbes. T h e level o f bloodN P N was significantly l o w e r in the d a m a g e d variant ( 1 8 9 _ 2 8 vs. 305_+44 mg/1; not s h o w n ) . T h e r e f o r e the e n r i c h m e n t of this fraction with 15N was higher (Fig. 1 ). In c o m p a r i s o n to the normal variant the 15N e n r i c h m e n t was higher in the bacterial N fraction of faeces and l o w e r in the total faecal N. T h e higher ~SNlabelling of b l o o d - N P N (Fig. 1 ) and the dilution o f the e n d o g e n o u s 15N with a large a m o u n t o f non-labelled dietary N ( T a b l e 3) are assumed to be reasons for this result. The incorporation o f 15N in the total N o f faeces ( expressed as p e r c e n t a g e o f the infused ~5N) a m o u n t e d to 2.1 ( n o r m a l ) and 2.9 ( d a m a g e d ) respectively ( T a b l e 3; P < 0.05). This difference can be explained by the different ~SN-excess in b l o o d - N P N . T h e small

1

•.c~

2,s

0,8

i2

@

C, Z

0,6

1,5

0,4

~,

0,2

0,5

24

48 72 96 120 144 168 Time after beginning of If'N-infusion [h]

192

0

,~ Z

-o Z

~- Faece~ B41ct..N, normal -t- FIf,oc4m, ElaCt,~, da~e~KI (.~ F u c u , To~-N, normal .,v-FMce4, ToqmJ-N,danl~ld

~ B4ood-NPIW,~

~Bkxx~-NPN,

Fig. l: Atom-% JSN-excessin bacterial N and total N of faeces and in the NPN-fraction of blood plasma in sheep fed normal and damaged grass.

incorporation o f 15N in the total N o f faeces is in accordance with results f r o m B e r g n e r et al. ( 1 9 8 5 ) and S o m m e t et al. ( 1 9 8 6 ) in cattle. M a s o n ( 1 9 8 4 ) assumed that m u c h o f the urea-N h y d r o l y z e d by m i c r o b e s adhered to the hindgut wall is rapidly re-absorbed as NH3 and

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J. Voigt et al. / Livestock Production Science 39 (1994) 93-96

is therefore unavailable to microbes located on teed particles. In contrast to the residual N the quantity o f endogenous N in faeces, calculated f r o m ~SN excess in total N o f faeces relative to ~SN-excess in blood p l a s m a - N P N was unaffected by the treatment o f the grass (3.99 vs. 4.03 g / k g D M I ; Table 3). The difference b e t w e e n e n d o g e n o u s N and residual N ( T a b l e 3) must h a v e been incorporated into microbial N. This value is smaller in the d a m a g e d variant (0.32 vs. 1.7 g N / k g D M I ) . F r o m these results, we conclude that the microbial portion of the e n d o g e n o u s N fraction a m o u n t e d to 42% ( 1 7 0 / 3 . 9 9 ) in the normal variant and 8% ( 3 2 / 4.03) in the d a m a g e d variant. This e m p h a s i z e s that after feeding o f d a m a g e d grass the microbes o f the large intestine used dietary residues as N source with high priority to urea N from the blood for their increased protein synthesis. In conclusion the increased faecal N after feeding o f heat d a m a g e d grass is o f bacterial and predominantly dietary origin.

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gen zum Stickstoffumsatz im Dickdarm von Wiederk~iuem. 1. Umsatz von i. v.-infundiertem ~SN-Harnstoffohne zus~itzliche Kohlenhydratversorgung des Dickdarms. Arch. Tierern~ihr., 35: 733-745. Mason, V.C., 1984. Metabolism of nitrogenous compounds in the large gut. Proc. Nutr. Soc., 43: 45-53. Mason, V.C. and Bech-Andersen, S., 1976. The estimation of 2,6diaminopimelic acid in digesta and faeces using acid ninhydrin reagent,. Z. Tierphysiol., Tierem~ihrg. u. Futtermitt., 36: 221229. Mason, V.C., Kessank, P., Ononiwu, J.C. and Narang, M.P., 1981. Factors influencing faecal nitrogen excretion in sheep. 2. Carbohydrate fermentation in the caecum and large intestine. Z. Tierphysiol., Tierem~hrg. u. Futtermitt., 45: 174--184. Naumann, K. and Bassler, R., 1988. Methodenbuch, Band III. Die chemische Untersuchung von Futtermitteln. Verlag NeumannNeudamm, Melsungen, Berlin, Basel, Wien, 265 pp. Orskov, E.R., Fraser, C., Mason, V.C. and Mann, S.O., 1970. Influence of starch digestion in the large intestine of sheep on caecal fermentation, caecal microflora and faecal nitrogen excretion. Br. J. Nutr., 24: 671-686. Prym, R. and Weissbach, F., 1985. Analytische Mfglichkeiten zur Kennzeichnung des Riickganges der Proteinverdaulichkeit bei der Heil31ufttrocknung yon Griinfutter. Arch. Anim. Nutr., 35: 515-529. Sommer, A., Ceresnakova, Z., Szakacs, J., Chrastinova, L., Bergner, H. and Simon, O., 1986. Untersuchungen zum Stickstoffumsatz im Dickdarm von Wiederktiuern. 2. Umsatz von i. v.-infundiertern 15N-Harnstoff bei zusatzlicher Versorgung des Dickdarms yon Bullen mit fermentierbarem Material. Arch. Anim. Nutr., 36:639-651. Voigt, J. and Piatkowski, B., 1987. Ruminal protein degradation and protein value of feeds. Arch. Anim. Nutr., 37: 63-68.