The Use Of N-15 For The Determination Of In-Sacco Bacterial-Contamination Of 3 Concentrate Feeds And N-Degradability Of Feeds In The Rumen

(1)

The

use

of

15 N

for

the determination of

in

sacco

bacterial contamination of 3

concentrate

feeds

and

N

_{degradability}

of feeds in the

rumen

Y Beckers

A Théwis

B Maudoux

E

_François

₂

1 _UER_de

Zootechnie,

faculté des Sciences

agronomiques,

passage des

Déportés,

B-5030

Gembloux,

2_Centre_{de Recherches}

_{agronomiques,}

_{station de Chimie}_et_de

_{Physique agricoles,}

chaussée de Wavre

115,

B-5030

_{Gembloux, Beigium}

The

aims

of

this

_experiment

were to

_quantify

the extent of

bacterial contamination

_(BC)

of

nylon bag

residues,

using

15 N

as a

microbial

marker and to establish its influence on the

effective

_{nitrogen degradability}

_(DT

_N

₎

of

3 concentrate

feeds.

The N

_{degradability}

of

soya-bean

meal

(SBM),

meat and bone meal

(MBM)

and wheat bran

(WB)

was measured

_using

the

nylon bag

proce-dure

_(0rskov

and

_McDonald,

₁₉₇₉₎

in 2 adult Friesian steers

receiving

twice

daily

a diet based

or concentrate and meadow

_{hay (50:50,}

13.5%

CP,

10

kg DM/d).

Two d before

starting

and

dur-ing

the incubation

period

(!SNH4)2S04

(99

atom% excess, 3

g/d)

was infused

continu-ously through

the rumen cannula. Rumen bacte-rial N in the residues

_(as

% of total residual

N)

was calculated from the ratio:

(

15 N

atom % ex-cess in the residue/!SN atom % excess in the

bacteria)

x 100. Whole rumen content was

sam-pled

in each steer

0, 3,

6 and 9 h after

feeding.

Liquid

and solid associated bacteria

_(LAB

and

SAB)

were extracted as described

by

Legay-Carmier and Bauchart

_(1989).

N was deter-mined

by

the

Kjeldahl

method.

Isotope-ratio

analyses

were

_{performed according}

to the

Kjel-dahl-Rittenberg

method

using

a double-collector mass

_{spectrometer (VG,}

SIRA

_12).

The

_isotopic

excesses

for LAB and SAB

averaged respectively

0.2430 ± 0.0247

and

0.1935 ±

0.0241 %. Rumen bacterial

N in the

MBM residues

was _very

low and

constant

_throughout

the

incubation

_period

(fig

1 As

a _consequence,

uncorrected

(Unc)

and corrected

_(Cor)

values of

_DT

_N

(k

= 6%)

for contamination

_by

SAB and

LAB

were

_respectively

54,

55 and

55%.

For

_WB,

rumen bacterial N increased with

time from

15 to

57%.

Also

Unc,

Cor

SAB

and

C

O

r

L

AB

values of

_DT

_N

were

_quite

different

(69,

78

and

_76%),

but

the

effect of

refer-ence bacterial

_sample

_(SAB

or

_LAB)

was

not

_important.

SBM had an intermediate be-haviour.

Until

16

h,

rumen

bacterial

N was

lower

than

11

%

but

_{substantially}

increased

after 24 and

48 h incubation

_(fig

₁

Howev-er,

DT

N

values were

_slightly

affected

(Unc

=

64,

Cor

SAB

= 67 and

Cor!B

=

66%).

In

conclusion,

the

BC of

_undegraded

residues

_markedly

affects the

_DT

_N

_only

for fibrous concentrate feedstuffs with a rather low N content. In our

_experiment,

the

choice of reference bacterial

_{sample only}

slightly

influenced

the results.

A

C

krlowledgmerlt -

This

_study

was

sup-ported by

the Institut pour

I’Encouragement

de la Recherche

Scientifique

dans I’Industrie et

l’Agriculture (Brussels, Belgium).

Orskov

ER,

McDonald

_{I (1979)}

J

_Agric

Sci

(Camb)

92, 499-503

Legay-Carmier

F,

Bauchart D

₍₁₉₈₉₎

Br J Nutr