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Original article
Effects of stage of harvest on the protein valueof fresh lucerne for ruminants
Javier GONZÁLEZ*, Jesús FARíA-MÁRMOL, Carlos A. RODRíGUEZ,María R. ALVIR
Departamento de Producción Animal, Escuela Técnica Superior de Ingenieros Agrónomos,Universidad Politécnica de Madrid, Ciudad Universitaria, 28040 Madrid, Spain
(Received 27 April 2001; accepted 13 September 2001)
Abstract — The ruminal degradation of dry matter (DM) and crude protein (CP) and the intestinalavailability of CP of four fresh lucerne (Medicago sativa L.) samples, corresponding to a 3rd grow-ing cycle and harvested at 2-week intervals, were determined. Rumen degradability, measured by thenylon bag technique, and rumen outflow rates were determined on three rumen-cannulated wethers.Intestinal digestibility was determined by the mobile bag technique on three duodenal fistulatedwethers. Both groups of animals were fed a 2:1 lucerne hay to concentrate diet at an intake level of40 g DM.kg–1 BW0.75. The effective degradability (ED) of DM decreased with maturity in linear andquadratic form, as a consequence of a decrease in the soluble fraction and a similar increase in theundegradable materials. The resultant values were 0.795, 0.661, 0.600, and 0.576 for harvests at 2,4, 6, and 8 weeks. The ED of CP showed the same trend. However, the variations (values of 0.896,0.832, 0.791, and 0.817, respectively), were moderate and mainly due to the reduction of the proportionof soluble CP. The intestinal digestibility of CP of all samples showed a downward trend with theincrease in the ruminal incubation time, as modelled according to a logistic function. The unde-graded CP digested in the gut (Di) and therefore the effective intestinal digestibility (EID) werederived from this function according to the rumen outflow of undegraded CP. The effects of matu-rity on the mean values of Di, expressed as a proportion of the original CP content, were the oppositeof those recorded for the ED of CP. These values were 0.067, 0.102, 0.115, and 0.089 for samples har-vested at 2, 4, 6, and 8 weeks, respectively. Nevertheless, when Di was expressed as g CP.kg–1 DM,these values (18.0, 17.4, 17.1, and 14.3, respectively) decreased in linear form. The same trend wasobserved for EID values, which represent 0.641, 0.609, 0.549, and 0.488, respectively. The changeof the digestion site produced by the reduction of ED of CP was also associated with an increase inthe undigested CP (values of 0.037, 0.066, 0.094, and 0.094, at the four harvesting times).
lucerne / stage of maturity / rumen degradability / intestinal digestibility / protein
Reprod. Nutr. Dev. 41 (2001) 381–392 381© INRA, EDP Sciences, 2002
* Correspondence and reprintsE-mail: [email protected]
J. González et al.382
1. INTRODUCTION
The sensible application of the new sys-tems for assessing protein nutrition in rumi-nants needs suitable information about therumen effective degradability and intesti-nal digestibility of the undegraded protein offeeds, as well as of the synthesis of microbialproteins promoted in the rumen. However,the variability existing for some of theseparameters within a given feed type is notusually considered in these systems. Lucerne(Medicago sativa L.) is a major forage forfeeding productive ruminants in temperatecountries. Nevertheless, its extensive rumendegradation may reduce its protein value asa consequence of important nitrogen lossesassociated with the ammonia absorptionfrom the rumen [9]. On the contrary, thestage of maturity is a main factor related tothe protein value of forages through its influ-
ence on microbial synthesis and the diges-tion site (rumen vs. small intestine) of thefeed nitrogenous compounds.
The mobile nylon bag technique, basedon the incubation of preincubated-rumensamples in bags inserted in the intestinesthrough a duodenal cannula, is a simple andadaptable method that estimates the intesti-nal digestibility of rumen undegraded pro-tein in feedstuffs. In vivo essays cannot carryout these systematic estimations, since it isan indirect, complex, laborious and time-consuming process. On the contrary, themobile bag technique can be used in sys-tematic studies with a large number of feeds.This technique can be used even if theintestinal digestibility values change withrumen preincubation time as occurs in manytypes of feeds, because the effectivedigestibility can be estimated based on solu-tions by integration of the functions which
Résumé — Effets du stade de maturité sur la valeur protéique du fourrage vert de luzerne.La dégradation dans le rumen de la matière sèche (MS) et des matières azotées totales (MAT), ainsique l’utilisation dans l’intestin des MAT de 4 fourrages de luzerne (Medicago sativa L.) recoltésen vert au 3e cycle à 2 semaines d’intervalle, ont été determinées. La dégradabilité effective (DE), mesu-rée en sachets de nylon, et le taux de sortie de particules du rumen ont été determinés sur 3 moutonsfistulés dans le rumen, tandis que la digestibilité dans l’intestin a été mesurée, à l’aide des sachetsmobiles, sur 3 moutons munis d’une canule du duodénum. Les deux groupes d’animaux ont été nour-ris avec une ration mixte de foin de luzerne et concentré (2:1 sur MS) distribuée à 40 g MS.kg–1 P0,75.La DE de la MS a diminué de façon linéaire et quadratique avec l’âge du fourrage, par suite d’une dimi-nution de la fraction soluble et d’une augmentation similaire de la fraction non dégradable. Lesvaleurs de DE ont été : 0,795, 0,661, 0,600 et 0,576 pour la fauche à 2, 4, 6 et 8 semaines. Une évo-lution similaire, mais plus modérée, a été observée pour la DE des MAT (valeurs de 0,896 ; 0,832 ;0,791 et 0,817, respectivement). Ceci est principalement la conséquence de la réduction des pro-portions des MAT solubles. L’augmentation du temps de séjour dans le rumen a entraîné pour tousles échantillons une diminution de la digestibilité dans l’intestin, qui a pu être convenablementdécrite par une fonction logistique. La proportion des MAT de l’aliment digérées dans l’intestin(Di) et la digestibilité intestinale effective (DIE) résultante ont été déduites de cette fonction et du fluxde sortie du rumen des MAT non dégradées. L’évolution en fonction de l’âge, des valeurs de Di– expri-mées comme proportion des MAT – a montré des effets inverses à ceux observés pour la DE des MAT.Ainsi, ces valeurs ont été : 0,067, 0,102, 0,115 et 0,089 pour les fourrages recoltés à 2, 4, 6 et8 semaines, respectivement. Par contre, si Di est exprimé en g MAT.kg–1 MS, les teneurs respectives(18,0, 17,4, 17,1 et 14,3) ont montré une diminution linéaire. Une évolution similaire a été observéepour les valeurs de DIE: 0,641, 0,609, 0,549 et 0,488, respectivement. Le déplacement du lieu de diges-tion, associé avec la réduction de la DE des MAT, donne lieu aussi à une augmentation des MAT nondigérées. Ainsi, ces valeurs représentent 0,037, 0,066, 0,094 et 0,094, respectivement.
luzerne / stade de maturité / dégradabilité ruminale / digestibilité intestinale / protéine
Protein value of fresh lucerne for ruminants
basal bud regrowth. These forage sampleswere frozen (–20 °C), freeze-dried andgrounded and passed through a 2-mm screenfor nylon bag assays and passed through a1-mm screen for chemical composition anal-yses (Tab. I).
2.2. Animals and feeding
Two groups of three wethers fitted withrumen cannulas (group 1) or “T” duodenalsimple cannulas (group 2) were used. Theanimals were housed in metabolism cagesand fed with a mixed diet consisting oflucerne hay and concentrate in a 2 to 1 ratio(on a DM basis). The hay and the concen-trate contained (per kg DM) 188 and 172 gof crude protein (CP), 488 and 188 g of neu-tral detergent fibre (NDF), 356 and 49 g ofacid detergent fibre (ADF), and 81 and 11 gof acid detergent lignin (ADL), respectively.This diet was offered at an intake level of40 g DM.kg–1 BW0.75 in two equal meals(8:00 and 16:00 h) from 15 days before andthroughout the experimental period.
2.3. Rumen degradability
Bags of 11 × 7 cm (inner dimensions)were made by heat-sealing (Preci-PackP30N, Dover Pack S.A., Barcelona, Spain)
describe this behaviour and the outflow fromthe rumen of undegraded protein [15]. Thesimplicity of this technique can be enhanced,in addition, by suppressing the incubationin the abomasum (or the pepsin–HCl prein-cubation), since different works have shownthat the lack of this step does not affect thevalues of intestinal protein digestion [7, 30,34]. Also, the recovery of bags directly fromthe faeces instead of the ileum does not rep-resent an important source of error [18, 34].
The objective of this research was to eval-uate the effect of maturity on the ruminaldegradation and the intestinal digestibility offresh lucerne.
2. MATERIALS AND METHODS
2.1. Lucerne samples
Four samples of fresh lucerne corre-sponding to a 3rd growing cycle were har-vested in a field located near Madrid (Spain)at intervals of two weeks from the previouscut (June 2 of 1998). The lucerne crop wasin its fourth growing season and was man-aged under irrigated on-farming conditions.The samples of 2, 4, 6 or 8 weeks were col-lected at vegetative, bud, full flowering, andearly pod setting stages, respectively. How-ever, the sample harvested at 8 weeks par-tially included vegetative biomass from
383
Table I. Chemical composition (g.kg–1 DM) of fresh lucerne samples.
Harvesting treatment (weeks)
Items 2 4 6 8
Organic matter 880 895 907 913 Crude protein 270 170 149 160 Neutral detergent fibre 294 411 471 488 Acid detergent fibre 242 329 369 379 Acid detergent lignin 60.5 71.7 78.6 72.1 NDIN1 14.1 16.2 11.2 11.7ADIN1 12.1 14.6 16.5 17.3
1 Percentage of total nitrogen.
J. González et al.
from nylon cloth with a pore size of 46 µm(Saatilon, SAATI Serigrafia Iberica S.A.,Almazora, Castellón, Spain). The bags werefilled with approximately 3 g (freeze-dried)of feed samples and incubated in the rumenof each animal of group 1 for intervals of2, 4, 8, 15, 24 and 48 h, with an additionalperiod of 72 h for the sample collected at8 weeks. Two series of incubation withduplicate bags were performed for each feed.At each series of incubation, all bags wereplaced simultaneously in the rumen justbefore the sheep were offered their first mealin the morning. After being collected fromthe rumen, the bags were washed with tapwater and stored at –20 °C. Later, the bagswere thawed and washed three times for5 minutes in a turbine washing machine.The same procedure was applied to twoseries of two bags of each feed to obtain thezero hours value. For each sheep and incu-bation time, one bag of each incubationseries was oven dried for 48 h at 80 °C andanalysed for DM and nitrogen. The otherbag was stored at –20 °C, freeze-dried anddestined to intestinal digestibility studies.
The disappearance of DM and CP fromthe nylon bags at each incubation time wascalculated from their respective amountsremaining after incubation in the rumen.The pattern of DM or CP disappearance (p)with the incubation time (t) was describedfor each animal using the model proposed byØrskov and McDonald [21]:
p = a + b (1 – e–kdt). (1)
In this model, the constants a and b repre-sent, respectively, the soluble fraction andthe non-soluble degradable component,which disappears at a constant fractional ratekd per unit of time. The fraction of undegrad-able CP (r) was estimated as 1 – (a + b).Effective degradability (ED) values wereestimated, in accordance with the sameauthors, using the passage rate through therumen (kp) of the lucerne hay included inthe diet, which had been marked by immer-sion with 10 mg Yb.g–1 of feed as describedby González et al. [14]. To determine
kp values, a pulse dose (40 g) of labelledlucerne hay was fed to each animal imme-diately before the first daily meal. A total of22 samples of faeces were obtained fromthe rectum of each animal, the first samplebefore supplying the marker and the remain-der between 12 and 144 h afterwards. Thesesamples were dried, milled and analysed forYb. The pattern of Yb concentrations in thefaeces over time was described for each ani-mal by fitting to the model of Dhanoa et al.[11] and rate constants derived from thedecreasing phase of concentrations wereused as kp values for all samples.
2.4. Intestinal digestibility
For each tested lucerne sample, thefreeze-dried residues were pooled for eachincubation time and the resulting sampleswere analysed for DM and nitrogen. Sixsub-samples of 0.2 g were taken from eachpooled sample and placed in heat-sealednylon bags of the above indicated materialhaving an approximately round shape (diam-eter 3 cm). Two bags from each rumen incu-bation time of each feed and three additionalempty bags (used as blanks) were insertedusing a spindle through the duodenal can-nula into the small intestine of each animalof group 2. Six bags per sheep per day wereinserted at random, at a rate of one bag every15 min. Once recovered from the faeces,the bags were washed with tap water anddeep-frozen. After thawing, they weremechanically washed, as indicated for thebags incubated in the rumen, and destinedintact to nitrogen analysis. Blanks contain-ing a known weight of nylon were used tocorrect for nitrogen content. The intestinaldisappearance (ID) of undegraded CP wascalculated as the amount of CP lost fromthe bag divided by the amount of CP in thebag before the intestinal passage. Then, theevolutions of ID values with the rumen pre-incubation time were fitted for each animalto a logistic function (see later) to determinethe effective intestinal digestibility by themethod proposed by González et al. [15].
384
Protein value of fresh lucerne for ruminants
as maturity progressed (Tab. I). Neverthe-less, in the last harvest time the concentra-tion of CP increased and that of ADLdecreased, probably due to the presence inthe sample of vegetative material corre-sponding to the regrowth of basal buds. Theobserved evolutions agree with previousresults [6, 12, 16] and should be related tothe main morphologic and chemical changesassociated with maturity such as the reduc-tion of the cell content to cell wall and leafto steam ratios. The evolution of the ADINproportions was similar to those of fibre.On the contrary, those of NDIN did notshow a clear trend with maturity.
3.2. Ruminal degradability
The parameters of the degradation kinet-ics and the effective degradabilities of DMand CP in the rumen are shown in Table II.Mean values and standard error of kp val-ues used to establish the ED estimates were0.0226 ± 0.0021 (h–1). These values agreewith those proposed by the AFRC [1] as afunction of the intake level and they are veryclose to those obtained in similar conditionsfor dehydrated lucerne by Repetto et al. [26].
As maturity advanced, opposite linearand quadratic variations for the soluble andundegradable DM fractions were observed,in agreement with the reduction of cell con-tents and the increase of cell walls andlignin. A slight variation of the potentiallydegradable materials was observed with aquadratic effect. A tendency (P = 0.055) toan opposite quadratic evolution was alsoobserved for the fractional degradation rateof this fraction. Therefore, the linear andquadratic decrease recorded for the ED ofDM (from 0.795 to 0.576) was only the con-sequence of the variations induced by matu-rity on the a and r fractions. The decreaseobserved for the ED of DM with maturityagrees with other reported results [6, 12, 16]and showed a reduction of the fermentationlevel and therefore of the rumen microbialsynthesis promoted by fresh lucerne.
2.5. Chemical analyses
Samples of tested lucerne were analysedfor DM, ash, and CP (Kjeldahl N × 6.25)by AOAC methods [3] and for NDF [31],ADF, and ADL [27]. Insoluble nitrogen inneutral detergent (NDIN) and in acid deter-gent (ADIN) solutions was determined byKjeldahl analysis of the NDF and ADFresidues, respectively. Rumen incubationresidues and intestinal mobile nylon bagswere also analysed for nitrogen by the Kjel-dahl method. Samples of faeces collectedfor transit studies were analysed for ytter-bium by atomic absorption spectrometry asdescribed by González et al. [14].
2.6. Statistical methods
The different kinetics associated with theemployed models were fitted using a non-linear regression programme. Statisticalanalyses of rumen degradation parameterswere made by variance analysis with ani-mals and samples as factors in the model.Then, linear and quadratic effects of matu-rity were established by orthogonal poly-nomials. The results of intestinal digestibil-ity for each lucerne sample were analysedconsidering animals and rumen pre-incuba-tion times as factors in the model, beforemodelling these data. Then, maturity effectswere studied on the effective parameters ofintestinal digestion as indicated above forrumen studies. All the statistical analyseswere performed using the Statistical Anal-ysis System for Windows software, version6.12 (SAS Institute Inc., Cary, NC, USA).
3. RESULTS AND DISCUSSION
3.1. Chemical composition of lucernesamples
Maturity leads to a decrease in the CPconcentration and to an increase in the con-tents of OM and fibre fractions, but the rateof variation of all these parameters decreased
385
J. González et al.386
Tab
le I
I.M
ean
valu
es o
f rum
en d
egra
datio
n pa
ram
eter
s an
d ef
fect
ive
degr
adab
ility
of d
ry m
atte
r and
cru
de p
rote
in o
f fre
sh lu
cern
e sa
mpl
es (v
alue
s ar
eex
pres
sed
as a
pro
port
ion
of th
e or
igin
al f
eed)
.
Har
vest
ing
trea
tmen
t (w
eeks
)E
ffec
ts
Item
2 4
68
M.S
.E.
L
Q
Dry
mat
ter
a 0.
506
0.37
0 0.
314
0.30
20.
0033
< 0
.001
< 0
.001
b0.
355
0.36
90.
365
0.33
50.
0082
0.
135
0.03
9
r0.
139
0.26
20.
321
0.36
30.
0059
< 0
.001
0.00
5
k d(p
er h
)0.
104
0.08
40.
085
0.10
30.
0079
0.93
30.
055
ED
0.79
5 0.
661
0.60
0 0.
576
0.00
48
< 0
.001
<
0.0
01
Cru
de p
rote
in
a0.
595
0.47
4 0.
471
0.51
5 0.
0052
<
0.0
01<
0.0
01
b 0.
342
0.41
10.
374
0.34
80.
0149
0.78
10.
019
r 0.
064
0.11
40.
156
0.13
7 0.
0152
0.00
90.
063
k d(p
er h
)0.
179
0.14
90.
140
0.16
80.
0299
0.76
9 0.
372
ED
0.89
6 0.
832
0.79
10.
817
0.00
96
< 0
.001
0.
003
a, b
, and
rre
pres
ent s
olub
le, n
on-s
olub
le d
egra
dabl
e, a
nd u
ndeg
rada
ble
frac
tions
, res
pect
ivel
y,k d:
fra
ctio
nal d
egra
datio
n ra
te o
f fr
actio
n b.
ED
: eff
ectiv
e de
grad
abili
ty. M
.S.E
.: m
ean
stan
dard
err
or. L
and
Q: s
igni
fica
nce
of li
near
and
qua
drat
ic e
ffec
ts, r
espe
ctiv
ely.
Protein value of fresh lucerne for ruminants
samples and their ED, values of the rumenescape protein of 28, 28.5, 31, and 29 gCP.kg–1 DM were estimated for samplesharvested at 2, 4, 6 and 8 weeks.
3.3. Intestinal digestibilityof undegraded crude protein
The effect of ruminal pre-incubationtime on ID of undegraded CP is shown inTable III. As previously indicated, the 72-hrumen incubation was only considered nec-essary for the 8-week sample. A decreasein the ID values with the increase of rumenincubation time was observed for all sam-ples. This reduction should be a consequenceof the progressive enrichment in indigestiblenitrogenous compounds of feed particlesproduced by the extent of rumen degradation[15]. In this last study, the use of an expo-nential equation to fit these changes hasbeen proposed in order to obtain the effec-tive proportion of feed protein digested inthe intestine, which is carried out consider-ing both this function and that whichdescribes the outflow of undegraded pro-tein from the rumen. Nevertheless, for sam-ples harvested at 2 and 4 weeks, the resultsrecorded up to 2 h were similar. In theseconditions, the use of an exponential modelwould lead to an overestimation of the ini-tial intestinal digestibility and therefore ofthe effective proportion of feed proteindigested in the intestines. For this reason,the evolution of these values for all sam-ples was fitted to a logistic equation:
ID = 1 – n (n + h) . (2)n + he–kit
In this model, the 1 – n value represents theintestinal digestibility of the total insolublefeed protein, whereas the asymptotic value1 – (n + h) corresponds to the intestinaldigestibility of the protein fraction that wasapparently undegradable in the rumen. Theconstant ki is a rate of decrease of intesti-nal digestibility derived from the sampleenrichment in undigestible compounds with
Effects of maturity on a and r fractions ofCP showed a similar trend as for DM. How-ever, the extreme values for both parame-ters were obtained in this case for the harvestat 6 weeks, suggesting an offsetting effect ofthe lucerne regrowth observed at 8 weeks. Inaddition, the increase of the undegradablefraction was lower than for DM, speciallyconsidering that an important part of thisfraction is composed by adherent microor-ganisms. Therefore, based on the predictiveequation obtained in similar conditions byRodríguez et al. [29], this microbial pro-portion increases with maturity and repre-sents 37.6, 53.4, 61.6, and 61.6% for 2, 4, 6,and 8 weeks, respectively. Therefore, therespective corrected fractions of undegrad-able CP from the lucerne samples were0.040, 0.053, 0.060 and 0.053. The evolutionof the potentially degradable fraction (b)showed, as for DM, a quadratic effect. Nev-ertheless, the variation between samplesfor these values or for their contribution toED values was greater than for DM. Matu-rity had a linear and quadratic effect on theED of CP. However, this variation was mod-erate (from 0.896 to 0.791), and even moreso when the values were corrected by micro-bial colonisation using the equation ofRodríguez et al. [28]: 0.923, 0.886, 0.854and 0.880 for 2, 4, 6, and 8 weeks at har-vest, respectively. The mean value of appar-ent ED of CP compares closely withreported in situ [4, 5, 6, 12, 26] or in vivo[13] values. In addition, the effect of matu-rity is in agreement with that of previousresults [2, 6, 12, 16].
The results of this experiment showedthat plant maturity leads to an importantdecrease of rumen use of the whole forage bythe reduction of the cell contents to cell wallsratio, but it only has a little effect on therumen degradation of CP. The results alsoshowed that the contents of insoluble by-pass protein in fresh lucerne is small andthey support the observation of Elizalde et al.[12] that the rumen escape protein of freshlucerne is not affected by maturity. So, con-sidering the CP concentration of the
387
J. González et al.
the extent of rumen degradative actions. Theuse of this model showed a better fitting forall samples than an exponential equation,although the more important differencesbetween both models were obtained for thesamples harvested at 2 and 4 weeks. Meanvalues of these parameters (n, h, and ki) forthe different samples are also shown inTable III.
The utilisation of a function to describethe evolution of the ID of undegradedresidues allows the estimation of the effec-tive intestinal digestibility from this func-tion and from the post-ruminal flow of unde-graded CP. González et al. [15] haveestablished this last function consideringthat undegraded nitrogen is defined by u = r+ b e–kd t and that the rumen outflow of anyfeed constituent is defined by f = 1 – e–kp t.Thus, the corrected outflow rate from therumen undegraded nitrogen is u(df/dt).Therefore, the corrected rate of digestedprotein in the intestine can by obtained asIDu(df/dt), and their cumulative proportionup to time t (Di(t)) can be derived from:
Di(t) = IDu df
dt = (1 – n (n + h) )
Di(t) = ∫ IDu dt dt = ∫ (1 – n + re–kit
Di(t) × (r + be–kdt) kpe–kpt dt. (3)
As the time from feeding increases, the frac-tion of CP flowing into the intestines tendsto zero, so that the proportion of CP fromthe feed digested in the intestines tends to:
Di = r + bkp
–∫n (n + h)
(r+be–kdt)
= ∫ IDukd
+ kp n + he–kitdt = ∫ (1 – × kpe–kpt dt. (4)
The primitive function of the last term ofthis equation has not been found. Never-theless, it can be easily calculated by numer-ical approximation using a mathematicalsoftware such as Derive 2 (Soft WarehouseInc., Honolulu, USA).
From the resultant value of equation (4),the effective intestinal digestibility of
undegraded nitrogen (EID) can be obtained as:
EID = Di
EID = (1 – ED) .
(5)
The important variations of ID associatedwith the rumen incubation time (Tab. III)show that the estimation of this parameterbased on a single rumen incubation timecan lead to errors. The use of a function todescribe the evolution of the ID of CP inrumen undegraded residues together withestimations of the ruminal flow of unde-graded CP allows for the estimation of theprotein effectively digested (Di) and solvesthe problems of accuracy derived from theuse of a fixed but arbitrary time of ruminalpre-incubation.
Mean values of the initial digestibility(1 – n) and the final digestibility (1 – (n + h))showed a linear decrease in all samples withmaturity (Tab. IV). The rate of changebetween initial and final values (ki) alsoshowed a linear downward trend with matu-rity. Nevertheless, this reduction was mainlyobserved between the sample harvested at2 weeks and the remainder samples as indi-cated by the quadratic tendency (P = 0.06)observed. The values of Di varied in linearand quadratic form, showing its maximumfor the 6 weeks harvest. This variation wasopposite that observed for the ED of CPand evidenced a change of the digestionsite from the rumen to the intestines withthe reduction of ruminal degradability. Thesame effect was also observed on differentforages [32] or when the reduction of theED of CP was produced by the thermictreatment of different concentrates [15, 22,24, 25].
When Di values were expressed as con-centration (g CP.kg–1 DM), a linear decreasewas observed with maturity (Tab. IV). Nev-ertheless, this reduction was small andmainly associated with the last harvesttreatment. This low variation shows that dif-ferences due to lucerne quality would bemainly derived from the synthesis of micro-bial protein. A superior forage quality is notonly associated with higher levels of rumen
388
0
∫
t t
0
∫
t
0
∫
∞
n + re–kitd
Protein value of fresh lucerne for ruminants
could be expected if a fraction of the feedsoluble CP escapes rumen degradation.Aufrère et al. [5] observed an accumulationof protein in the rumen liquid fluid of sheepfed fresh lucerne ad libitum, which repre-sent some evidence of rumen escape of sol-uble proteins. However, this accumulationwas small, short and associated with a largermeal. Therefore, the contribution of the sol-uble proteins to the EID value should below. On the contrary, the mathematical cal-culations from present results of intestinaldigestibility for total protein (undegradedprotein plus microbial protein) are higherthan in vivo measurements. So, assumingthat the effective degradability of OM shouldbe similar to that of DM and based on thespecifications of the PDI system [33], theestimated mean value for the intestinaldigestibility of total protein was 0.730, whichis higher than the value (0.587) recorded by
fermentation and therefore of microbial syn-thesis, but also with a higher intake and,consequently, with an additional increaseof the microbial protein synthesised. Thelow values obtained in all samples for thetotal protein digested in the intestine alsoshow that the main part of the supply ofamino acids to the animal from this forage isderived from microbial protein.
Maturity was associated with a linearreduction of the EID (Tab. IV), at a meanrate of 2.6 percentage points per week. Thiseffect shows that intestinal digestibility offresh lucerne cannot be considered as a con-stant value. Present EID values were lowerthan those estimated in an indirect form byINRA, who propose intestinal digestibilityvalues of 0.75 for fresh forages [33]. Valuesof intestinal digestibility higher than thoseobtained with the mobile bag technique
389
Table III. Effect of rumen incubation time on intestinal digestibility (ID) of undegraded crude pro-tein and resulting fitting parameters for different fresh lucerne samples.
Harvesting treatment (weeks)
RPT (h) 2 4 6 8
0 0.876 0.784 0.760 0.720 2 0.851 0.790 0.707 0.547 4 0.812 0.715 0.725 0.633 8 0.692 0.729 0.616 0.634 15 0.519 0.517 0.572 0.47124 0.533 0.548 0.424 0.419 48 0.531 0.498 0.464 0.39672 – – – 0.384M.S.E. 0.0174 0.0248 0.0500 0.0395Significance < 0.001 < 0.001 0.003 < 0.001
Fitting parameters1
n 0.090 0.193 0.235 0.291 h 0.402 0.343 0.327 0.347 ki 0.276 0.124 0.128 0.101
RPT: rumen pre-incubation time.M.S.E.: mean standard error.1 Parameters obtained by fitting the equation ID = 1 –
n (n + h); where, 1– n: intestinal digestibility of total
Di(t) = ∫ n + he–ki t
insoluble feed CP, 1– (n + h): intestinal digestibility of rumen undegradable CP; ki: rate of decrease of the intesti-nal digestibility.
J. González et al.390
Tab
le I
V.T
he e
ffec
ts o
f m
atur
ity o
n in
test
inal
dig
estio
n of
cru
de p
rote
in o
f fr
esh
luce
rne
(val
ues
are
expr
esse
d as
pro
port
ions
of
the
orig
inal
fee
d C
P).
Har
vest
ing
trea
tmen
t (w
eeks
)E
ffec
ts
Item
2 4
68
M.S
.E.
L
Q
Initi
al d
iges
tibili
ty (
1 –
n)
0.91
0 0.
807
0.76
50.
709
0.02
60
0.00
2 0.
409
Fina
l dig
estib
ility
(1–
(n +
h))
0.
508
0.46
40.
439
0.36
20.
0168
<
0.0
010.
362
k i(h
–1)
0.27
60.
124
0.12
8 0.
101
0.02
710.
005
0.06
0
Inte
stin
al d
iges
ted
CP
(Di)
– as
pro
port
ion
of o
rigi
nal C
P0.
067
0.10
2 0.
115
0.08
90.
0044
0.
006
< 0
.001
–
as g
. kg–1
DM
18.0
17
.417
.1
14.3
0.67
0.
010
0.15
2
Eff
ectiv
e di
gest
ibili
ty (
EID
) 0.
641
0.60
9 0.
549
0.48
8 0.
0224
0.00
2 0.
543
Who
le tr
act u
ndig
este
d C
P0.
037
0.06
60.
094
0.09
40.
0044
<
0.0
010.
016
Protein value of fresh lucerne for ruminants
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4. CONCLUSION
The ED of fresh lucerne markedlydecreased with maturity for DM and onlyslightly for CP. Considering both effectsand considering the high ED of CP, it can bestated that the differences associated withlucerne quality are mainly related to themicrobial protein synthesis promoted in therumen. Although the reductions of proteindegradability produced by maturity shiftedthe site of protein digestion from the rumento the intestine, the progression of maturityalso implied an increase of the undigestedCP and, therefore, a reduction of its EID.Nevertheless, the most important changesassociated with the protein value took placeup to 4 weeks and, consequently, for theusual harvesting stages, the negative effectof maturity on the total supply of intestinaldigestible proteins seems to be small.
ACKNOWLEDGEMENT
This work has been supported by the CICYTfunded Project AGF 1998-0842.
391
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