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Original article Influence of the method of forage conservation on feeding behaviour, intake and characteristics of reticulo-rumen content, in sheep fed ad libitum V Chiofalo JP Dulphy R Baumont M Jailler JM Ballet 1 Istituto d i Zootechnica, Via S, Cecilia 30,98100 Messina /fa/ /; 2 INRA, CRZV de Theix, Station de Recherches sur la Nutrition des Herbivores, 63122 Saint-Genès-Champanelle, France (Received 10 February 1992; accepted 2 July 1992) Summary ― The influence of silage conservation methods on eating behaviour and the characteris- tics of reticulo-rumen contents was studied in sheep by comparing 3 forages, a silage without additive (WAS), one with additive (FAS) and hay (H), prepared from the same cut green forage. The sheep were rumen fistulated. WAS was very badly and FAS poorly preserved while the hay was dried under favourable conditions. The forages were fed ad libitum and the dry matter (DM) intake was 1 054, 1 241 and 1 469 g/day for WAS, FAS and H respectively. There was a single feeding in the morning. At the main meal, DM intake was 270, 317 and 388 g/day and ingestion rate 4.76, 4.56 and 4.16 g of DM/min for WAS, FAS and H respectively. There was slight recovery in ingestion around 16-19 h with hay and FAS but not with WAS. With hay, rumination lasted much longer than with the silages and be- gan sooner after the end of the main meal. In contrast, overall rumination efficiency was the same for hay and FAS. There were fewer contractions of the reticulo-rumen with both WAS and hay. The amounts of reticulo-rumen contents were comparable for the 2 silages and higher for hay. The amounts of NH 3 in the contents were the same with WAS and hay but greater with F4S. With WAS, contents were richer in butyric, valeric and caproic acids. The DM turnover rate of the contents was the same for FAS and hay and lower, but not significantly, for WAS. Accordingly, with silage, satiety seems to be rapidly reached but there was no evidence that organoleptic factors were involved. There were no problems with the digestion rate of silages. However, the factors limiting silage intake persisted throughout the diumal cycle, and reticulo-rumen fill was affected. The more poorly preserved the silage the more these factors reduced the duration and volume of the meals, or both. intake / feeding behaviour / rumen content / forage conservation / sheep Résumé ― Influence de la méthode de conservation du fourrage sur le comportement alimen- taire, l’ingestion et les caractéristiques du contenu réticulo-ruminal, chez le mouton nourri à volonté. L’influence du mode de conservation sur les activités alimentaires et les caractéristiques du contenu réticulo-ruminal a été étudié chez le mouton par comparaison de 3 fourrages (un ensilage * Correspondence and reprints
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Page 1: forage on feeding behaviour, characteristics content ...

Original article

Influence of the method of forage conservationon feeding behaviour, intake and characteristicsof reticulo-rumen content, in sheep fed ad libitum

V Chiofalo JP Dulphy R Baumont

M Jailler JM Ballet

1 Istituto di Zootechnica, Via S, Cecilia 30,98100 Messina /fa//;2 INRA, CRZV de Theix, Station de Recherches sur la Nutrition des Herbivores,

63122 Saint-Genès-Champanelle, France

(Received 10 February 1992; accepted 2 July 1992)

Summary ― The influence of silage conservation methods on eating behaviour and the characteris-tics of reticulo-rumen contents was studied in sheep by comparing 3 forages, a silage without additive(WAS), one with additive (FAS) and hay (H), prepared from the same cut green forage. The sheepwere rumen fistulated. WAS was very badly and FAS poorly preserved while the hay was dried underfavourable conditions. The forages were fed ad libitum and the dry matter (DM) intake was 1 054,1 241 and 1 469 g/day for WAS, FAS and H respectively. There was a single feeding in the morning.At the main meal, DM intake was 270, 317 and 388 g/day and ingestion rate 4.76, 4.56 and 4.16 g ofDM/min for WAS, FAS and H respectively. There was slight recovery in ingestion around 16-19 h withhay and FAS but not with WAS. With hay, rumination lasted much longer than with the silages and be-gan sooner after the end of the main meal. In contrast, overall rumination efficiency was the same forhay and FAS. There were fewer contractions of the reticulo-rumen with both WAS and hay. Theamounts of reticulo-rumen contents were comparable for the 2 silages and higher for hay. Theamounts of NH3 in the contents were the same with WAS and hay but greater with F4S. With WAS,contents were richer in butyric, valeric and caproic acids. The DM turnover rate of the contents was thesame for FAS and hay and lower, but not significantly, for WAS. Accordingly, with silage, satiety seemsto be rapidly reached but there was no evidence that organoleptic factors were involved. There wereno problems with the digestion rate of silages. However, the factors limiting silage intake persistedthroughout the diumal cycle, and reticulo-rumen fill was affected. The more poorly preserved the silagethe more these factors reduced the duration and volume of the meals, or both.

intake / feeding behaviour / rumen content / forage conservation / sheep

Résumé ― Influence de la méthode de conservation du fourrage sur le comportement alimen-taire, l’ingestion et les caractéristiques du contenu réticulo-ruminal, chez le mouton nourri àvolonté. L’influence du mode de conservation sur les activités alimentaires et les caractéristiques ducontenu réticulo-ruminal a été étudié chez le mouton par comparaison de 3 fourrages (un ensilage

* Correspondence and reprints

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sans conservateur WAS, un ensilage avec conservateur FAS et un foin H) préparés à partir du mêmefourrage vert sur pied. Les moutons étaient munis d’une fistule du rumen. L’ensilage WAS a été très malconservé et l’ensilage FAS a eu une qualité de conservation médiocre alors que le foin avait été séchétrès correctement. Dans ces conditions, les quantités de MS ingérées ad libitum ont été respectivementégales à 1054, 1241 ef 1469 9o. Les fourrages ont été distribués en une seule fois le matin. Lors dugrand repas les quantités de MS ingérées ont été de 270, 317 et 388 9o alors que les vitesses d inges-tion étaient respectivement de 4,76, 4,56 et 4,16 g de MS/min. Au cours de la joumée une très légèrereprise de l’ingestion a été observée vers 16-19 h pour le foin, également pour l’ensilage FAS, mais paspour l’ensilage WAS. Avec le foin, la durée de rumination a été nettement plus élevée qu’avec les ensi-lages et cette rumination a démarré toujours plus vite après la fin du grand repas. Par contre, l’efficacitéglobale de la rumination a été la même pour le foin et l’ensilage FAS. Il y a eu moins de contractions duréseau avec l’ensilage WAS, mais également avec le foin. Les quantités de contenu du réticulo-rumenont été comparables pour les 2 ensilages et plus élevées pour le foin. Dans les contenus réticulo-ruminaux les teneurs en NH3 ont été identiques pour l’ensilage WAS et le foin, mais plus élevées pourl’ensilage FAS. La plus importante augmentation au cours du repas a été observée avec l’ensilageWAS. Avec cet ensilage, les contenus ont été plus riches en acides butyrique, valérianique et caprofque. Enfin, le taux de renouvellement de la matière sèche des contenus a été identique pour l’ensilageFAS et le foin et plus faible, mais non significativement, pour l’ensilage WAS. L ingestion d’ensilage en-traîne donc un rassasiement rapide des animaux sans que, apparemment, des facteurs organolepti-ques ne soient en cause. Il n ÿ a pas non plus de problèmes dans la vitesse de digestion des ensilages.Par contre, les facteurs limitant l’ingestion des ensilages ont un effet permanent tout au long du nycthé-mère. Cet effet limite le remplissage du réticulo-rumen. Les facteurs en cause réduisent d autant plus ladurée etlou le volume des repas que les ensilages sont plus mal conservés.

ingestion / comportement alimentaire lrumen / conservation de fourrage / mouton

INTRODUCTION

In Europe, grass silage is increasingly pre-ferred to hay as animal feed. The variationsin intake between these two types of foragedepend on how finely they are chopped,how well they are preserved and on theirdry matter (DM) content. All these factors

have been thoroughly studied (Demarquillyet al, 1981; Dulphy and Michalet-Doreau,1981; Dulphy and Demarquilly, 1991). How-ever, despite the large number of publishedreports, the mechanisms involved in the

regulation of silage intake have not yetbeen completely elucidated. Two main ap-proaches, descriptive or experimental,emerge from studies in this field. The first isbased essentially on 3 complementarymethods of investigation: 1) examination ofthe regressions between the intake and

characteristics of silage (Wilkins etal, 1971;Demarquilly, 1973); 2) observation of eating

behaviour (Deswysen et al, 1978; Dulphy etal, 1984); 3) observation of the extent of re-ticulo-rumen fill (Campling, 1966; Dulphy etal, 1975a; Thiago and Gill, 1986). The sec-ond approach is based on the study of theeffects of experimental modifications to si-

lage characteristics: chopping of silage afterremoval from the silo (Dulphy and Demar-quilly, 1973) and addition of different acidsof nitrogen compounds at the time of feed-ing (Hutchinson and Wilkins, 1971; MartinClancy et al, 1977; Buchanan-Smith andPhilipp, 1986; Buchanan-Smith, 1990).

In sheep receiving grass silage, the

amounts of DM ingested during the mainmeal after distribution were low (Dulphy,1985) but this was offset by an increase inthe number of small meals taken duringthe day, if green forage (Dulphy et al,1984) or hay (Dulphy et al, 1975a) weretaken as controls. These findings suggestthat the animals did not use their rumen

capacity to the full (Thiago and Gill, 1986).

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In the present trial we studied both thebehaviour of the animals and the physicalas well as the chemical characteristics ofruminal digestion in relation to the natureof the preserved forage. For that purposewe compared 3 preserved forages, all pre-pared at the same time from the same

green plant, but with very different charac-teristics: poorly preserved silage, well pre-served silage and a good hay. Our objec-tive was to try to understand the dailyvoluntary intake of these 3 forages by theanalysis of the kinetics of intake, of the eat-ing and ruminating activities, of the extentof the rumen fill and of the characteristicsof reticulo-rumen content. This approach isnot common in the literature and is usedhere to study the effects of silage (by com-parison to hay) and of quality of silagepreservation.

MATERIAL AND METHODS

Animals and feeds

In July 1990, a cocksfoot meadow in the secondcycle of vegetation was mown to preparethree differently preserved forages: a direct-cutsilage, without additive and harvested with a

precision chop forage harvester (WAS); thesame forage, but treated with 4.4 1 of formic acid(FAS); and hay (H), field cured in favourableconditions.

The 3 forages were distributed from Septem-ber to December to six 4-year-old castrated Tex-el sheep (initial live weight 60 ± 3 kg) fitted witha rumen cannula 75 mm in diameter.

The required amounts of silage were re-

moved from the silo twice a week and storedat +4 °C. Throughout the experimental periodthe animals were maintained with lights offfrom 21.00 h to 07.00 h and with lights on for therest of the time, to achieve a constant photoperi-od. The animals were fed ad libitum (10% refu-sal) with a single feeding a day at 8.30 h. Theyhad free access to the forages, water and saltlicks.

Experimental design

The experimental design was a replicated 3 x 3Latin square, using 2 sheep simultaneously pertreatment. Each of the 3 periods lasted 5 weeks,divided as follows: 1 week of adaptation to thediet; 1 week of adaptation to the metabolism

crates; 1 week of measuring digestibility, the kin-etics of intake during the day and eating and ru-minating behavior; 2 weeks with the animals stillin metabolism crates, during which 4 completeemptyings of the reticulo-rumen were performed.

Measurements

The digestibility of the 3 forages in vivo was de-termined by collecting the total faecal produc-tion. Concomitantly, but in 3 different sheep re-ceiving a good quality lucerne hay, the kineticsof the DM degradation of the 3 forages wasstudied in situ (Demarquilly and Chenost, 1969)at incubation times in the rumen of 0, 4, 8, 16,24, 48 and 72 h. On removal from the rumen,the nylon bags (2 per incubation period and persheep) were washed in cold water and placed ina pepsin solution for 48 h.

Daily intake was calculated by measuring thedifference between the amounts offered and re-fused. Mangers were placed on sensors fittedwith strain gauges (Baumont et al, 1988) coupledto a computer so that the intake could be deter-mined for each meal throughout the day withoutdisturbing the animals. At the same flme, eatingand ruminating behaviour was observed accord-ing to the technique of Ruckebush (1963), asadapted by Baumont et al (1988). The pressuresignals transmitted by a balloon filled with poly-urethane foam were immediately read into the

computer and analyzed with software developedby Brun et al (1984). Likewise, we recorded themotility of the reticulo-rumen by means of anotherballoon placed in the dorsal sac of the rumen andconnected to a pressure transducer.

The total reticulo-rumen contents were with-drawn by hand at 8.30, 10.30, 15.30 and 20.30 h.Emptyings were made at intervals of at least 72h, to allow digestion to retum to normal (Aitchi-son, 1985). For a given time, all animals weretreated together. All these contents were weighedand sampled after homogenization to determineDM content (48 h at 80 °C), dry samples beingkept for analysis, particle size, and to measurepH, VFA (volatile fatty acids) and ammonia con-tent, osmotic pressure and viscosity in the fluid.

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The disappearance rate of digesta was calcu-lated in g/h between 2 different times accordingto the formula: disappearance rate = (initial con-tent + amount ingested-final content) / duration.

Chemical analyses

During the measurement periods, a representa-tive sample of silage and hay was taken at eachfeeding. The DM content of the samples wasdetermined after drying at 80 °C for 48 h andcorrected for silage according to the recommen-dations of Dulphy et al (1975b), which take intoaccount losses in volatile DM during oven dry-ing. Crude protein content was determined inundried silage. The other analyses were madeon samples after drying: ash was determined af-ter incineration for 6 h at 550 °C, nitrogen by themethod of Kjeldahl and total cell walls (NDF) ac-cording to Goering and Van Soest (1970).

The fermentation characteristics of the silag-es (pH, VFA, lactic acids, ammonia and solublenitrogen) were determined in fluid expressedunder pressure. Ammonia was immediately as-sayed according to the method of Conway(1957) and soluble nitrogen by that of Kjeldahl.The fluid sample for lactic acid assay was con-served frozen without additives and then anal-

yzed by the method of Noll (1974). Another fluidsample was treated with orthophosphoric acidand stabilizing agent and frozen for VFA and al-cohol assay (Jouany, 1981 ).

The characteristics of the reticulo-rumen con-tents were determined either overall (particlesize) or on dry matter, or on fluid filtered througha 1-mm mesh grid. Particle size was estimatedafter sieve separation in a liquid flow accordingto the method of Grenet (1970). The dry sam-ples were ground (0.8 mm grid) for estimation ofthe plant cell walls. In the fluid, VFA were anal-yzed according to the method of Jouany (1981)and ammonia by the method of Berthelot (fluidtreated with NaCl and frozen). In the fluid, wealso estimated osmotic pressure by cryometry,and viscosity from the flow time into a capillarytube (Dardillat and Baumont, 1992).

Statistical analysisStatistical significance of the feed intake, eatingand ruminating activities, motility of reticulo-

rumen, extent of rumen fill, characteristics of thereticulo-rumen contents and some parametersof the digestion were tested using an analysis of

variance with the effects of animal (5 DF), typeof forage (2 DF) and period (2 DF) and with 8DF in the error term. The calculations were per-formed using the general linear model proce-dure of the Statistical Analysis System Institute(SAS, 1985).

RESULTS

Chemical compositionand digestibility of forages

Table I shows the chemical composition ofthe 3 forages. The 2 silages were verywet. Overall, ash content was rather highand crude protein rather low for regrowths(Andrieu et al, 1988). In vivo digestibili-ty of the silage with additives was slightlyhigher than that of the other 2 forages(+ 3.6 points for OM). The kinetics of di-gestion in sacco, in the rumen, were al-most identical: the variables A and B, cal-culated from the model of Qlrskov and MacDonald (1979) were comparable for the 2silages. In contrast, the value of B waslower for hay. The degradation rate C washighest for silage with FAS and lowest forWAS.

Table II shows the fermentation charac-teristics for the 2 silages. The silage with-out additive contained practically no lacticacid but a high level of VFA, including 9 gof propionic lactic acid, 5.5 g of valeric acidand 10.5 g of hexanoic acid per kg DM.

Intake, eating behaviourand motility of the reticulo-rumen

Silage intake was considerably smallerthan hay intake (table 111). Although the in-take of silage with additive was markedlyhigher (+ 18%) than that of silage without,there was no significant difference be-

tween the 2. In relation to the metabolic

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weight of the sheep, the amounts ingestedwere 48.8, 57.6 and 66.2 g of DM/kgW0.75 respectively for WAS, FAS and H.

The daily time spent eating hay waslonger than with the silages (+ 25%) butthere was less difference between inges-tion rates (+ 12% between WAS and H,which is not significant).

Intake during the main meal was signifi-cantly higher for hay than for the silages,+ 22 and 44%, compared to FAS and WASrespectively. The rates of intake were com-parable, although slightly lower for hay, sothat the duration of the main meals differed

significantly according to the treatment. In

contrast, these rates were much higher forsilages at the beginning of the main meal.For hay, they were regular throughout themeal.

There was no significant difference be-tween the number of secondary meals aday.

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The daily duration of rumination variedsignificantly, with a difference of 28% be-tween WAS and H.

Figure 1 shows the daily pattern of eat-ing and rumination at 1 h intervals. The re-sults confirm the importance of the mainmeal for hay intake. They also show a sig-nificant recovery in hay intake between 16 6and 19.00 h. There was a similar recoveryfor FAS only at 18.00 h, but not for WAS.

Although forage was distributed onlyonce, there were 2 peaks of rumination in

the day (fig 1 With hay, the latency timewas shorter and rumination always beganmore quickly (table 111). The rumination cy-cles had an average duration of 50 s and

although the difference in times between

FAS and H was significant, the values

measured were very similar.

The time spent chewing daily was com-parable on the 2 silages (on average 750min) but significantly higher (23%) with

hay. In contrast, chewing efficiency wasthe same for FAS and H and slightly lower

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(-8%) for WAS. However, if we report thetime spent ruminating per unit DM digestapresent in the reticulo-rumen we obtain thesame figure for the 3 forages: 0.36 min/gDM digesta.

Table IV shows the number of contrac-tions of the reticulo-rumen per min. Over-

all, this number was higher for FAS andlower for hay. If the numbers are offset bytaking account of the real duration of eachactivity, we find 1 741, 1 830 and 1 752 of

type A contractions per day for WAS, FASand H respectively. The values for the totalcontractions were 2 893, 3 181 and 3 052.

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Extent of reticulo-rumen fill

and physical characteristicsof the contents

Table V shows the amounts of fresh mat-

ter, DM and total cell wall contents. Therewas no significant difference between the2 silages: an average of 10 266 g freshmatter for WAS and 10 668 g for FAS

(+ 4%) and 1 153 g DM for WAS and1 224 g for FAS (+ 6%). The values forhay were much higher, with significant dif-ferences at 10.30 and 15.30 h comparedto silages (on average, 11 988 g freshmatter and 1 479 g DM over the day). Onthe basis of total cell wall contents, the re-ticulo-rumen contents were higher with

hay (on average, 994 g vs 775 and 814 gwith WAS and FAS).

The DM level of the contents was high-est with hay, but overall there was little dif-ference between the forages (table VI).The levels were low before the morning

meal (107 g/fresh kg) and remained steadyat 119 and 123 g after the main meal andin the evening respectively (fig 2). The cellwall levels of the contents were very simi-lar with only slight variations during theday: an average maximum of 677 at

15.30 h and a minimum of 656 g/kg DM at20.30 h.

There was little difference between the2 silages in the size of ruminal particles(table VI), except for a slight inversionbetween the small particles (0.05-1 mm)and the soluble compounds with the latterbeing a little higher with the well preservedsilage. In contrast there were fewer largeparticles with hay (-9% left on the 8 mmmesh sieve) but more medium-sized parti-cles (+ 70% on mesh between 1-8 mm)and hence more particles 1 mm (fig 2).When observed with the naked eye, these

particles generally seemed to be larger forhay than for the silages, but no precisemeasurement was made.

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The viscosity of the centrifuged rumenjuice (fig 2) was practically the same forthe 2 silages but a little higher (+ 21%) forhay. On average, viscosity went from 1.20before the meal to 1.08 afterwards andthen to 1.06 and 1.12 at the subsequentemptyings. Likewise, the osmotic pressureof the rumen fluid (fig 2) was similar for the2 silages but higher (+ 7%) for hay.

Chemical characteristicsof the reticulo-rumen contents

There was a significant difference in pHbetween the forages, and the level waslower for hay than for silages (table VI). Onaverage, pH decreased regularly from 8.30h (6.71) to 20.30 h (6.29) (fig 3). Ammoniacontents were the same for WAS and Hbut higher for FAS. At 8.30 h, the level was

132 mg/I and at 10.30 h 137, after which itdecreased. The most clear-cut effect of themain meal (an increase in ammonia levelof 47 mg/I in 2 h) was observed with theWAS (fig 3).

VFA contents were about the same forthe 2 silages and slightly higher for hay.The levels increased gradually throughoutthe day, from 84 mmol at 8.30 h to 109mmol at 20.30 h (fig 3). Overall, the pro-portions of acetic and propionic acid werelowest with WAS. In contrast, it was this si-

lage that had the highest amounts of buty-ric, valeric and caproic acid. The propor-tions of valeric and caproic acid were low-est with hay. The most notable modifica-tions were observed with WAS after themain meal, at which time the proportions ofacetic, propionic, butyric, valeric and

caproic acids reached levels of 65.5, 15.3,13.7, 3.5 and 0.8% respectively.

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Rate of disappearance of digesta

During the day there was no great differ-ence in the rates of DM disappearance be-tween the 3 forages (average of 48 g/h)but a marked increase at night for hay(table VII). The disappearance rates of theplant cell walls were also very similar ex-

cept again for an increase with hay duringthe night. The DM turnover rate of the con-tents during the nycthemeral cycle wastherefore the same for FAS and H and low-

er, but not significantly for WAS. The varia-tions in the cell wall turnover rates, al-

though slightly greater, were still not

significant.

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DISCUSSION

The green forage from which the silagesand hay were prepared was rich in waterand probably poor in soluble carbohy-

drates (Jarrige, 1981). As a result, the si-lage with additive was of mediocre qualityand that without was very poorly pres-erved. Hence, in contrast to what is usuallyobserved (Dulphy and Demarquilly, 1991 ),

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the intake of the silages was far lower thanthat of the hay. Likewise, the differencebetween the 2 silages themselves wasgreater than the standard dissimilarity(Dulphy and Michalet-Doreau, 1981). Thedifference corresponds however to that

predicted by the equations given by theseauthors, in which the intake of the silage isin relation to the pH or the proportion ofammonia nitrogen. Finally, in comparisonwith the tables of Andrieu et al (1988), weobtained high, normal and low intake re-

spectively for H, FAS and WAS.

Differences in intake were observedfrom the main meal. With the WAS, thesheeps’ appetite was quickly satisfied, asobserved in a previous study (Dulphy,1985) and in agreement with the findingsof Gill et al (1988) in young cattle. Howev-er, contrary to what has often been report-ed (Dulphy et al, 1975a; Martin Clancy etal, 1977; Dulphy et al, 1984), the animalsdid not compensate for rapid satiety by in-creasing the number of their meals.

Yet the number of meals taken with the

silage diets was not so different from that

usually reported (Dulphy et al, 1984). Withhay it was higher (Dulphy, 1972; Dulphy etal, 1975a), perhaps because there wasjust 1 feeding per day. The same increasein the number of meals with a hay diet hasbeen observed elsewhere (Dulphy et al,1988). These findings therefore suggestthat the effects of the factors limiting intakeof the 2 silages were persistent throughoutthe diurnal cycle, since there was a lowerintake at each small meal.

There are several factors involved in the

regulation of silage intake: oral (taste,odour, prehensibility; cf Buchanan-Smith,1990), digestive (rate of digestion, func-

tioning of the reticulo-rumen; cf MartinClancy et al, 1977; Sissons et al, 1986)and metabolic, via mechanisms that main-tain the homeostasis of the animals.

One approach to organoleptic problemsis to study the rate of ingestion, a variablewhich often sharply discriminates betweenthe different types of silage. The rate de-creases when the silage particles are long-er (Dulphy and Demarquilly, 1973) and if

there is no additive (Dulphy et al, 1984).

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Strangely, in the present trial, this rate wasnot affected by the silage conservationmethod. Moreover, the silages were inject-ed during the main meal at a faster ratethan the hay. Problems of palatability didnot therefore seem to limit intake of the 2

silages studied. In this respect, Buchanan-Smith (1990) stresses the negative role ofacetic acid, of which there were similarlymoderate amounts in our 2 si-lages. In

agreement with the findings of the sameauthor, we observed no effet of ammoniaor of butyric, valeric and caproic acids.

Three aspects of digestion can be stud-ied; the efficiency of rumination, the rate ofdigestion and reticulo-rumen fill. The effi-

ciency of rumination was a little lower withthe silage containing no additive. The rea-sons for this are difficult to determine sincethere was little difference between the di-

gestion rates and digestibilities of the 3 for-ages. The digestion rate may have been alittle slower for WAS. However, these ob-servations are not sufficient to explain thedifferences in intake. Elsewhere, per unitof rumen digesta, the efficiency of rumina-tion was exactly the same for the 3 treat-ments. For the same silage, the latencytime between the end of the main mealand the beginning of rumination was muchlonger. The intake of poorly preserved si-

lage therefore tended to ’disturb’ rumina-tion more than that of silage with additive.However, this may simply have been dueto the fact that rumen fill was lower with si-

lage than with hay.Our findings showed that reticulo-rumen

fill was greater for hay than for silages, andare consistent with previous reports (Cam-pling, 1966; Thiago and Gill, 1986; Waldoet al, 1966), confirming that the animals’rumen capacity, at least in these trials, wasnot a factor that limited silage intake. In ra-tio to the metabolic weight of the animals,the average amounts of fresh content

reached highest levels of 510, 540 and

600 g per kg BWO.75 respectively for the 3forages. These values were high com-pared to those of several other reports(Dulphy et al, 1975a; Baumont et al, 1988;Dulphy et al, 1988) but comparable (595 g)for hay to the results of Baumont et al

(1990). The fact that the forage was well orpoorly preserved had little effect on rumenfill, in contrast to results of a trial per-formed by Dulphy et al (1975a).

The fall in silage intake therefore result-ed in a decrease in the reticulo-rumen con-tents. For WAS, a lower turnover rate ofthe contents was also observed. However,there is no evidence that the intake of the2 silages studied was limited by a de-crease in the animals’ digestive capacity.

Given the composition of the grass si-lages, 2 series of compounds may be in-volved in metabolic changes: acids, or theproducts of the decomposition of nitrogenmatter (Miettinen et al, 1991), or both. Be-cause of this, it is often difficult to identifythe compounds involved and to determinewhere and how their effects are achieved.

Certain end-products of the fermenta-tion of finaly chopped silages quickly satis-fy the animal’s appetite before the rumen isfilled. Several compounds may play a role:VFA, nitrogen compounds and probablylactic acid also (Morgan and L’Estrange,1977; Thomas et al, 1980). Since the phe-nomenon occurs with all silages, the acidsmust be involved. Satiety is even more

quickly reached if the silage is poorly con-served, so compounds from the degrada-tion of nitrogen have an effect. When theyare given well-preserved silage, animals

may offset early satisfaction of hunger byeating more often, and in sizeable

amounts, so that they ingest as much si-lage as hay (Dulphy and Demarquilly,1991). This did not happen in our trial be-cause the silage with additive was of medi-ocre quality. With poorly conserved sil-

ages, which hence are rich in VFA or am-

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monia or in both, 2 sets of mechanismsmay come into play simultaneously. Thetime spent eating and the amount ingestedper meal may decrease, as in our experi-ment, in proportion to the poor state ofconservation of the silage. The receptorareas are probably situated in the stom-achs. The animals’ lack of appetite, as evi-denced by a slow intake rate, can alsoshorten the duration of meals or reducethe amount ingested for the same time

spent eating (Dulphy et al, 1984; Bucha-nan-Smith and Philipp, 1986). The numberof meals themselves may also decrease.While this decrease is only slight with wellchopped silages, it is clear-cut in thosewith long particles (Dulphy and Demarquil-ly, 1973). The limiting effects of chemicalor physical factors last longer in the rumenin this case, or eating may be delayed bythe animals’ lack of motivation. The latter

possibility is supported by the findings ofGatel (1984) and Baumont et al (1990),who observed that the unpalatability ofdiets could be largely responsible forshort-term satiety. In the present trial, how-ever, this factor did not seem to be impor-tant.

An interesting question is how these

end-products of the silage limit intake. Wecan suggest a direct action on chemical re-ceptors, but it is also possible that silagefeeding may elicit a signal which does notpermit the rumen to fill to the same levelas with hay.

In conclusion, organoleptic factors arenot involved in the low intake of our si-

lages. In the same way, the digestive ca-pacity of the animals, their ability to reduceparticles, the rate of digestion of the si-

lages and their flow out of the rumen donot limit intake. The observation of the

feeding activities of sheep indicates a

strong action of the fermentation productsleading to rapid satiety. This behavioral

approach is very original and must beused in a more systematic way in the fu-

ture to obtain a better understanding of themechanisms modifying intake of the con-served forages.

ACKNOWLEDGMENT

We thank J Lefaivre for animal surgery.

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