Numéro #155

15 septembre 1998

Récolter et conserver l'herbe aujourd'hui (1ère partie)

Valeurs nutritive et alimentaire des fourrages selon les techniques de conservation : foin, ensilage, enrubannage

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La valeur nutritive et l'ingestibilité des fourrages conservés sont déterminées avant tout par celles du fourrage vert au moment de la fauche. La synthèse de nombreux essais montre quel est l'effet des différentes techniques de conservation sur les performances des animaux (génisses et vaches laitières) ?
La fenaison et l'enrubannage en balles rondes (BRE) entraînent la diminution de la digestibilité de la matière organique (dMO) et souvent de l'ingestibilité. L'ensilage diminue très peu la dMO mais diminue, parfois beaucoup, l'ingestibilité et la valeur azotée réelle. Les ensilages d'herbe récoltés en coupe directe avec une ensileuse à coupe fine et additionnés d'acide formique ont une excellente qualité ; distribués à volonté, ils assurent 1) à des génisses laitières de 1 an des croissances hivernales de 600 à 1 000 g/j en liaison étroite (r = 0,875) avec la dMO de l'ensilage, 2) à des vaches laitières une production de 9 à 20 kg de lait 4 % en liaison étroite avec la dMO (r = 0,64). Le préfanage (32% MS) ou le mi-fanage (45% MS) entraînent une légère diminution des croissances (10%) malgré l'augmentation (5-10%) des quantités ingérées. Les foins et les BRE récoltés à la même date que les ensilages permettent des croissances proches de celles permises par les ensilages avec acide formique.

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Nutritive and feeding value of conserved forages according to harvest and conservation methods : hay, silage, round bale wrapping

The nutritive value and DM intake of conserved forages are determined primarily by the quality of the fresh forage at cutting. Hay-making reduces organic matter digestibility (OMd) and often also voluntary intake, especially in conditions of bad weather and protracted field drying. This also holds true for wrapped round bales (WRB). Ensiling hardly reduces OMd, if at all, but results in a decrease, sometimes dramatic, of voluntary intake and above all of the protein value when conditions are bad, especially with uncontrolled butyric fermentation. Direct-cut silage, made with a precision-chop harvester and preserved with formic acid, is nearly always of excellent quality, provided ensiling conditions are good.
Three samples of silage were fed ad lib., without any complementation, except for minerals, to one-year old Holstein heifers ; winter liveweight gains ranged from 600 to 1 000 g/day, with a close relationship to silage OMd (r = 0.875). An increase of 1 point in this OMd improved liveweight gain by 21 g/day. But, if no additive was used, liveweight gains were lower by 200 g/day ; wilting to 30-35% DM or to 45% (haylage) resulted in slightly lower gains (by 10%), despite a better intake (by 5 to 10%) ; hay and WRB, made in excellent conditions, at the same date as silage, gave gains comparable to those with silage preserved with formic acid.
The same types of silage, also fed ad lib. to dairy cows, but complemented for energy and crude protein according to their production requirements, cover maintenance requirements and the equivalent of 9 to 20 kg fat-corrected milk, with a good relationship to silage OMd (r = 0.64). The milk production covered by this forage increased by 0.4 kg for each point of OMd. By contrast, if no additive was used, milk production was only slightly modified, but liveweight gain was diminished ; hay and WRB, made at the same date as the silage, gave a milk production lower by 2 kg, but without any negative effect on liveweight gain. There is therefore an effect of the methods of conserving forage on the performance of stock, but this effect is smaller than that of digestibility at cutting.

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