Lors du chantier d'ensilage, le fourrage est récolté, haché, puis tassé dans un silo pour éviter la présence d'air. Pour le maïs-fourrage, on se contente habituellement de ramasser la plante entière, de hacher au champ et de transporter le produit finement haché dans un silo horizontal rapidement recouvert d'une bâche en plastique après tassement au tracteur. Si l'ensilage se fait en silo-tour, le maïs haché est chargé pneumatiquement dans le silo dans lequel il se tasse par gravité. Les graminées et les légumineuses sont d'abord hachées soit en coupe fine, soit en drins longs. Ce hachage est souvent précédé d'un préfanage (35 % de matière sèche) ou d'un ressuyage (25 % de matière sèche) qui ont pour objectif de limiter l'écoulement des jus dans le silo et de favoriser la fermentation lactique. L'ensilage mu-fané (50 % de matière sèche), encore appelé « hayage », est réalisé en silo hermétique ou en balles enrubannées. L'ensilage en brins longs se fait avec de l'herbe préfanée et pressée, sous film plastique. Le silo est généralement bâché de façon hermétique pour maintenir les conditions propices au développement des processus fermentaires. Au-delà d'une période de 2 à 3 semaines nécessaire à la stabilisation du produit ensilé, le silo est débâché uniquement sur le front d'attaque pour utiliser le fourrage dans l'alimentation des animaux.
La mise en place rapide des conditions d'anaérobiose permet le développement de bactéries lactiques qui utilisent les sucres solubles du fourrage pour leur métabolisme fermentaire. Ces phénomènes produisent de l'acide lactique et entraînent une baisse de pH dans le silo permettant la conservation du produit ensilé. En cas de présence d'air prolongée en début d'ensilage, des bactéries coliformes se développent en provoquant une accumulation d'acide acétique et d'azote soluble. Si le pH ne baisse pas assez vite pour atteindre une valeur seuil de 4 en l'espace de quelques jours ou si l'humidité du fourrage est trop grande (teneur en matière sèche inférieure à 30-35 %), une population de micro-organismes sporulés lactofermentaires apparaît. La consommation d'acide lactique par ces agents s'accompagne d'une production d'acide butyrique et d'une protéolyse intense conduisant à l'accumulation d'ammoniaque. Ces phénomènes sont néfastes à la qualité du produit ensilé.
La qualité de conservation d'un ensilage peut donc être appréciée par les teneurs en acide acétique et acide butyrique, et par la proportion d'azote soluble et sous forme ammoniacale. La chute rapide de pH, et par conséquent l'« ensilabilité », du fourrage dépend d'une part de sa teneur en glucides fermentescibles (faible pour le dactyle, élevée pour le ray-grass, le maïs...) et d'autre part de son pouvoir tampon (plus élevé pour les fourrages riches en protéines, luzerne par exemple). Dans des conditions peu favorables, il peut être nécessaire d'utiliser des conservateurs d'ensilage permettant de réduire le pH (acide formique...) ou de compenser la carence en sucres solubles (mélasse...).
La valeur énergétique des fourrages ensilés est comparable à celle des fourrages verts, mais la valeur azotée est plus faible en raison des pertes d'azote liées à la conservation.
L'ingestibilité des ensilages de graminées ou de légumineuses par les ruminants dépend avant tout du stade végétatif du fourrage à la récolte. Cependant, les fourrages conservés sous forme d'ensilage sont en général moins bien consommés que les foins, et surtout que les fourrages verts correspondants. Le niveau d'ingestion dépend également du mode de conditionnement du fourrage (les fourrages conditionnés en brins courts sont mieux ingérés que les fourrages présentés en brins longs), de son taux de matière sèche au moment de l'ensilage (l'ingestibilité est plus élevée pour les ensilages préfanés que pour les ensilages en coupe directe) ainsi que de la qualité de conservation de l'ensilage (plus faible consommation dans le cas d'une accumulation d'acides acétique et/ou butyrique).
Chapoutot/Schmidely
ensileuse
1. Récolteuse des fourrages destinés à l'ensilage. 2. Appareil de hachage et de chargement pneumatique utilisé à poste fixe pour remplir un silo-tour de fourrage à ensiler.
Aubineau
ensoleillement
Temps pendant lequel un lieu est ensoleillé.
La durée d'ensoleillement au cours d'une journée se mesure en durée d'insolation ; l'intensité de l'énergie solaire reçue est donnée par la valeur du rayonnement global, valeur qui doit être corrigée par la position de la parcelle (orientation et pente). En moyenne annuelle, l'ensoleillement sera caractérisé par les deux grandeurs classiques de durée et d'intensité du rayonnement solaire reçu.
Perrier
entérite
Inflammation de l'intestin grêle, fréquemment associée à une inflammation de l'estomac (gastro-entérite) et parfois à une inflammation du côlon (entérocolite).
L'entérite provoque des douleurs abdominales et de la diarrhée. Elle peut être due à une infection, à du parasitisme, à une intoxication, à une congestion intestinale, à un refroidissement, etc. On distingue des entérites bactériennes (colibacillose, salmonellose, vibriose, pasteurellose, clostridiose, maladie de Johne des bovins, charbon bactéridien...), des entérites virales (fréquentes et en général banales chez les bovins adultes, redoutables chez le veau ; chez le porc et les animaux de compagnie, il s'agit surtout de gastro-entérites), des entérites parasitaires dues à des protozoaires (coccidies) ou à des vers (douves du foie, ascaris, strongles), et des entérites toxiques qui font suite à des ingestions de produits chimiques, de plantes, de moisissures, etc.
Plusieurs causes prédisposent les animaux aux entérites. Elles dépendent soit de l'individu (jeune âge, surmenage, constipation...), soit des conditions de milieu (refroidissement, déséquilibre alimentaire...). Selon la nature de l'agent en cause, il peut y avoir légère inflammation ou destruction de la muqueuse avec hémorragie.
L'entérite aiguë se caractérise par de la diarrhée : les excréments sont très liquides, sentent très mauvais et peuvent contenir du sang et du mucus ; l'animal ne mange plus.
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