15 mars 2002
auteur
L'éleveur doit en permanence concilier 2 objectifs contradictoires : maximiser l'ingestion et les performances animales individuelles, et maîtriser la hauteur d'herbe résiduelle. Pour gérer au mieux le pâturage, il est utile de connaître les répercussions de la hauteur et du rythme d'exploitation sur la repousse et la composition morphologique du couvert.
Un essai conduit pendant 2 ans met en évidence que, après des coupes moyennes à hautes, la repousse est rapide et la sénescence précoce des limbes impose une exploitation à un rythme rapide (2 semaines environ). Mais après des coupes rases, la prairie repousse lentement et son entrée en sénescence est retardée, ce qui permet d'allonger l'intervalle entre exploitations d'une semaine. Ce mode d'exploitation ras génère peu de tiges, de gaines et de sénescence, et préserve la qualité des repousses. Le taux de défoliation, intégrant les hauteurs du couvert à l'entrée et à la sortie des animaux, permet de situer la fin du pâturage en préservant les performances animales. Un rapport limbes/gaines élevé assure les meilleures productions animales.
Good grazing management requires the knowledge of the effects of cutting height and cutting rate on the regrowth and the morphological make-up of the sward. A trial carried out for 2 years showed that the harvested production was significantly larger when the cutting frequence was low, especially with close (3 cm) cuts. The residual biomass of culms varied from 878 to 2280 kg DM/ha when the cutting height increased from 3 to 9 cm ; delayed cuttings magnified these differences. Besides, Perennial Ryegrass, when cut frequently, gave few ears ; with delayed cuts, the number of ears was linked directly to the cutting height.
After medium-low or high cuts, regrowth was fast and early senescence of the blades dictated a frequent cutting rate : intervals of 2 weeks in spring, of 20 days at the end of summer. On the contrary, after close cuts, regrowth was slow, and the beginning of senescence delayed, so that the intervals between cuts could be lengthened by one week. This type of management with close cuts created few ears, few sheaths, and little senescence, and the quality of aftermaths was preserved. In successive grazings, the animals ate that part of the herbage which was above the residual height of the previous cycle ; the herbage thus ingested had a good digestibility.
A high blade/sheath ratio leads to the largest animal yields. The defoliation rate, which takes into account the residual height and the sward height at turnout to the pasture, indicates when the grazing should be ended if the animal performances are to be preserved.
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