Use of an plant materials, plant extracts or nature-identical components selected from Lonicera japonica, Gentiana asclepidea, Gentiana lutea, Eugenia caryophyllata, Bellis perennis, Olea europaea, Symphytum officinale, Carduus pycnocephalus, Paeoniae alba radix, Populus tremula, Prunus avium, Salix caprea, Rheum nobile, Helianthemum canum, Arctostaphylos uva-ursi, Peltiphyllum peltatum, Epilobium montanum, Knautia arvensis, Latuca sativa and Urtica dioica and extracts thereof, and &bgr;-myrcene to affect the rumen fermentation and to increase the availability of energy and nitrogen source for a ruminant.