Forages are extensively used to feed domesticated farm animals, notably cattle and sheep, and comprise a wide variety of plant species. They are predominantly grasses or legumes and can either be fed fresh or conserved. When fed fresh, the harvesting is usually left to the animal | 26 Pasture Characteristics and Animal Performance P. Chilibroste 1 M. Gibb2 and S. Tamminga3 Facultad de Agronomía Estación Experimental M. A. Cassinoni Ruta 3 km 363 CP 60000 Paysandu Uruguay 2Institute of Grassland and Environmental Research North Wyke Research Station Okehampton Devon EX20 2SB UK 3Animal Nutrition Group Wageningen Institute of Animal Sciences Marijkeweg 40 6709 PG Wageningen The Netherlands Introduction Forages are extensively used to feed domesticated farm animals notably cattle and sheep and comprise a wide variety of plant species. They are predominantly grasses or legumes and can either be fed fresh or conserved. When fed fresh the harvesting is usually left to the animal. Conserved forages vary from wet silage through various degrees of wilting to hay. The bulk component of forages is p-linked polysaccharides. Other components in forages include proteins soluble sugars lipids minerals and vitamins. The p-linkages in the structural carbohydrates cannot normally be split by the hydrolytic enzymes inherently present in the digestive tract of animals. Due to a highly adapted digestive system with holding and mixing compartments that slow down passage of the feed and accommodate dense populations of microbes ruminants can use microbes for the breakdown of the structural carbohydrates. Hence extraction and utilization of nutrients from forages by ruminants uses a three-way interaction between the herbivore the plant and the microbial population. Important aspects of this interaction are characteristics of the forage and ingestive behaviour of the animal. Success depends on the extent to which this combination can accommodate the microbial population such that it executes a maximum of activity and provides its host with sufficient quantities of the required nutrients in microbial biomass or in its waste products the volatile fatty acids VFA . This chapter focuses on the utilization by farm animals of nutrients present in forages and the role .
