Introduction

Ethiopia is an area rich in germplasm of many plant species and was considered as a primary centre of crop diversity by Vavilov (1951). Among the most important plant genetic resources of the East African region, and indeed of all of Africa, are forages and especially forage grasses (Zeven & Zhukovsky, 1975). Large areas of Africa are covered with tropical savannah with a great diversity of grasses which are vigorous and polymorphic (Clayton, 1983). Tropical forage legumes and browse species are also endemic. In particular, Africa has been described as the centre of diversity of the browse shrubs of the subfamily Caesalpinioideae (Williams, 1983).

The genetic resources of forages are usually found in wild populations since they have only been cultivated on a commercial scale for about 50 years and no landraces are available (Williams, 1983). This is very different from crop species and therefore the collection strategies for forages differ to accommodate the population structures found in the wild. Marshall & Brown (1983) have defined the objective of forage plant exploration as the collection of material with the maximum amount of useful genetic variability within a strictly limited number of samples. The strategy of the International Livestock Centre for Africa (ILCA) is to collect representative population samples from the wild, although in some cases only a few plants can be found growing together as a population and sampling is therefore limited (Lazier, 1984).