Overwintering of Trifolium repens L. and succeeding growth: results from a common protocol carried out at twelve European sites

A common experimental protocol was followed at 12 sites in Europe to study the development of two clover cultivars (AberHerald and Huia) grown in association with a ryegrass cultivar throughout their annual cycle. The duration of the experiment was between 1 and 3 years at each site. Detailed information about the morphogenesis and carbohydrate reserves of white clover during winter and the subsequent productivity of clover plants during the growing season was collected. Yield of the companion ryegrass was also measured at several harvests during the growing season and grass tiller density was recorded several times during winter. There was wide variation among sites and between years in climatic conditions, in the growth characteristics and chemical composition of the two cultivars, and in the tiller density of the ryegrass. The relative performance of the two cultivars varied among sites but AberHerald generally outperformed Huia. Major changes in plant characteristics (morphology, population size, chemical composition, etc.) occurred during overwintering. This paper presents a preliminary analysis of the effects of site, clover cultivar and sward age (years) on various plant characteristics. It is established that the data provide a unique basis for modelling the effects of a wide range of environmental conditions and plant properties on the performance of white clover in mixed swards. A modelling approach that seeks to replace site and year by climatic variables characterizing each site by year combination is developed in two companion papers.