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RESEARCHW hite clover (Trifolium repens L.) is a major contributor of productivity and ecological services in pastures and rangelands worldwide. The advantages of including and maintaining white clover in a pasture are well documented (Chapman et al., 1993). In New Zealand alone, it is estimated that white clover adds approximately NZ$3 billion attributable to the seed industry, N fixation, and honey production . In grazed swards, white clover provides up to 400 kg N ha â1 via biological N fixation (Crush, 1987). In terms of area, white clover is the predominant temperate forage legume serving as a companion to temperate grasses in grazing systems (Laidlaw and Teuber, 2001). White clover in mixed sward pastures is estimated to occupy ?15 million ha in Australasia and 5 million ha in the United States. Globally, renewal of white clover is estimated at 3 to 4 million ha yr â1 (Mather et al., 1996).White clover breeding is important worldwide and is recognized as a priority in many countries with a pastoral-based livestock productivity system ( Jahufer et al., 2012). For example, the last checklist of white clover varieties (Caradus and Woodfield, 1997) described a total of 326 cultivars in relation to their maintainer institution, origin and breeding procedures used, agronomic performance, and disease susceptibility, among other characteristics, depending on the information available. This checklist comprises information collected from cultivars from 32 ABSTRACT White clover (Trifolium repens L.) is a major contributor of productivity and ecological services in pastures and rangelands worldwide. White clover breeding is recognized as a priority in many countries with a pastoral-based livestock productivity system. The objective of this study was to provide an updated estimate of the rate of change in genotypic value attributable to population improvement in white clover, using a set of 80 cultivars released between 1920 and 2010 by public and private plant breeding programs across 17 countries, in an experiment to evaluate forage yield across three locations in New Zealand. Overall, some New Zealand cultivars released in the 2000s were highly adapted to more stable testing environments. Geographic origin resulted in differential rates of gain. A segmental regression comparing the pre-1965 and post-1965 rates of gain was performed. The pre-1965 regression line for white clover dry matter yield resulted in an absolute rate of increase of 0.031 g m â2 yr â1 decade â1 or 0.087% decade â1 , whereas the post-1965 line resulted in an absolute rate of increase of 0.058 g m â2 yr â1 decade â1 or 0.162% decade â1 . White clover content resulted in a pre-1965 absolute rate of increase of 0.011% content decade â1 or 0.032% decade â1 on average, and the post-1965 line indicated an absolute rate of increase of 0.04% content decade â1 or 0.121% decade â1 on average. The comparisons between pre-1965 and post-1965 indicated a twofold increase for white clover dry matter yield and a fourfold increase in rates of gain for white c...