Received: 14 May, 2010. Accepted: 20 August, 2010. Invited Review International Journal of Plant Breeding ©2010 Global Science Books Progress Made in Improving Red Clover (Trifolium pratense L.) Through Breeding Heathcliffe Riday * USDA-ARS, US Dairy Forage Research Center, Madison, WI 53706, USA Correspondence: * Heathcliffe.riday@ars.usda.gov ABSTRACT Red clover (Trifolium pratense L.) is a major forage legume grown on approximately 4 million hectares worldwide. It has a long and varied history in agriculture. Active breeding efforts began at the end of the 19 th century. Since this time, significant improvement in red clover cultivars has occurred. The major focus of breeding and accompanying research efforts has involved increasing persistence. Improved persistence has been achieved through general persistence selection, pathogen resistance breeding, and breeding for tolerances to other agronomic stresses. Dry matter yield has received less attention, but has nevertheless been improved. More recently, selection for adaptation and performance of red clover in a grass-legume mixture used especially in grazing has received attention. Genomic resources in red clover have matured with the availability of numerous genetic maps and publicly available molecular markers. Transformation techniques have been developed to allow creation of genetically modified plants. Numerous opportunities still exist to improve agronomic characteristics of red clover, particularly by continuing efforts to increase persistence and dry matter yield. In a world with expensive fossil fuel-based fertilizer, breeding to develop and maintain improved red clover will continue to remain important. _____________________________________________________________________________________________________________ Keywords: dry matter yield, genetics, persistence, plant breeding CONTENTS INTRODUCTION........................................................................................................................................................................................ 22 BREEDING FOR PATHOGEN RESISTANCE ........................................................................................................................................... 23 BREEDING FOR INCREASED DRY MATTER YIELD ........................................................................................................................... 24 BREEDING FOR MIXTURES IN PASTURES .......................................................................................................................................... 25 GERMPLASM DIVERSITY AND MOLECULAR GENETICS ................................................................................................................ 25 CONCLUSIONS .......................................................................................................................................................................................... 26 REFERENCES............................................................................................................................................................................................. 26 _____________________________________________________________________________________________________________ INTRODUCTION Red clover (Trifolium pratense L.) is a major forage legume grown on approximately 4 million ha worldwide (Table 1). It has a long and varied history in agriculture (Westgate and Hillman 1911; Fergus and Hollowell 1960; Smith et al. 1985; Undersander et al. 1990; Lacefield and Ball 1999). It is uncertain when red clover entered active cultivation; evi- dence of red clover in pastures dates to the Bronze Age in Europe (Hodgson et al. 1999; Rasmussen 2005). Berseem clover (Trifolium alexandrinum L.) was actively cultivated in ancient Egypt (Cagle 2001). Early herbalist manuscripts describe red clover and its medicinal purposes as early as at least Anglo-Saxon Britain (Cockayne 1961; Zeven 1991). Clover’s agricultural benefits were extolled by Albert Mag- nus in the 12 th century (Jessen 1982). In Medieval Spain Ibn al Awwam described the cultivation of a bright red flowered forage legume (with larger more “gracious” leaves than alfalfa) which he termed “burgundy grass (or for- age)“ and which later translators and authors associated with berseem clover (Awwam ~1200; Bolens 1981). Al- though it should be noted that the Burgundy region of Europe would likely be too cold for berseem clover, sug- gesting the possibility of a cultivated red clover originating from the Burgundy region during the medieval period. Definitive records of active red clover cultivation appear in the 16 th century and are associated with intensification of agriculture (Blomeyer 1889; Merkenschlager 1934a, 1934b; Lane 1980; Zeven 1991; Hopcroft 2003; Kjærgaard 2003). By the 1800s almost all temperate agricultural regions in the world used red clover as an integral part of cropping rotations to improve soil and provide fodder. With the rise of widespread synthetic fertilizer use during the 20 th century, a steep decline in red clover acreage was observed (Rhodes and Ortega 1996; Kjærgaard 2003). This decline is evident in the United States during the 20 th century from red clover seed production data (Fig. 1). Today red clover is grown in pure stands or mixed with grass for hay, haylage, silage, or grazing (Undersander et al. 1990; Lacefield and Ball 1999). Red clover is known for rapid establishment, shade tolerance, and tolerance to low ® Table 1 Red clover (Trifolium pratense L.) seed production amounts. a – 2006/07 growing season estimates (unpublished), b – 2008 growing season (EU, 2009). Country or Region Kg (Million) Argentina 0.07 a Canada 1.23 a European Union-27 5.30 b New Zealand 0.16 a Norway 0.10 a Switzerland 0.05 a United States of America 4.57 a Total 9.88 a