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Genetic Improvement in White Clover Representing Six Decades of Plant Breeding

New Zealand Pastoral Agric. Res. Inst., AgResearch Grasslands, Private Bag 11008, Palmerston North, New Zealand

^* Corresponding author (woodfieldd{at}agresearch.cri.nz).

Improvements in white clover (Trifolium repens L.) performance during the past 60 yr can be attributed to increased use of fertilizer, better management practices, and development of genetically superior cultivars. The rate of genetic improvement during the past six decades was evaluated in New Zealand using a world collection of 110 white clover ecotypes and cultivars. Genetic differences were assessed by evaluating all cultivars in the same environment. Field trials conducted from 1985 to 1989 were managed using rotational grazing by sheep (Ovis aries L.) with an average of nine grazings per year. Cultivars were grouped according to decade of release, the climate in which they originated (cold, cool, or warm), and plant type (small-leaved, medium-leaved, large-leaved non-ladino, and large-leaved ladino). Regression analyses were used to determine the rate of genetic improvement. In absolute units, white clover yield has increased at an annual rate of 1.44 g m^–2, whereas the percentage clover by weight in the sward has increased at an annual rate of 0.14%. This equates to a genetic gain of 6% per decade for both traits, which is higher than rates reported for other forage crops. Genetic gains for clover yield and percentage clover in the sward have been greatest in small-leaved (7.5 and 8.7% per decade) and large-leaved, non-ladino types (6.6 and 8.3% per decade), whereas cultivars originating in cool climates (8.3 and 8.1% per decade) have improved at a faster rate than those originating in either cold (3.5 and 3.2% per decade) or warm climates (1.5 and 2.6% per decade).

Received for publication August 10, 1993.

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