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Genetic Improvement for Yield and Fertility of Alfalfa Cultivars Representing Different Eras of Breeding

Dep. of Crop Science, Box 7620, North Carolina State Univ., Raleigh, NC 27695-7620
Dep. of Agronomy, 1575 Linden Dr., Univ. of Wisconsin-Madison>, Madison, WI 53706-1597

^* Corresponding author.

Alfalfa (Medicago sativa L.) exhibits inbreeding depression for forage yield and also tends to set fewer seeds following self-pollination than following cross-pollination. The objectives of this study were to estimate changes in yield potential, inbreeding depression for yield, and self- and cross-fertilities of alfalfa cultivars adapted to Wisconsin and representing different eras of breeding from 1898 to 1985. In addition, two University of Wisconsin-Madison experimental cultivars with well-known breeding histories were included in the study. The S₀ and S₁ populations derived from each cultivar were evaluated for 3 yr for forage yield. Self- and cross-fertilities of some S₀ populations were measured in a greenhouse study. Both S₀ and S₁ populations of modern (Era 3) cultivars yielded greater than those of the oldest (Era 1) cultivars. Era 3 S₀ populations generally yielded more than those of cultivars released in the 1940s and 1950s (Era 2) but this was not true for S₁ populations. These results suggest that favorable alleles have accumulated in modern alfalfa cultivars but that this mostly occurred between Eras 1 and 2. Inbreeding depression decreased between Eras 1 and 2 and increased between Eras 2 and 3. Increased heterozygosity or exploitation of nonadditive genetic effects may account for much of the improvement in cultivar yield potential that occurred between Eras 2 and 3. Era 3 cultivars had a significantly lower ratio of self-fertility to cross-fertility than Era 1 cultivars. Therefore, modern cultivars appear to have improved capacity to produce high proportions of cross-pollinated seed when nonself pollen is available, compared with older cultivars.

Received for publication January 21, 1993.

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