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Midparent Values as Indicators of Performance of Alfalfa Strain Crosses¹

Alfalfa (Medicago sativa L.) breeders strive to incorporate an increasing number of pest resistance factors into cultivars in a minimal time period. Strain crossing appears to be a desirable procedure that also minimizes the effects of inbreeding. Objectives of this study were to determine pest resistance, forage yield, and fall dormancy of strain crosses originated by bee pollinations of diverse parental sources and to determine the effectiveness of the midparent value as an indicator of performance. Twenty-three two-parent strain crosses were produced from 24 germplasm sources in field isolation cages and pollinated by honeybees (Apis mellifera L.). Parental sources were planted within crossing blocks in a checkerboard pattern with four to six replications. Seed of each parent was harvested and processed separately. Strain crosses were formulated by mixing equal quantifies of seed from each parent. Strain crosses were evaluated for resistance to four insects and four diseases, forage yield, and fall dormancy. Pest resistance evaluations used standardized testing procedures. Forage yield was measured for a 2-yr period in field plots. Fall dormancy was based on natural plant height measurements taken in the forage production plots 3 weeks after the last fall harvest. Performances of the strain crosses were near the midparent values in 84% of the evaluations. Deviations from midparent values were most frequent for resistance to the spotted alfalfa aphid and for fall dormancy. Deviations from midparent values for one trait were not indicative of expected results for other traits in specific crosses. It was concluded that bee-pollinated strain crosses of widely divergent parental sources offers an effective method of incorporating multiple-pest resistance into alfalfa cultivars while avoiding some inbreeding effects associated with a narrow genetic base.

Key Words: Medicago sativa L. • Breeding methods • Inbreeding • Multiple-pest resistance • Pollination patterns

² Former graduate student (now, alfalfa breeder-West, AGRIPRO, Nampa, ID 8365), professor, and assistant professor, respectively, Dep. of Crop and Soil Sciences, New Mexico State Univ., Las Cruces, NM 88003.

Received for publication October 3, 1985.