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A Comparison of Pea Aphid-resistant and Susceptible Alfalfas, for Field Performance, Saponin Concentration, Digestibility, and Insect Resistance¹

Six pea aphid [Acyrthosiphon pisum (Harris)]-resistant (PAR) and five pea aphid-susceptible (PAS) alfalfa (Medicago sativa L.) varieties were compared for foliage and root saponin concentration, forage yield, lodging, foliage color, pea aphid resistance, in.vitro digestibility, and clover root curculio [Sitona hispidula (F.)] resistance.

The PAR varieties were characterized by higher yields, but lower digestibility than the PAS varieties under noneconomic levels of pea aphid infestation. When yields of hay were corrected for digestibility, the PAR varieties had an advantage of 0.9 metric ton/ha over five cuttings. Lodging was not affected by level of pea aphid resistance, but was much higher in some varieties than others. Foliage and root saponin contents were not significantly correlated. Pea aphid resistance had a significant negative correlation with root saponin level; clover root curculio damage had a significant positive correlation with foliage saponin level. There was no evidence that breeding for pea aphid resistance had changed the foliage saponin concentration. However, it may have reduced the saponin in the roots. ‘Lahontan’ had high resistance to clover root curculio feeding (a feature that undoubtedly contributes to its longevity in infested areas).

Key Words: Disease resistance • In vitro digestibility • Clover root curculio resistance • Medicago sativa L. • Acyrthosiphon pisum (Harris) • Sitona hispidula F.

² Research agronomists and research animal scientist: crops res. lab., Utah State U., Logan, UT 84322; Dept. of Agron., Kans. State U., Manhattan, KS 66506; and Dairy Sci. Dept., Utah State U., Logan, UT 84322, respectively.

Received for publication October 21, 1974.