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Combining Ability for Protein Degradability in Alfalfa

Dep. of Soil & Crop Sciences, Texas A&M Univ., College Station, TX 77843-2474
Agronomy Dep.
USDA-ARS and Agronomy Dep., Kansas State Univ., Manhattan, KS 66506

^* Corresponding author.

The protein in alfalfa (Medicago sativa L.) forage is degraded extensively and quickly by ruminant animals, and N may be lost through NH₃ formation. Recent research has indicated that the degradability of forage protein varies among plants. The objective of this study was to determine the relative importance of additive and nonadditive genetic effects for protein degradability traits in an alfalfa population derived from major germplasm sources. Eight alfalfa plants were intercrossed in a half-diallel to produce 28 crossed progenies. Three replications of the crosses and parental clones were transplanted and grown in Riley County, Kansas. Protein percentages were estimated with microKjedahl, and protein degradability was estimated by degradation with the proteolytic enzyme ficin prior to microKjedahl digestion. Specific (nonadditive genetic effects) and general (additive genetic effects) combining abilities were determined for crude and soluble protein, buffer-insoluble and undegradable insoluble protein, and the proportion of undegradable protein/total insoluble protein. Data for each variable on two harvest dates in 1993 and 1994 were used in a combined analysis. Harvest date within years was a significant source of variability for all traits. A significant genotype effect was detected for all traits except undegradable insoluble protein. General combining ability effects were significant for the remaining traits, but specific combining ability effects were significant only for soluble protein. The relative ratios of additive to nonadditive effects were 0.86 for crude protein, 0.70 for soluble protein, 0.74 for insoluble protein, and 0.80 for proportion of undegradable insoluble protein/insoluble protein. The results indicate that genetic effects for alfalfa protein degradability exist and are mostly additive. Recurrent selection procedures should be effective in reducing protein degradability in alfalfa.

Received for publication May 25, 1995.