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Changes in Forage Quality of Improved Alfalfa Populations¹

Forage yield and forage quality have often been reported to be negatively correlated. African 32 (AF32) and Hairy Peruvian 32 (HP32) alfalfa (Medicago sativa L.) germplasm pools were developed from ‘African’ (AF) and ‘Hairy Peruvian’ (HP), respectively, by two cycles of phenotypic recurrent selection for increased dry weight and increased N concentration. Individual plants were screened by growing them sequentially under N₂- and NH₄NO₃-dependent growth conditions. Because of the relationship between forage yield and forage quality, this study was undertaken to determine the effect of selection under this protocol on forage quality. Glasshouse studies in which the source of N could be controlled showed that 69 to 41% increases in total forage dry matter yield (g plant^–l) of HP32 relative to HP on N₂- and NH₄NO₃-dependent conditions were associated with 71 and 36% increases in total digestible dry matter (TIVDDM, g plant^–1). The AF32 population increased forage yield and TIVDDM by more than 20% relative to AF under NH₄NO₃-dependent conditions, but 9% improvements on N₂-dependent conditions were not significant. Neutral-detergent fiber, acid-detergent fiber, and cellulosic fractions were not increased significantly (P < 0.05) in either AF32 or HP32 under either N regime, and in several cases the selected populations had lower concentrations than the original cultivar. Overall marked differences occurred in the manner in which forage quality in the two germplasm sources responded to selection and N-regime tests. For example, a major improvement occurred in stem quality with selection in AF32 relative to AF, but not in HP32 relative to HP, under the NH₄NO₃ environment. Combining data from both N environments, much more of the variation in whole-plant IVDDM (g kg^–1) could be attributed to measured parameters in HP, HP32, and AF than in AF32 (92, 91, 89, and 54%, respectively). This study suggests that forage quality can be increased simultaneously with forage yield because, over the range of forage yields studied here, the larger plants did not require a higher proportion of indigestible components to support the additional shoots.

Key Words: Dinitrogen fixation • Medicago sativa L. • Lucerne • Neutral-detergent fiber • Acid-detergent fiber • In vitro digestibility • Yield

² Assistant and associate professor, Univ. of California; associate professor, Univ. of Arizona; and professor, Univ. of California, respectively.

Received for publication December 2, 1985.

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