Improved forage quality in alfalfa (Medicago sativa L.) via selection for increased stem fiber digestibility

Authors: Xu, Zhanyou; Heuschele, Deborah - Jo; LAMB, JOANNE F.S. - Retired ARS Employee; Jung, Hans Joachim; Samac, Deborah - Debby

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/6/2023
Publication Date: 3/7/2023
Citation: Xu, Z., Heuschele, D.J., Lamb, J., Jung, H.G., Samac, D.A. 2023. Improved forage quality in alfalfa (Medicago sativa L.) via selection for increased stem fiber digestibility. Agronomy. 13(3). Article 770. https://doi.org/10.3390/agronomy13030770.
DOI: https://doi.org/10.3390/agronomy13030770

Interpretive Summary: Alfalfa is the most important forage crop used in ruminant animal production in North America. While alfalfa is generally considered to be a high-quality forage, it is also recognized that the stem fraction of alfalfa is of limited digestibility, which decreases feed intake and animal performance. Because poorly digested stem material increases with plant age, the quality of stems becomes a greater issue in more mature alfalfa. This pattern of alfalfa development has led to the common practice of harvesting the crop when immature; however, this practice reduces long-term forage yield and stand persistence. The value of alfalfa as a forage crop could be improved by breeding alfalfa with improved stem digestibility. This report summarizes the development of unique alfalfa lines from two breeding cycles of selecting plants with increased stem digestibility. Results showed that stem digestibility traits are highly heritable and genetic gain was achieved in each breeding cycle without changing the proportion of stems to leaves. Selection increased stem digestibility at the later maturity stages and decreased seasonal variability in forage quality. The methodology developed was a very effective approach for improving forage digestibility and biomass yield. This approach can be incorporated into alfalfa breeding programs for developing improved cultivars with greater persistence and improved animal performance.

Technical Abstract: Low fiber digestibility in alfalfa stems can limit dry matter intake and energy availability in ruminant animal digestion. Breeding for improved stem fiber digestibility could increase net digestible biomass yield. Previously, alfalfa (Medicago sativa L.) plants were identified for a breeding program based on either low or high rapid (16-h) and low or high potential (96-h) in vitro neutral detergent fiber digestibility (IVNDFD) of plant stems. Two cycles of bidirectional selection for plants with low or high stem 16-h IVNDFD and low or high stem 96-h IVNDFD were carried out. The resulting populations were evaluated for herbage biomass, leaf to stem ratio, IVNDFD, and detergent fiber components in field trials with plants harvested at three maturity stages. The 96-h IVNDFD trait was highly heritable (H2 = 0.71) with a genetic gain rate of 5.05% per selection cycle. The patterns of continuous increase of stem fiber digestibility from each cycle of selection for high 16-h and high 96-h IVNDFD digestibility and a decrease in digestibility from each cycle in the low 16-h and low 96-h populations suggested that additive gene effects may control stem fiber digestibility. The non-unidirectional change of digestibility from the high x low and low x high populations indirectly affirms the additive gene effects. Divergent selection did not alter leaf to stem ratio nor other plant morphological traits. Selection for stem IVNDFD was a highly effective strategy for developing alfalfa cultivars with improved nutritional quality.