Divergent phenotypic selection for alfalfa cell wall fractions and indirect response in digestibility

An alfalfa (Medicago sativa L.) breeding strategy to decrease slowly digestible or indigestible fiber and simultaneously increase digestible fiber could improve forage quality without reducing total fiber. The objectives were: (1) to estimate selection responses from divergent and opposite direction selections of (i) hemicellulose (HEM) and acid detergent fiber (ADF), (ii) acid detergent lignin (LIG) and HEM + cellulose (CEL) and (iii) CEL and HEM + LIG, and (2) to determine correlated responses in in vitro true digestibility (IVTD). Selection progress was evaluated in replicated plot trials at two locations, sampled for 2 or 3 yr. Selection for divergent HEM and ADF resulted in change only for ADF [10.9 g kg-1 dry matter (DM)]. Selection for divergent LIG and HEM + CEL, resulted in same direction change in LIG (3.3 g kg-1 DM). Selection for divergent CEL and HEM + LIG resulted in change only in CEL (5.1 g kg-1 DM). Low LIG and high HEM + CEL, and low ADF and high HEM populations had 9.7 and 8.3 g kg-1 DM higher IVTD than their counterparts, respectively. The first cycle of selection for the fiber components simultaneously in the opposite directions was not successful. However, reduced LIG or ADF concentration appears to increase alfalfa forage digestibility and decrease total fiber concentration. Key words: Alfalfa, cell wall, hemicellulose, cellulose, lignin, digestibility