Most attempts to improve alfalfa forage quality have increased leaf-to-stem ratio (LSR). Yet, increased LSR may not result in improved quality because of leaf losses. Increased stem quality may be a more stable improvement, but little is known about difference in stem morphology. The objective of this research is to determine genetic differences in stem morphology for diverse alfalfa cultivars. Alfalfa genotypes have been divergently selected for the mass ratio of branches to main stems (BMR). Six high- and six low-BMR genotypes were selected from two populations. Half-sib seed was gathered from four divergent genotypes in 1991.

A single-row nursery was established in 1992. The alfalfa entries included: (i) progeny of lines selected for high or low BMR from both '120' and 'Travois'; (ii) the cultivars Travois, 120, 'Arc', 'Green Genes', 'WL 322 HQ', 'Cimmaron VR'; and (iii) lines selected for high (HL) or low (LL) herbage lignin concentration. The experimental design was a randomized complete block with three replicates. Hand harvest of 1.2 m sections of each plot occurred in June and August 1993 and June 1994.

Open pollinated progeny from divergent selections for BMR expressed their phenotypic traits (Fig. 1). High-BMR progeny derived from 120 had 29% higher BMR than the low-BMR progeny. For divergent progeny derived from Travois, high-BMR progeny had 35% higher BMR than the low-BMR progeny. Divergent traits were also expressed in June 1994.
 
 

Divergent progeny did not differ in concentrations of either neutral detergent fiber (NDF) or acid detergent lignin (ADL) in any of the stem fractions. Also, divergent progeny did not differ from the cultivar entry from which they were derived, except that branches of high-BMR progeny derived from Travois had 16% lower ADL concentration than Travois. There was a positive relationship between BMR and NDF of main stems (r = 0.83**), but BMR and total stem NDF were not significantly correlated. There was a positive correlation between BMR and ADL of branches (r = 0.71). The NDF and ADL concentrations of total stems were closely related to NDF and ADL of the main stems. Even in high-BMR progeny, little dilution of the main stem occurred by the presence of branches.

Cell wall composition of the total stem fraction is mostly affected by the main stem. The proportion and composition of branches in the total stem fraction had little effect on NDF and ADL concentration of total stems. Concentrations of NDF in the main stem were positively correlated with BMR.