HERITABILITY AND IMPROVEMENT OF PROTEIN AND NITROGEN CONCENTRATIONS IN WILTED ALFALFA FORAGE

During wilting of alfalfa (Medicago sativa L.) before preservation as silage or hay, proteinases in plant cells hydrolyze soluble protein (SP) into nonprotein N (peptides and amino acids), thus decreasing postharvest quality of alfalfa. Our objectives were to compare (i) expected gain in SP concentration remaining in wilted, lyophilized alfalfa from direct selection with expected and actual gain from indirect selection for N concentration and (ii) phenotypic selection with half-sib (HS) progeny test for gain in both SP and N concentrations in wilted, lyophilized forage. In Exp. 1, heritabilities for SP and N concentrations in wilted, lyophilized alfalfa were estimated by parent-offspring regression and by HS progeny analysis. Samples of alfalfa forage from HS families and individual plants were wilted for 24 h, chopped, frozen, and lyophilized. Near-infrared reflectance spectroscopy (NIRS) was used to predict SP and N concentrations in samples. Heritability estimates for N concentration were moderate in magnitude (0.29 to 0.76), and those for SP concentration were low (-0.03 to 0.46). The expected gain in SP concentration was higher from selection for N concentration than from direct selection. In Exp. 2, two alfalfa populations previously selected for higher N concentration and two unselected cultivars were grown in sward plots. Unwilted forage had 25% (P less-than-or-equal-to 0.01) higher SP concentration in 1987 than forage that had been wilted for 24 h and had 13% (P < 0.01) higher SP in 1988. The two populations bred for higher N concentration were higher in SP concentrations, both before and after forage wilting, than the two unselected cultivars. Among the selection methods studied, pre- and postwilting SP concentration in lyophilized alfalfa could be most efficiently improved indirectly by phenotypic selection for higher N concentration.