GENETIC VARIANCE OF PROTEOLYTIC ACTIVITY IN ALFALFA HERBAGE

Abstract

Protein hydrolysis can be extensive during ensiling of alfalfa (Medicago sativa L.) herbage. Such hydrolysis lengthens the ensiling phase and reduces the quality of the resultant feed. The objectives of this study were: (1) to estimate the genetic variance and narrow-sense heritability (via a half-sib analysis) of proteolytic activity in an alfalfa breeding population (OAC-A83); and (2) to estimate the genetic diversity among related Medicago subspecies (M. falcata, M. varia, M. praefalcata and M. glomerata), M. sativa introductions, and two cultivars, Iroquois and OAC Minto. An in vitro procedure was used to quantify proteolytic activity, which was expressed as milligrams of trichloroacetic acid (TCA)-soluble leucine equivalents per gram fresh leaf weight, released during a 7-d incubation. Significant (P < 0.05) differences were found among the half-sib families of OAC-A83. The narrow-sense heritability of proteolytic activity was low (13.9%). The expected response to selection for low proteolytic activity, if the top 10% of the plants were recombined, was 5% of the OAC-A83 population mean. Although significant (P < 0.05) differences were obtained among the Medicago introductions, none of the introductions or subspecies tested had lower proteolytic activity than that found in the two cultivars tested (Iroquois and OAC Minto). It was concluded that, although possible, genetic improvement of proteolytic potential in alfalfa may not be easily obtained. In the short term, rapid lowering of silage pH (e.g., bacterial inoculants or acid additives) will likely remain the most effective means of controlling protein degradation in alfalfa silage.Key words: Medicago sativa L., silage, ensiling, soluble nonprotein nitrogen, heritability, alfalfa