Proteolysis and rumen degradability of alfalfa silages preserved with a microbial inoculant, spent sulfite liquor, formic acid or formaldehyde

Abstract

The efficacy of biological additives and direct acidifiers on the preservation and extent of proteolysis in unwilted alfalfa silage was investigated using a 2 × 3 × 5 factorial design. Alfalfa, harvested as unwilted second-cut (21% dry matter (DM)) and third-cut (17% DM) material was ensiled untreated or treated with formic acid (0.4%) or a microbial inoculant (Ecosyl: ICI Biologicals, Mississauga, ON), with or without formaldehyde (0.2%) or three levels of spent sulfite liquor (SSL) (0.25, 0.50 and 0.75%) in laboratory silos for 90 d. Addition of direct acidifiers (formic acid with or without formaldehyde or SSL) reduced pH and enhanced levels of water-soluble carbohydrates (WSC) and lactic acid. Proteolysis of unwilted second- and third-cut silage as measured by soluble nitrogen (N), nonprotein N (NPN) and ammonia N (NH 3 -N) concentration was reduced (P < 0.05) relative to controls (untreated, formaldehyde and SSL). Biological additives (Ecosyl with or without formaldehyde or SSL did not differ (P > 0.05) from controls in second-cut silage. However, addition of biological additives reduced (P < 0.05) proteolysis of third-cut forage compared with controls. No linear response in extent of acidification or proteolysis by increasing levels of SSL was found. In a subsequent experiment, three rumen-fistulated Hereford steers were used in a randomized incomplete block design to investigate whether the chemical profile of N constituents of unwilted alfalfa silages could be used to predict protein degradability of silage incubated in nylon bags. Individual silages which demonstrated a marked difference in NPN content from untreated controls were incubated. Addition of formic acid and formaldehyde increased (P < 0.05) the potentially degradable insoluble N fraction while decreasing (P < 0.05) the rumen soluble N fraction, which was found to be positively correlated (r = 0.72, P < 0.0001) with NPN content of silage. There was no relationship between NPN and extent of ruminal protein disappearance (r = 0.08, P < 0.65); however, total N, total soluble N and NPN were found to be positively correlated to protein degradability (r = 0.68, P < 0.0001; r = 0.61, P < 0.0001; and r = 0.67, P < 0.0001, respectively). Key words: Alfalfa, silage, inoculant, degradability, steers