Silage

Ensilage as a method of forage conservation was known to the ancient Egyptians around 3000 years ago and throughout recorded history there have been periods when innovative European agriculturalists have enthused about the technique (Schakking, 1976).In Britain, silage-making gained widespread acceptance after the Second World War but as late as 1968 only 12-15% of conserved grass was silage.During the following period, however, the amount of silage made increased sharply; by 1973 it accounted for 28% of conserved forage and the present figure is about 45% (Wilkinson, 1980).Thus, silage is now a conserved grass product of major importance.This paper deals briefly with technical aspects of the production of grass silage, reviews current information on its feeding value and considers the prospects for the 1980s. Silage-ma king Silage-femrentation.Uncut grass carries low numbers of lactic acid bacteria but the numbers increase during harvesting (McDonald & Whittenburg, 1973), and following the sealing of the silo, when trapped air has been used up and aerobic bacteria inhibited, homofermentative and heterofermentative lactate producing organisms normally become dominant (Woolford, 1972;Beck, 1979).The organisms ferment water-soluble hexose and pentose carbohydrates and there is a partial degradation of hemicellulose (McDonald & Whittenburg, 1973;Edwards & McDonald, 1979; Momson, 1979).The main fermentation products are lactate and acetate which reduce the pH in the silo to about 4 units, inhibit further fermentation and preserve the crop.Mannitol and ethanol are also produced from sugar fermentation and a range of compounds-lactate, acetate, formate, ethanol, acetoin and 2, 3-butanediol-from fermentation of plant citrate and malate.Saccharolytic and proteolytic clostridial bacteria are also present in the silo.These organisms ferment sugars and lactate to butyrate with small amounts of ethanol, butanol and formate and metabolize amino acids to give acetate, propionate, isobutyrate, isovalerate, phenylacetate, phenylpropionate, indole, indoleacetate, indolepropionate, a-aminobutyrate, y-amindutyrate and amines such as histamine, tryptamine, tyraminc, cadaverine and putrescine.Their action raises the pH of the silage and if unchecked leads to putrefactive degradation.The clostridial type organisms are discouraged through a combination of the correct moisture conditions and pH in the silo.Their activity is reduced in wilted crops, *Present address: BOCWSilcock (NI) Ltd., 35-39 York St., Belfast.