The influence of conservation methods on digestion and utilization of forages by ruminants.

Biochemical changes during conservation Drying. During field drying the cut crop respires and sugars, malate, citrate and succinate are oxidized (Sullivan, 1973). Plant protease activity increases the soluble nitrogenous components, peptides, amides and amino acids, with marked increases in proline, glutamine and asparaghe (Kemble & Macpherson, 1954). The extent of these changes is influenced by the length of the drying period, which is dependent upon the plant resistances, envitonment and mechanical treatment. In barn-drying, baling is carried out earlier and oxidation and nutrient losses are reduced. In hightemperature drying of forage, biochemical changes are minimal and the composition of the dried herbage is almost identical to that of the original material. Ensiling. The fermentations in the silo can be classified broadly into five types: lactate, acetate, butyrate, wilted and chemically restricted. The lactate type of silage, in which the lactic acid bacteria have dominated the fermentation, is characterized by a low pH (c. 3.7-4.2), a high concentration of lactate (c. 80-120 g/kg dry matter (DM)), with smaller amounts of formic, acetic, propionic and butyric acids (Table I). Variable quantities of mannitol and ethanol derived from the activities of heterofermentative lactic acid bacteria and yeasts are present. The nitrogen fraction of lactate silage is mainly in a non-protein, soluble form with low concentrations (<I 2%) of ammonia-N. Residual water-soluble carbohydrates (WSC) in the DM are low (usually <2%. The buffering capacity (Bc) within the pH range 4-6 is about three to four times that of the original herbage. Under certain conditions, acetic acid-producing bacteria may dominate the fermentation. Such acetate-type silages have been reported by Catchpoole (1972) working with tropical grass and Henderson & McDonald (1975) in the UK. Apart from low lactate and high acetate values the fermentation characteristics appear to be similar to those of lactate silages. The conditions under which butyrate-type silages are produced have been well defined (McDonald & Whittenbury, 1973). Lactic acid and residual WSC are converted to butyric acid, resulting in silages with pH values normally between 5 and 6. Proteolytic clostridia are usually active; decarboxylation of amino acids leads to the formation of amines, while deamination results in free ammonia, ketoacids and higher volatile fatty acids (VFA) (Table I).