The influence of physical processing on the intake, digestion and utilization of dried herbage

The effects of physical processing of forage diets on their value as feedstuffs for ruminants has been reviewed by Minson (1963)~ Beardsley (1964), Moore (1964), Campling & Milne (1972), Greenhalgh & Wainman (1972), Jarrige, Demarquilly, Journet & Beranger (1973) and Wilkins (1973).In this review we have therefore attempted to concentrate attention upon the chemical and physical changes produced in forages by harvesting and processing, to indicate briefly the effects upon voluntary intake and the net energy (NE) of the feedstuffs, and finally to review the mechanisms which might explain the effects of processing. Chemical and physical changes induced by harvesting and processing foragesAt the high temperatures achieved normally during hot-air drying and sometimes during milling and packaging, non-enzymic browning occurs mainly as the result of the formation of enzyme-resistant linkages between the carbonyl groups of reducing sugars and amino groups of the protein fraction.The chemistry of these Maillard reactions has been reviewed by Hadge (1953) and Bjarnason & Carpenter (1969, 1970) and they conclude that lysine is the amino acid most susceptible to the formation of such linkages.A major consequence of these reactions is a reduction of the solubility of the protein.The oven-drying of a grass reduces the solubility of protein when compared with the fresh material and gives rise to increases in both the dietary protein escaping degradation (49%) and the microbial protein synthesized in the m e n (57%).The net result of these changes is to increase the total protein entering and absorbed from the small intestine by 51 and 73% (Beever, Cammell & Wallace, 1974).Increasing amino acid absorption in response to decreasing solubility of the protein in forages has been demonstrated by MacRae & Ullyatt (1974) and Beever, Thomson & Cammell (1976) and in commercial drying is attributed by Israelsen (1973) mainly to overdrying (product moisture less than 207%).