Introduction major differences due to level of intake. Over the last

254 T . Yan et al. Livestock Production Science 64 2000 253 –263

1. Introduction major differences due to level of intake. Over the last

2 decades cow genetic merit has substantially in- creased Coffey, 1992 and these animals can The metabolisable energy ME concentration in produce milk yields of over 50 kg day and consume the diet of ruminant animals is determined as the more than 300 MJ day of ME Yan et al. 1997a, a difference between gross energy GE intake and level which can be over five times their maintenance energy outputs in faeces, urine and methane. The requirement as estimated from the Agricultural and measurement of methane energy output CH -E 4 Food Research Council 1990. Beever et al. 1998 requires complex and expensive equipment and 0.75 hence prediction equations are widely used to calcu- also reported a mean ME intake of 1.89 MJ kg late CH -E. A number of these equations have been with lactating cows, which is approximately four 4 published since 1930, using total dry matter DM levels of feeding calculated from the Agricultural intake Kriss, 1930; Axelsson, 1949, digestible and Food Research Council 1990. The effect of carbohydrates Bratzler and Forbes, 1940; Moe and feeding level on methane production is therefore Tyrrell, 1979, energy digestibility and feeding level becoming of increasing importance and there is Blaxter and Clapperton, 1965, and a range of therefore a need to re-examine this effect. animal and dietary factors Holter and Young, 1992. At this Institute, between 1992 and 1997 a number However, the data used to develop these equations of experiments have been completed with lactating were collected from ruminant animals offered diets dairy cows n 5247 and beef steers n575 offered containing mainly dry or high DM forages, rather grass silage-based diets and subjected to gaseous than low DM grass silages, typical of those used in exchange measurements in calorimetric chambers. many areas of Western Europe. The fermentation The objective of the present study was therefore to process in low DM grass silage results in a low use the energy metabolism data from these studies to concentration of water soluble carbohydrates and evaluate the relationship between methane product- high levels of fermentation products, such as volatile ion and a number of animal and dietary factors. fatty acids VFAs, lactate, alcohol and ammonia. Furthermore, the concentration of fibre in the DM of silage can also differ from that in dried grass due to

2. Material and methods