Impact of recent research on energy feeding systems for dairy cattle

A considerable volume of research in the energy metabolism of dairy cows has been undertaken over the last 2 decades. The purpose of the present review is to reflect on the impact of these studies on the UK metabolisable energy (ME) system, and other net energy (NE) systems, and validate these systems using published calorimetric data. The NE requirement for maintenance (NE ) in the UK ME system is based on the fasting metabolism of cattle, while in other NE systems the NE m m was obtained from regression techniques. The NE values currently used in Europe and North America, which were m developed from the data published 30 years ago, have been demonstrated, in recent studies, to be lower than for present dairy cattle. Maintenance metabolic rate can increase with increasing dietary fibre concentration, possibly being due to increasing gut mass and metabolic activity in organs. Grazing cattle require more time and greater physical efforts for eating the same amount of feed as housed animals and thus require extra energy for grazing activity. The NE of cattle may be a function of m body protein mass, rather than total liveweight of the animal. The efficiency of utilisation of ME for lactation (k ) can be l derived from regression techniques or calculated by relating milk energy output and energy balance to ME available for production. Dietary fibre concentration has little effect on k , although it can influence the composition of volatile fatty acids l produced in the rumen and consequently shift milk composition and energy partition between milk and body tissue. There is no evidence to show an effect of cow genetic merit on k , but high genetic merit cows have the ability to partition more l energy into milk than medium or low genetic merit cows. The k has been shown to be higher than the efficiency of l utilisation of ME for tissue retention (k ) for dry cows, but ME was utilised with similar efficiency for milk production and g concomitant tissue retention. The energy value per unit of liveweight gain or loss should not be fixed as it depends on gut fill and composition of fat, protein and water in the gained or mobilised liveweight. The energy value per mobilised liveweight can also differ with stage of lactation. The above effects are important for dairy cattle feeding and should therefore be incorporated in the future revision of an energy feeding system. The current energy feeding systems used in Australia, the Netherlands, UK and USA have also been validated using calorimetric data of lactating dairy cows published since 1976.  2000 Elsevier Science B.V. All rights reserved.