Energy metabolism of dairy cows fed on grass

Production performance of grass-fed dairy cows is often lower than expected from the estimated energy supply. To explain the overestimation of the energy content of grass for dairy cows, data from energy balance trials from three different laboratories (Wageningen, Lelystad and Hillsborough) were collected. The trials in Wageningen and Lelystad were carried out in the 1970s and those in Hillsborough in the 1990s. Regression analyses were carried out with the complete data set as well as per laboratory to identify differences per laboratory. Average net maintenance requirements per kg 3/4 (NE m) were 0.573 MJ, whereas the efficiency of metabolizable energy utilization for lactation ( k l) was 0.777. When NE m was fixed at the presently used value of 0.293 MJ/kg 3/4, k l was 0.60. Between-laboratory NE m varied between 0.294 (Lelystad) and 0.786 MJ/kg 3/4 (Hillsborough), whereas k l varied between 0.57 (Lelystad) and 0.84 (Hillsborough). For Wageningen and Hillsborough, NE m was high, whilst k l was also high. With the intercept fixed at 0.293 MJ/kg 3/4, efficiency varied between 0.53 (Hillsborough) and 0.62 (Wageningen). The k l and NE m are interrelated. Based on these data we surmise that the maintenance requirements for grass-fed dairy cows are 10% higher than presently assumed, with no change in k l.