Two experiments were carried out in the springs of 1986 and 1987 to investigate the effect of energy concentration in complete diets on the performance of heifers and mature cows. In 1986, all animals were on trial for an 8-week indoor period. In 1987, 14 heifers were on trial for 8 weeks, four heifers were on trial for 7 weeks and six heifers were on trial for 4 weeks. Carry-over effects were compared while animals were on pasture. Two diets offered ad libitum were compared. Diet H contained 0·7 concentrates and 0·3 grass silage. Diet L contained 0·5 concentrates and 0·5 grass silage. In 1986, the concentrate portion of diet H had a higher metabolizable energy (ME) content than diet L whereas in 1987 the same concentrate mixture was used in both diets.
Increasing the energy concentration of the diet from 10·66 MJ ME per kg dry matter (DM) to 11·21 MJ ME per kg DM resulted in a significant proportional increase of 0·36 in DM intake for heifers and mature cows in 1986. The proportional increase in DM intake in 1987 for heifers was 0·17. DM intake (kg DM per day) for heifers and mature cows in 1986 and for heifers in 1987 for diets H and L was 11·8, 8·7, 15·0, 11·0, 14·9, 12·7 respectively. The energy concentration of the complete diet did have a significant effect on average milk yield during the treatment period in the 1986 trial for mature cows but not for heifers in either year. Milk yields were 19·1, 17·5, 26·3, 240, 19·8, 18·9 kg/day respectively for heifers and mature cows in 1986 and for heifers only in 1987. In vivo digestibility coefficients in 1986 for energy, DM, organic matter, modified acid-detergent fibre and protein for diet H (0·708, 0·702, 0·724, 0·678, 0·671) were significantly higher than those for diet L (0·677, 0·666, 0·694, 0·653, 0·625).
The total lactation yield for the heifers and mature cows was not significantly influenced by the energy concentration of the diet given in early lactation.