Four groups of six cows were fed from 12 weeks before calving to achieve condition scores at calving of 3·23 (F) and 1·98 (T). For the first 20 weeks of lactation all cows were given 10 kg/day of isonitrogenous (180 g crude protein per kg dry matter (DM)) compounds containing either a high (13·0 MJ/kg DM; HE) or a low (9·8 MJ/kg DM; LE) concentration of metabolizable energy, together with 3 kg/day molassed sugar-beet pulp and hay offered ad libitum.
The mean milk yield of cows receiving compound HE (27·7 kg/day) was higher (P > 0·05) than that of cows receiving compound LE (25·6 kg/day). Cows in group FHE yielded 27·0 kg/day, compared with 25·5 kg/day for FLE (P > 0·05); cows in group THE yielded 28·4 kg/day compared with 25·17 kg/day for TLE (P <0·05). After peak lactation, milk yields were maintained better in group THE but declined at a faster rate in group TLE than in groups FHE and FLE. Neither dietary energy concentration nor condition score at calving significantly affected milk composition (butterfat 43·5, protein 29·1, lactose 50·2 g/kg). Cows receiving compound HE consumed significantly (P < 0·001) more energy (208 MJ/day) than cows receiving compound LE (188 MJ/day). Over the first 10 weeks of lactation, changes in condition were –0·83, –0·88 +0·08 and –0·25 (s.e.d. 0·22, P < 0·05) condition score units for cows in groups FHE, FLE, THE and TLE respectively.
It is concluded that with diets of high energy concentration intake is mainly limited by physiological mechanisms so that thin cows eat more than fat cows and produce similar amounts of milk. With diets of lower energy concentration, intake is limited by rumen capacity and thin cows eat the same as fat cows. This results in increased fat mobilization and a slight decrease in milk yield by cows which are fat at calving but the limited fat reserves of cows which are thin at calving are insufficient to compensate for reduced energy intake so large reductions in milk yield are observed.