Four groups of six cows were fed from 12 weeks before calving to achieve condition scores at calving of 3·15 (F) and 2·15 (T). For the first 20 weeks of lactation all cows were given 8 kg concentrate (13·93 MJ metabolizable energy (ME) per kg dry matter (DM), 200 g crude protein (CP) per kg DM) which contained either a high or a low dietary concentration of undegradable dietary protein (UDP) (70 g/kg DM, H, and 49 g/kg DM, L) and 3 kg sugar-beet pulp. Hay was also offered ad libitum.
There was no significant effect of treatment on milk yields over the first 20 weeks of lactation (overall mean yield 27·3 (s.e. 3·65) kg/day) but cows in group FL tended to produce less (P > 005). Milk composition was similar for all groups (48·8 (s.e. 5·12) g fat per kg, 27·2 (s.e. 1·48) g protein per kg and 49·0 (s.e. 2·52) g lactose per kg). DM intakes for groups FH, FL, TH and TL were 18·1, 16·9, 17·5 and 18·2 (s.e.d. 0·58) kg/day, respectively (TL v. FL, P < 0·05; FH v. FL, P < 0·05). Mean condition scores for groups FH, FL, TH and TL respectively, were 3·T7, 3·13, 2·21 and 2·08 (s.e.d. 0·18) at calving; 2·25, 2·50, 2·21 and 1·83 (s.e.d. 0·27) in week 10; and 2·46, 2·83, 2·46 and 1·96 (s.e.d. 0·35) in week 20, of lactation. Groups FH, FL, TH and TL lost proportionately 0·09, 0·02, 0·01 and 0·04 (s.e.d. 0·035) of their calving live weights, respectively, over 20 weeks of lactation.
It is concluded that increasing the supply of UDP to cows which are fat at calving results in a greater negative energy balance and a small though non-significant increase in mobilization of body fat reserves. At the dietary ME and protein concentrations used, the thin cows could not respond to increased UDP supply and their intake was limited by physical restriction.