1. The effect of energy and protein intake on the nitrogen and energy utilization and carcass composition of artificially-reared pigs was studied between 8 and 32 d of age in an experiment employing a 5 × 3 × 2 factorial design. The factors were initially energy:N value (I) (250, 355, 460, 565 or 670 kJ/g N), rate of increase of I (R) at 8 d intervals (0, 12.5 or 25%) and plane of nutrition (three times daily to appetite or 75% of this intake).
2. The range of energy:N values was obtained by formulating five diets based on dried skim milk, lactose and casein and feeding appropriate combinations of two diets.
3. The metabolizable energy (ME) intake, carcass dry matter (DM) content, carcass protein gain and carcass fat gain exhibited significant (P < 0.001) quadratic responses to I.
4. The carcass fat content in the DM increased from 200 to 342 g/kg (P < 0.001) and the carcass crude protein (N × 6.25) content decreased from 657 to 519 g/kg with increasing level of I (P < 0.001). The fat and protein contents were 309 and 556 g/kg and 242 and 610 g/kg respectively on the high and low plane of nutrition (PN) and were significantly different (P < 0.001).
5. There was a significant I × R interaction in relation to carcass protein gain with the maximum gain occurring at 460 kJ/g N when I was constant and at 355 kJ/g N when R was 25% per 8 d.
6. The proportion of apparent digested N intake retained (N retention (NR): apparent digested N (ADN)) was significantly (P < 0.001) affected by all three factors and there were significant I × R (P < 0.001), I × PN (P < 0.01) and R × R × PN (P < 0.01) interactions. The maximum value of NR:ADN was 0.80.
7. Carcass fat gain and carcass energy gain exhibited quadratic responses to I reaching maximum values at 460 kJ/g N. The energy content of the live-weight gain increased linearly (P < 0.001) from 5.79 to 7.90 MJ/kg with increasing level of I. PN and R also produced significant (P < 0.001) responses the means being respectively 7.52 and 6.39 MJ/kg on the high and low PN and 6.55, 6.94 and 7.38 kJ/kg with increasing value of R.
8. Multiple regression analysis of ME intake on live-weight (W), protein gain (PG) and fat gain (FG) yielded the equation: <m:math><m:mrow><m:mtext>ME</m:mtext><m:mo stretchy="false">(</m:mo><m:mtext>MJ</m:mtext><m:mo stretchy="false">)</m:mo><m:mo>=</m:mo><m:mn>0</m:mn><m:mo>.</m:mo><m:mn>644</m:mn><m:msup><m:mtext>W</m:mtext><m:mrow><m:mn>0</m:mn><m:mo>.</m:mo><m:mn>75</m:mn></m:mrow></m:msup></m:mrow><m:mo>+</m:mo><m:mn>32</m:mn><m:mo>.</m:mo><m:mn>6</m:mn><m:mtext>PG</m:mtext><m:mo>+</m:mo><m:mn>48</m:mn><m:mo>.</m:mo><m:mn>2</m:mn><m:mtext>FG</m:mtext><m:mo>.</m:mo></m:math>
This result is discussed in relation to published values for the energy requirements of the young pig.