Energy and protein nutrition of early-weaned pigs. 1. Effect of energy intake and energy: protein on growth, efficiency and nitrogen utilization of pigs between 8-32 d.

1. The effect of energy and protein intake on the growth, food efficiency and nitrogen retention of artificially-reared pigs was studied over three 8 d periods between 8-32 d of age in an experiment employing a 5 x 3 x 2 factorial design. The factors were initial energy: N value (I; 250, 355, 460, 565 or 670 kJ/g N), rate of increase of I 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. The diets, which were pelleted, contained 100 g maize oil/kg and the gross energy content was approximately 20 MJ/kg. 3. N digestibility was high at all three age intervals, reaching 0.99 on the diet containing the highest dietary crude protein (N x 6.25) level. Metabolic faecal N excretion was found to be I.I g/kg dry matter (DM) intake. 4. Growth rate, feed conversion ratio (kg food intake/kg wt gain; FCR), N retention (NR) and the proportion of digested N retained (NR: apparent digested N (ADN)) were significantly (P less than 0.001) affected by I values at all age intervals and the responses were quadratic. Response curves were calculated by the least squares method and optimum values of I determined for each of the criteria. A constant energy: N value of approximately 400 kJ/g N was indicated by growth, FCR and NR optima but the NR:ADN value fell from 0.77 for the 8-16 d period to 0.60 for the 24-32 d period at this I value. It is concluded that a suitable compromise would be an I value of 470 kJ/g N increasing by 10%/week. 5. There was a significant interaction between plane of nutrition and I values on FCR between 16-24 d (P less than 0.001) and 8-32 d (P less than 0.01) indicating that FCR was better at high protein levels and worse at low protein levels when the diets were fed on the lower plane of nutrition.