The influence of protein:energy value of the ration and level of feed intake on the energy and nitrogen metabolism of the growing pig. 1. Energy metabolism.

1. The heat losses and energy and nitrogen balances of thirty-six individually-housed, entire male pigs (initial body-weight 18-30 kg) were measured over 7 d periods, when they were fed on rations containing 153, 201 and 258 g crude protein (nitrogen x 6.25; CP)/kg dry matter (DM). The rations also contained 16.29, 16.96 and 17.24 mJ metabolizable energy (ME)/kg DM so that the CP:ME values were 9.4, 11.8 and 15.0 g CP/MJ ME respectively. Each ration was given at three levels, 20, 35 and 50 g feed/kg body weight per d, thus giving nine dietary treatments. The experiments were carried out at an environmental temperature of 22 (+/- 1) degree. 2. Heat loss (H) increased significantly (P less than 0.01) with increase in ME intake. The rate of increase in H was not, however, influenced by the protein content of the ration. Thus, energy retention (ER) at any given level of ME intake was independent of the ration offered. From the relationship between ER and ME, estimates of the maintenance energy requirement (MEm) and the partial efficiency of energy utilization (k) were determined. MEm varied within the range 494-568 kJ/kg body-weight 0.75 pe d, while k varied from 0.70 to 0.76. 3. Both energy and protein intakes had a significant influence upon the rates of protein (P) and fat (F) deposition, and hence body-weight gain. At any given level of feed intake P was higher and F lower the higher the protein content of the ration. However, when compared at similar levels of protein intake, both P and F were reduced the higher the protein control of the ration. 4. From the multiple regression equations relating P and F to ME, individual estimates of MEm and the energetic efficiencies of protein (kP) and fat (kF) depositions were determined. Using an overall mean kF value of 0.86, it was calculated that MEm ranged from 462 to 525 kJ/kg body-weight 0.75 per d while kP varied from 0.48 to 0.55. The significance of these estimates of kP are discussed in the light of their derivations and in relation to theoretical values.