The prediction of body composition in bacon pigs from measurements of feed intake and live-weight gain

Model responses of the growing pig to the dietary intake of energy and protein

A simple model is described which enables the prediction of the magnitude and direction of the responses of growing pigs to different energy and protein intakes. The model calculates daily live-weight gain from the conversion of the dietary supply of crude protein and energy into protein, lipid and ash in the body of the growing pig. Values were also determined for the energy and protein balances, the composition of the body and the efficiency of feed conversion. The model was formulated with factors drawn from published findings and validated by comparison with independent feeding trials.

Theoretical aspects of a flexible model to stimulate protein and lipid growth in pigs

A model designed to simulate growth in pigs has been developed to include the following aspects of protein and energy use.

1. It is proposed that the ratio of protein accretion to protein synthesis is a function of protein mass. Synthesis rate influences the efficiency of protein use, the energy cost of protein accretion and the energy cost of maintenance. A calculated energy cost for protein synthesis of 7·3 MJ ME/kg is suggested; the calculated energy yield from deaminated protein is 11·5 MJ ME/kg.

2. Urinary losses of nitrogen are derived from estimates of protein quality by essential amino acid index, endogenous losses and the rate of protein accretion.

3. A minimum fat to protein ratio in the gain of growing pigs of 1: 1 is assumed.

4. An estimate of critical temperature which is dependent upon live weight and heat output is used to calculate energy expenditure for cold thermogenesis.

Objectives in pig breeding

1. Some criticisms of the classical approach to constructing a selection index are advanced. This approach (the economic model) is contrasted with an alternative one (the biological model) which is derived from the concept of the biological efficiency of production of lean tissues. The selection objective which is proposed from the biological model is the improvement of lean tissue feed conversion. The most important single means of achieving this is increasing the lean tissue growth rate.

2. The approaches were compared in three studies: (a) the effects of errors in economic and genetic parameters of the economic index; (b) phenotypic relationships between biological objectives and other measures of performance; (c) a theoretical model based on the utilization of metabolizable energy.

3. In a fixed situation the results were similar for both models, but the main advantage of the biological model is that it gives good indications of what happens, and what selection policies should be, in different situations. In addition, lean tissue feed conversion and lean tissue growth rate are closely related to concepts widely applied in biology, and their use as selection objectives should be helpful in enabling workers in many disciplines to make use of each other's knowledge.

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.