In the present study, a linear regression analysis between lysine intake and lysine retention was conducted to investigate the efficiency of lysine utilisation (kLys) at marginal lysine intake of either protein-bound or free lysine sources in juvenile turbot (Psetta maxima). For this purpose, nine isonitrogenous and isoenergetic diets were formulated to contain 2·25–4·12 g lysine/100 g crude protein (CP) to ensure that lysine was the first-limiting amino acid in all diets. The basal diet contained 2·25 g lysine/100 g CP. Graded levels of casein (Cas), fishmeal (FM) and l-lysine HCl (Lys) were added to the experimental diets to achieve stepwise lysine increments. A total of 240 fish (initial weight 50·1 g) were hand-fed all the experimental diets once daily until apparent satiation over a period of 56 d. Feed intake was significantly affected by dietary lysine concentration rather than by dietary lysine source. Specific growth rate increased significantly at higher lysine concentrations (P< 0·001). CP, crude lipid and crude ash contents in the whole body were affected by the dietary treatments. The linear regression slope between lysine retention and lysine intake (kLys) was similar between all the dietary lysine sources. The kLys values for the diets supplemented with Cas, Lys or FM were 0·833, 0·857 and 0·684, respectively. The bioavailability of lysine from the respective lysine sources was determined by a slope-ratio approach. The bioavailability of lysine (relative to the reference lysine source Cas) from FM and Lys was 82·1 and 103 %, respectively. Nutrient requirement for maintenance was in the range of 16·7–23·4 mg/kg0·8 per d, and did not differ between the treatments. There were no significant differences in lysine utilisation efficiency or bioavailability of protein-bound or crystalline lysine from the respective sources observed when lysine was confirmed to be the first-limiting nutrient.

A slaughter experiment was carried out to study the effect of dietary protein quality on maintenance energy requirements and energy costs for protein accretion and fat deposition in fast-growing broiler-type male chickens. Three isonitrogenous (200 g crude protein/kg DM) and isoenergetic (14 kJ metabolizable energy (ME)/g DM) semipurified diets based on soyabean meal unsupplemented (diet S) or supplemented with 20 g L-lysine/kg (diet SL) or 2 g DL-methionine/kg (diet SM), in order to promote a decrease or an increase in growth rate respectively, were selected and given at four feeding levels (ad lib. or restricted to 40, 28 and 18 g DM/d, on average) to 10-d-old fast-growing male broiler-type chicks for 2 weeks. Both the efficiency with which ME was used to support growth (kg) and the maintenance requirements (MEm) significantly decreased inversely to the biological value of the dietary protein (kg = 0·660, 0·600 and 0·572; MEm = 597, 522 and 464 k.J/kg W0·75 per d, for diets SI, S and SM respectively). The partial efficiencies of use of ME for protein accretion (kp) or fat deposition (kf) were also inversely related, the former increasing with the quality of the protein offered. An alternative procedure was used to try to overcome the statistical problems inherent in the partition of ME between fat and protein.

The objective of the experiment was to test whether maintenance energy requirements of non-lactating, non-pregnant beef cows increased with body fatness. Twelve Charolais cows were initially fed to attain either lean or fat body condition. Subsequently, over a period of 66 days food allowances were regularly adjusted for zero live-weight change. For the following 76 days, lean and fat cows received either 400 or 553 kJ metabolizable energy (ME) per kg initial live weight 0·75 per day, respectively. Fat cows maintained their body weight and composition whereas lean cows lost 296 g body weight, 221 g lipid and 65 g protein daily. Calculated maintenance energy requirements averaged 516 and 536kJ ME per kg live weight 0·75 per day for lean and fat cows respectively but were not significantly affected by body fatness. Diet dry matter, organic matter, crude protein and energy apparent digestibilities but not acid-detergent fibre digestibility were significantly higher in fat animals.