This experiment was conducted to investigate the effects of guanidinoacetic acid (GAA) on productive performance, intestinal morphometric features, blood parameters and energy utilisation in broiler chickens. A total of 390 male broiler chicks (Ross 308) were assigned to six dietary treatments based on a factorial arrangement (2×3) across 1–15 and 15–35-d periods. Experimental treatments consisted of two basal diets with standard (STD; starter: 12·56 MJ/kg and grower: 12·97 MJ/kg) and reduction (LME; starter: 11·93 MJ/kg and grower: 12·33 MJ/kg) of apparent metabolisable energy (AME) requirement of broiler chickens each supplemented with 0, 0·6 and 1·2 g/kg GAA. Supplemental 1·2 g/kg GAA decreased the negative effects of feed energy reduction on weight gain across starter, growing and the entire production phases (P<0·05). Energy retention as fat and total energy retention were increased when birds received LME diets supplemented with 1·2 g/kg GAA (P<0·05). Net energy for production (NEp) and total heat production increased in birds fed LME diets containing 1·2 g/kg GAA (P<0·05). A significant correlation was observed between dietary NEp and weight gain of broilers (r 0·493; P=0·0055), whereas this relationship was not seen with AME. Jejunal villus height and crypt depth were lower in birds fed LME diets (P<0·05). Serum concentration of creatinine increased in broilers fed LME diets either supplemented with 1·2 g/kg GAA or without GAA supplementation (P<0·05). Supplemental GAA improved performance of chickens fed LME diet possibly through enhanced dietary NEp. The NEp could be preferred over the AME to assess response of broiler chickens to dietary GAA supplementation.

A foundation diet, an intermediate blend and a summit diet were formulated with different levels of soyabean meal, casein and crystalline amino acids to compare ‘slow’ and ‘rapid’ protein diets. The diets were offered to male Ross 308 chicks from 7 to 28 d post-hatch and assessed parameters included growth performance, nutrient utilisation, apparent digestibility coefficients and disappearance rates of starch and protein (N) in four small intestinal segments. Digestibility coefficients and disappearance rates of sixteen amino acids in three small intestinal segments and amino acid concentrations in plasma from portal and systemic circulations from the foundation and summit diets were determined. The dietary transition significantly accelerated protein (N) disappearance rates in the distal jejunum and ileum. The transition from foundation to summit diets significantly increased starch digestibility coefficients in the ileum and disappearance rates in all four small intestinal segments. These starch responses were associated with significant enhancements in nutrient utilisation. The dietary transition linearly increased digestibility coefficients and disappearance rates of amino acids in the majority of cases. The summit diet increased plasma concentrations of five amino acids but decreased those of four amino acids relative to the foundation diet to significant extents. Plasma concentrations of free amino acids were higher in the portal than systemic circulations. Rapid protein disappearance rates advantaged poultry performance and influenced post-enteral availability of amino acids. If the underlying mechanisms are to be identified, further research into the impact of protein digestive dynamics on broiler performance is required but appears justified.

A total of ten experimental diets with protein concentrations ranging from 154 to 400 g/kg and two lipid levels (46 and 85 g/kg) with identical energy densities were offered to 240 male Ross 308 broilers from 7 to 28 d post-hatch. Growth performance was monitored and nutrient utilisation (apparent metabolisable energy (AME), N-corrected AME (AMEn), AME daily intake, AME:gross energy ratios, N retention) was determined. The weight gain response of broiler chickens to dietary protein concentrations in diets containing high and low lipid levels was diverse, with the relevant quadratic regressions being significantly different (P<0·05). With low lipid levels, the predicted maximum weight gain of 1809 g/bird equated to 342 g/kg dietary protein, whereas, for high lipid levels the predicted maximum weight gain of 1694 g/bird equated to 281 g/kg dietary protein. AME was linearly correlated with dietary protein concentration but regressions in diets with different lipid content were not significantly different (P>0·05). AMEn was also linearly (P<0·0001) increased with dietary protein concentrations but regressions in diets with low and high lipid content were significantly different (P<0·03). Carcass protein content increased linearly with dietary protein content in diets containing high lipid concentrations (r 0·933, P<0·0001); by contrast, this relationship was quadratic (R2=0·93, P<0·0001) in diets with low lipid levels. In conclusion, predictably, the effects of dietary protein concentrations on broiler performance were profound; however, the impact of dietary protein on performance in broiler chickens was modified by dietary lipid concentrations.

The right-angled triangle mixture experiment was designed to include fourteen diets with different concentrations of starch, protein and lipid. Experimental diets were offered to male Ross 308 broiler chickens from 10 to 23 d after hatching, and response curves and surfaces were generated to illustrate the influence of macronutrients on growth performance and nutrient utilisations. Despite the primary function of macronutrients, especially protein, may not be providing energy, macronutrients were expressed as energy derived from starch, protein and fat for statistical purposes in the mixture design. Energy derived from lipid had a greater impact on feed intake than energy derived from starch and protein. When we compared the influence of starch and protein on feed intake, ‘equal distance rule’ was observed, which means the animal consumes feed to the point on its respective nutritional rails where the shortage of starch exactly equals the surplus of consumed protein. Increasing the protein-derived energy intake increased weight gain in broiler chickens, whereas energy intake derived from starch and lipid had little impact on weight gain. Feed conversion ratio (FCR) may be reduced by either increasing protein energy intake or decreasing starch energy intake. As the slope of the contours was less than 1, the influence of starch energy intakes on FCR exceeded that of protein energy intakes. In conclusion, energy derived from protein is more important than non-protein energy in terms of weight gain, and a balance between protein and energy supplies is required for efficient muscle protein deposition.

Two experiments were conducted to evaluate the nutritional value of three cultivars (Promore, Kiev mutant and Ultra) of white lupins (Lupinus albus L.) for broilers. In experiment 1, the apparent metabolisable energy (AME) and ileal amino acid digestibility coefficients of the three cultivars were determined. The cultivar effects were significant (P < 0.05) for AME, but the ileal amino acid digestibility coefficients were similar (P > 0.05) between cultivars. The AME value of Ultra cultivar was lower (P < 0.05) than those of Promore and Kiev mutant cultivars. In Experiment 2, using the AME and ileal digestible amino acid values determined in Experiment 1, diets containing 200 g/kg of lupin were formulated and the effects of feeding these diets on performance, digestive tract development and excreta quality of broiler starters were investigated. Weight gain, feed intake and feed per gain of broilers fed diets containing white lupins were similar (P > 0.05) to those fed the maize–soybean meal diet. The performance of birds fed diets containing different cultivars of white lupins was similar (P > 0.05). Several digestive tract parameters were influenced by the dietary inclusion of white lupins. In particular, the relative liver weight and the relative empty weights of small intestine and caeca in birds fed diets containing white lupins were higher (P < 0.05) than those fed the maize–soybean meal diet. No differences (P > 0.05) were observed in the excreta quality scores between the birds fed the maize–soybean meal diet and those fed diets containing white lupins.