Abstract In Pakistan, broilers are usually reared in opensided houses and are thus exposed to heatstress conditions during most part of the year. Furthermore, the commercial broiler diets are formulated to contain low Metabolizable Energy (ME) content. A review of literature revealed that most of the protein/amino acid requirement studies were conducted under thermocomfort temperatures and using the high ME diets; and thus appeared to be of limited practical significance for the local feed formulators. Keeping in view this premise, the present study was designed to evaluate the effect of graded levels of dietary protien (CP), lysine, methionine + cystine, and threonine on the performance of broilers fed lowME diets under the summer conditions of Pakistan. The first objective was to study the extent to which dietary CP could be reduced at the expense of supplemental lysine, methionine, and threonine and yet matching the performance to that on "standard" diets. In experiment 1, diets containing 20.3, 19.3, 18.4, and 17.2% CP (2700 kcal ME per kg; 0.90% digestible lysine with all other essential amino acids to meet or exceed the Ideal Protein Ratios) were fed to six replicate pens of 17 chicks each during the 421 days of age. Similarly, experiment 2 used diets containing 17.9,17.2,16.6, and 16.0 % CP (2750 kcal ME per kg; 0.83% digestible lysine with. all other essential amino acids to meet or exceed the Ideal Protein Ratios). Each of the diet was fed to six replicate pens of 10 chicks during 2142 days posthatch. Minimummaximum house temperature was recorded twice daily and found to be 2537C (exp 1) or 2637C (exp 2). In both experiments, body weight gain, feed intake and feed efficiency on diets with lowest levels of CP were significantly (P<0.05) low compared to that of respective counterparts fed on higher levels of CP. In experiment 2, significantly better (P<0.05) feed efficiency was achieved at diet having the highest CP, compared with two intermediate levels that in turn resulted in an improved feed efficiency over the diet having the lowest level of CP. Objective of the experiments 3 and 4 was to evaluate whether supplemental arginine and/or potassium could correct the performance depression at lowCP diets in previous experiments. In experiment 3, supplementation of Arginine + Potassium to lowCP diet resulted in a significant (P<0.05) improved feed intake and a numeric improvement in body weight gain (7.8%) and feed efficiency compared with either normal or lowCP unsupplemented diets. Similarly the results of experiment 4 showed that mimicking the amino acid and the electrolyte balance across diets, lowCP diets could restore an equal body weight gain and feed efficiency to that recorded on standardCP fed groups. Experiments 5 and 6 evaluated the effect of further reducing dietary CP on the performance of starting and finishing broilers. In experiment 5, two levels of dietary CP viz. 19.3 and 15.6% were fed each to 10 replicate pens of 10 chicks during 521 days posthatch. In experiments 6, two experimental diets having dietary CP levels of 17 and 13% were fed to 5 replicate pens of 5 chicks each during 2142 days posthatch. LowCP (LCP) diets were supplemented with crystalline lysine, methionine, threonine, arginine, tryptophan, isoleucine, and valine to bring the level of these amino acids to meet the Ideal Ratio dictated by the respective levels of dietary lysine. In both experiments, birds fed on LCP diets realized similar feed intake to that of corresponding standardCP diets; Body weight gain and feed efficiency, however, was significantly (P<0.00l) low on LCP fed groups. Experiments 7 and 8 were conducted, respectively to evaluate the lysine requirement of broiler chicks for 421 and 2142 days posthatch. In experiment 7, a practicaltype lysine deficient basal diet (2700 kcal ME per kg; 18.6% CP) containing 0.85% total lysine (0.72% digestible lysine) was supplemented with Llysine HCL to get total lysine levels of 0.85, 0.90,0.95, 1.00, 1.05 and 1.10%. All other amino acids in basal diet were calculated to meet or exceed the Ideal Protein ratios calculated at 0.97% digestible lysine (the maximum level of digestible lysine used in supplemental series). Each diet was fed to 7 replicate pens of 20 chicks each during 421 days posthatch. Quadratic regression analysis revealed a requirement estimate (0.95 of the maximum or minimum response) of 0.98 and 0.97% total lysine (0.85 and 0.84% digestible lysine) for body weight gain and FCR, respectively. Similarly, in experiment 8, practicaltype lysine deficient basal diet (2750 kcal ME per kg; 17.1% CP) containing 0.72% total lysine (0.6% digestible lysine) was supplemented with Llysine HCL to get total lysine levels of 0.72,0.78,0.84, 0.90, 0.96 and 1.02%. All other amino acids in basal diet, were calculated to meet or exceed the Ideal Protein ratios calculated at 0.90% digestible lysine (the maximum level of lysine used in supplemental series). Each of the diet was fed to 4 replicate pens of 20 chicks each during 2142 days posthatch. Fitted quadratic curves revealed the requirement estimate of 0.87% total lysine (0.75% digestible lysine) for both body weight gain and feed efficiency. Experiments 9 and 10 were designed to evaluate the effect of different levels of dietary lysine and ME on the performance of broiler chicks during 321 days posthatch. As a total of four lysine levels were tested under each ME, it was also intended to determine lysine requirement with each level of ME, and thus to test the hypothesis that the lysine requirement is a constant proportion of dietary ME? In experiment 9, treatments were placed in a 3 x 4 factorial arrangement Le. 3 levels of dietary ME (2700,2900, and 3100 kcal ME per kg) and 4 levels of dietary lysine to ME ratios (2.90, 3.15, 3.40, and 3.65 g digestible lysine per Meal ME). Significant ME to lysine interaction was found for body weight gain (P<0.001) feed intake (P<0.044) and feed efficiency (P<0.001). At the lowest level of lysine to ME ratio (2.90), increasing ME from 2700 to 2900 kcal significantly (P<0.001) improved the gain and feed efficiency. Supplemental lysine resulted in quadratic improvement in the gain and feed efficiency in only the 2700 kcal diet. Quadratic regression analysis with 2700 kcal diet revealed a lysine requirement estimate of 3.25 g digestible lysine per Meal ME, for both the gain and feed efficiency. Absence of any effect of supplemental lysine in case of higher ME diets indicated that lysine requirement was no more that 2.90 g digestible lysine per kcal ME. Treatments in experiment 10 were also placed in a 3 x 4 factorial arrangements i.e. 3 levels of dietary ME (2700, 2900, and 3100 kcal ME per kg) and 4 levels of total lysine (0.84, 0.94, 1.04 and 1.14 % of diet). Significant ME to lysine interaction was found for body weight gain (P<0.002) and feed intake (P<0.00l). At lowest level of dietary lysine (0.84 % lysine) increasing ME from 2700 to 2900 kcal significantly (P<0.002) improved gain and feed intake. This effect, however, disappeared at higher levels of dietary lysine. Feed efficiency improved (P<0.001) by increasing dietary level of ME or lysine; while no interaction effect of ME to lysine was observed in case of feed efficiency. Using the quadratic regression, estimated requirement for 2700 kcal ME diet were 1.0 and 1.04 % lysine respectively for gain and feed efficiency. For 2900 kcal ME diet requirement was estimated to be 1.05 and 1.06 %, while for 3100 kcal ME diet estimated requirement was 1.0 and 1.05 % total lysine, respectively for gain and feed efficiency. Using the amino acid digestibility coefficients of individual rawmaterials, requirement estimates respectively for gain and feed efficiency were 0.86 and 0.90 % digestible lysine at 2700 kcal ME; 0.91 and 0.92 % at 2900 kcal; and 0.86 and 0.91 % digestible lysine at 3100 kcal ME. Methionine + Cystine (M+C) requirements were studied for 021 days (exp.13) and 2142 days (exp 14) posthatch. In experiment 11, a practicaltype M+C deficient basal diet (2750 kcal ME per kg; 20.1% CP) containing 0.64% total M+C (0.54% digestible M+C) was supplemented with DLmethionine to get total M+C levels of 0.64,0.69,0.74,0.79,0.84 and 0.89%. Quadratic regression curves showed requirement estimates of 0.77 and 0.75% total M+C, respectively for the gain and feed efficiency. Calculated on the digestible amino acids, these were equal to 0.67 and 0.65% digestible M+C, respectively for gain and feed efficiency. In experiment 12, a practicaltype M+C deficient basal diet (2780 kcal ME per kg; 17% CP) containing 0.55% total M+C (0.47% digestible M+C) was supplemented with DLmethionine to get total M+C levels of 0.54,0.59,0.64,0.69,0.74 and 0.79%. The estimated requirement was 0.67 total M+C for both the gain and feed efficiency. Calculated on the digestible amino acids, it was found to be 0.60% digestible M+C for both the gain and feed efficiency. Threonine requirements were studied for 021 days (exp 13) and 2142 days (exp 14) posthatch. In experiment 13, a threonine deficient basal diet (2700 kca1 ME per kg; 16.6% CP) containing 0.59% total threonine (0.48% digestible threonine) was supplemented with Lthreonine to get total threonine levels of 0.59, 0.64, 0.69, 0.74, 0.79 and 0.84%. Quadratic regression curves didn't represent a good fit to the data, as mentioned by very poor coefficient of determination (Rsq) values 0.013 and 0.116 respectively for gain and feed efficiency. The requirement of threonine, therefore, for both the gain and feed efficiency were assumed to be no more than the minimum levels of dietary threonine in this experiment (0.59% total threonine). Calculated using the digestibility coefficients for individual component ingredients, this was equal to 0.48% digestible threonine. In experiment 14, a threonine deficient basal diet (2780 kcal ME per kg; 15.9% CP) containing 0.55% total threonine (0.43% digestible threonine) was supplemented with Lthreonine to get total threonine levels of 0.55, 0.59, 0.63, 0.67, 0.71 and 0.75%. Quadratic regression curves showed requirement estimates (0.95 of the maximum response) of 0.67 and 0.66% total threonine, respectively for the gain and feed efficiency. Calculated on the digestible amino acids, these were equal to 0.55 and 0.54% digestible threonine, respectively for gain and feed efficiency.
