Broiler breeding companies continue to analyse the recommended nutrient levels of broiler diets due to the short production cycle and continuous genetic development of broilers. Modern broilers reach marketable weights very early, often at an immature body weight and without achieving maximum genetic potential in terms of absolute quantities, for example daily growth. As broilers age, their daily feed intake increases and nutrients are deposited in body tissue, mainly as protein and fat. An imbalance of energy or protein cause for excess fat deposited in carcasses due to excessive feed intake to satisfy a nutritional requirement. The cost of feed contributes substantially to the total production expenses, with energy alone contributing approximately 70% of the total cost of poultry diets. Therefore, the focus of feed companies remain to determine the energy and protein requirements of broilers, in order to feed a balanced diet, delivering a bird with an optimal carcass composition.
Two experiments of identical design were conducted in floor pens to evaluate the effect of metabolisable energy levels as well as crude protein levels on broiler performance. The first trial was an energy dose-response trial and the second trial a protein dose-response trial. Each study was conducted over a period of five weeks where five thousand seven hundred and sixty Hubbard Flex (mixed sex as hatched) broilers were housed in 60 pens. Ninety six (96) chicks were randomly allocated in a pen at a stocking density of 16 birds/m2. Each treatment was repeated once within a block, totalling to 10 replications/treatment. Water and feed were provided ad libitum.
In the energy dose-response experiment two iso-protein basal feeds were formulated and manufactured, one containing a high energy (HE) level, the other a low energy (LE) level. These basal diets were further diluted into four diets containing various percentages of energy (80% LE: 20% HE; 60% LE: 40% HE; 40% LE: 60% HE and 20% LE: 80% HE). The crude protein and amino acid balance was kept constant across all treatments. Six dietary treatment combinations were implemented in a 4-phase feeding programme: Pre-starter (0 to 10 d), Starter (11 to 18 d), Grower (19 to 28 d) and Finisher (29 to 35 d of age). Body weight (BW) and average daily gain did not show any significant difference between LE and HE treatments. A weekly numerical difference was recorded for BW between LE and HE treatments; where birds fed the HE diet weighed heavier on Day 35, at 1939.97 g compared to the 1898.10 g of birds fed the LE diet. FI increased as the energy concentration increased from the LE to HE treatments; and although the differences in FI was non-significant (NS), birds fed HE diets consumed 2882.11 g/bird by Day 35 and birds fed LE diets, only consumed 2830.90 g/bird in total. No significant difference was recorded for cumulative feed conversion ratio (CFCR) among treatments by Day 35 and birds fed the LE diet ended with 1.53 points compared to 1.52 points for birds fed HE diet. Despite the dietary energy increase in energy level from the LE diet to the HE diet, there was no significant effect on daily mortalities; birds fed the LE diet showed 5.10% mortality compared to 5.41% mortality for birds fed the HE diet.
In the protein dose-response trial two isocaloric basal feeds were formulated and manufactured, one containing a high protein (HP) level, the other a low protein (LP) level. These basal diets were further diluted into 4 diets containing various percentages of protein (80% LP: 20% HP, 60% LP: 40% HP, 40% LP: 60% HP and 20% LP: 80% HP). The dietary metabolisable energy (ME) was kept constant across all treatments. Birds fed the HP diet showed the greatest BW at Day 21 with 900.97 g compared to birds fed the LP diet on 858.85 g; and weighed 1937.48 g at Day 35 compared to 1869.80 g for birds fed LP diet. Cumulative feed intake (CFI) decreased as the protein content in the feed increased. Although the results from Day 28 only approached significance, birds fed the HP diet consumed significantly less feed (2840.29 g/bird) than birds fed the LP diet (2913.66 g/bird) by Day 35. CFCR for HP was 1.50 points by Day 35 compared to 1.59 points for LP. The increase in protein content from LP to the HP diet, did not cause a significant increase in mortalities; although a numerical increase can be seen of 6.24% for birds fed on the LP diet compared to 9.06% mortality in birds fed the HP diet.