Effect of phytase dose and reduction in dietary calcium on performance, nutrient digestibility, bone ash and mineralization in broilers fed corn-soybean meal-based diets with reduced nutrient density

The effect of reducing dietary Ca level (and Ca:P ratio) in combination with phytase supplementation, on broiler growth performance, nutrient digestibility, bone ash and mineralization, was investigated. A total of 2072 Ross 308 d-old male broilers were allotted to 7 dietary treatments with 37 birds/pen and 8 pens/treatment in a randomized block design. A positive control corn-soybean meal-based diet was formulated based on breeder's recommendations. Six test diets were evaluated in a 2 x 3 factorial arrangement including two levels of a Buttiauxella sp. phytase (500 and 1000 FTU/kg feed) and three Ca reduction levels (equivalent to 1.3, 1.6, 2.3 g/kg reduction and 1.6, 1.9 and 2.3 g/kg reduction in the 500 and 1000 FTU/kg phytase treatments respectively vs. PC). The reductions in available P (AvP) of the test diets were 1.46 and 1.74 g/kg in the 500 FTU/kg and 1000 FTU/kg phytase treatments respectively. Test diets were formulated by adding phytase, Ca and P supplements to a basal diet containing a reduction of 68 kcal ME/kg, on average 0.2 g/kg dig AA and 0.3 g/kg Na vs. PC. Diets were formulated in four phases. Excreta samples were collected on d 24 to 26 and 38 to 40 (from 5 birds transferred to cages on d 21 and 35 respectively) and ileal digesta and tibia samples were collected on d 10, 27 and 41. Compared to PC, feed conversion ratios (FCR) were reduced in growers fed medium or high Ca reduction and high phytase dose diets (P < 0.05). All phytase treatments improved energy efficiency (by up to 1.05 MJ/kg BWG) compared with the PC. Factorial analysis revealed a greater BWG and lower FCR with phytase supplemented at 1000 FTU/kg vs. 500 FTU/kg during starter and grower phases (P < 0.05). The high Ca reduction level (2.3 g/kg reduction vs. PC) reduced FCR in the finisher phase, and reduced overall (42 d) calorie consumption and feed cost per kilogram weight gain, compared with the low Ca reduction level (P < 0.05). Compared to the PC, phytase supplementation generally enhanced (by 10-30%) ileal, and total tract digestibility of P and, to a lesser extent Ca. Ileal digestibility of P at d 10 and d41 and Ca at d 41 were higher in the high- vs. the low-phytase groups. The high Ca reduction groups resulted in higher ileal P digestibility at d 10 and d41, ileal Ca digestibility at d27 and d41 vs. low Ca reduction groups (P < 0.05). Tibia ash was unaffected by dietary treatments, but bone Zn was increased at d 10 in all but one experimental diet vs. PC (P < 0.001) and was higher in the high- vs. the low-phytase groups (P < 0.01). Collectively, these findings indicate that a moderate (1.6 g/kg) to high (2.3 g/kg) reduction in dietary Ca in diets supplemented with 500-1000 FTU/kg of Buttiauxella phytase improved P and Ca digestibility, energy efficiency and productive performance in broilers fed corn-soybean meal-based diets.