Influence of dietary electrolyte balance on phytase efficacy in poultry

The aims of the studies reported in this thesis were to investigate the possible effects of changes in dietary electrolyte balance (dEB) levels on phytase efficacy on growth performance, bone mineralisation and nutrient utilisation; and the effects of phytase supplementation, alone or in combination with xylanase, on growth performance, bone ash, volatile fatty acids (VFA) and pH at different parts of the gastrointestinal tract (GIT) in diets marginally deficient in dEB. Two experiments were performed and each experiment used 336 day-old Ross 308 male broiler chicks which were allocated to 7 treatments in a randomised complete block design. The dietary treatments were corn-soybean meal based and were fed in a mash form. For both experiments, each treatment had 6 replicate pens with 8 birds each. A 3×2 factorial arrangement of treatments was used in each experiment along with a positive control (PC) with nutrient levels meeting breeder recommendation. Data in both experiments were analysed using the linear mixed model of GenStat. In experiment 1, three levels of dEB (214, 234 and 266 mEq/Kg) and 2 levels of phytase (0 FTU/Kg and 1500 FTU/Kg) were used. Birds and feed were weighed on day 0 and 21. Excreta were collected from pens on days 19 and 20. On day 21 one randomly selected bird from each pen was killed, blood was collected from the jugular vein and blood pH measured. The remaining birds in each pen were euthanised and ileal digesta were collected from each pen. The left tibia bones from two randomly selected birds in each raised pen were collected for bone mineralisation study. There were no dEB × phytase interactions for any of the growth performance and bone mineralisation responses. But an interaction (P<0.05) was observed for ileal digestibility and total tract retention for most of the nutrients. Increasing dEB from 214 to 266 mEq/Kg had no effect on growth performance or tibia bone mineralisation. Phytase supplementation increased (P<0.05) broilers growth performance and tibia bone mineralisation responses relative to the diets without phytase. Growth performance and bone mineralisation responses were greater (P<0.05) in PC compared with NC, except for feed intake. There were no differences in the response between PC and the diets containing equivalent dEB (234 mEq/Kg) but broilers fed the corresponding diets with phytase outperformed NC and PC broilers. Ileal and total tract P utilisation were greater (P<0.05) in birds fed NC and phytase-supplemented diets than the PC birds. Utilisation of DM, Ca, K, and P was also greater (P<0.05) in phytase-supplemented diets at 234 mEq/Kg but for P and K was greater in phytase-supplemented diets when dEB was 266 mEq/Kg. In Experiment 2, the factors included 3 levels of enzyme (no enzyme, phytase alone or combination of phytase and xylanase) and 2 types of negative control (NC1 and NC2). All the birds and feed were weighed on day 0 and 21. After slaughter on day 21, the left tibia bones were collected and the pH of the gizzard, jejunum and caeca (left and right) were taken. Caecal content were collected into tubes to be analysed for volatile fatty acids content. There were no significant matrix × enzyme interactions for any of the responses. Reducing the dietary levels of P and Ca in both NC1 and NC2 reduced (P<0.05) gain:feed compared with PC but had no significant effect on the other growth performance responses. Phytase alone had no effect on growth performance compared to diets with no enzyme supplementation, but when phytase was combined with xylanase an increase (P<0.01) in gain:feed was observed. Tibia ash was lower (P<0.01) in NC1 and NC2 compared to PC and improved (P<0.05) with supplementation of phytase alone or combined with xylanase. The treatments had no effect on the digesta pH or caeca VFA. It was concluded from experiment 1 that phytase promoted growth and tibia bone mineralisation independently of the dEB levels and that the extent of phytase effect on nutrient utilisation differs depending on dEB content of the diets, which indicates that treatments effects seen in nutrient utilisation do not always reflect effects observed in growth performance. It was concluded from the second experiment that dietary reductions of Ca and P negatively affected tibia mineralisation and gain:feed which were counterbalanced by supplementation of phytase plus xylanase.