Effect of drying temperatures on feed value of maize for broilers
The maize grain tested was received at a moisture content of 11.3 percent. Some of the maize was used as received (Fresh) while other batches were obtained through drying in large draught-ovens at 85 deg, 95 deg or 105 deg. Each batch was dried over 24 hours to yield varying moisture contents, followed by storage at 5 deg. Four diets were prepared, based on these grain batches and a commercial soybean isolate.
Three hundred and twenty day-old female broiler chicks (Ross) were allocated to the treatments. Each of the four diets, in mash form was fed with or without a microbial enzyme supplement, Avizyme 1500, included at 1 g/kg diet.
There were four replicates for each of the eight general treatments. The diets were fed for 28 days.
Feed intake and body weight were measured at the end of each week. At the end of the feeding period, six chicks were killed and dissected.
Dry matter (DM), crude protein (CP), lipid and phosphorus (P) contents increased with increase in drying temperature. The amino acid contents of the oven-dried maize were also generally higher than those of the sun-dried maize (Fresh), especially up to 95 deg of heating. Compared to the fresh maize, oven-drying at 105 deg improved the total starch and amylopectin contents by 8 and 5 percent, respectively while amylose content was reduced by about 12.4 percent.
The ratio of amylose:amylopectin also declined with increasing drying temperature. This reduction in amylose signifies an improvement in quality but is contrary to the classical response of wet starch to heating. The moisture content of the samples used in this study may be too low to cause retrograding of starch through heating.
Although the samples were stored at low temperature prior to use, this was not done immediately after oven-drying. Therefore, the grains would have cooled slowly, which would reduce the chances of annealing.
Feed intake during the entire trial period was increased (P<0.05) as a result of heattreating the maize up to 95 deg. Further heating did not have any significant effects on intake of the diets.
There were no significant effects of microbial enzyme supplement on feed intake at any period of assessment. The 7-day body weight of chicks was reduced (P<0.05) in chicks on the control (without Avizyme) diet in which maize had been heated at 85 deg.
At 14 days of age, body weight was also reduced (P<0.05) by oven-drying at up to 95 deg but this was observed only on diets supplemented with the microbial enzyme. The final body weight of chicks on the diet based on fresh maize was improved (P<0.05) by the microbial enzyme supplement. There was no effect of the enzyme supplement on body weight assessed at earlier ages.
Heat treatment had some effects on the chemical composition of maize grain but this did not impart enough variation in the samples to stimulate a response to the microbial enzyme supplement tested.
Heat treatment tended to also improve the starch quality of the grain, thereby obviating the need for microbial enzyme supplement. It may be necessary to test samples obtained from actual commercial processes or more closely simulate the commercial conditions in future studies.
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Article made possible through the contribution of the Australian Poultry Science Symposium (APSS) 2006.