The influence of drying regimes on the digestibility of starch for broilers fed semi-synthetic diets

Starch digestibility is commonly used to indicate the nutritional value of a cereal; the higher the starch digestibility, the more energy is available to the animal. Two broiler trials were conducted to assess digestibility of wheat starch, each employing a standard semi-synthetic diet with test wheat comprising 750 g/kg. In Experiment 1, two varieties of wheat were employed; E, a hard wheat and C, a soft wheat. Batches were dried separately at 70 °C, 85 °C and 100 °C in a large forced air oven, on mesh-bottomed metal trays. An untreated sample of each variety was also tested as a control. In Experiment 2, wheat was of one variety; E, grown at two locations harvested at two dates, being the date of grain maturity and maturity plus 14 days. Each of these four samples was then dried either at 100 °C for 72 h, or at ambient temperature (less than 30 °C). In Experiment 1, there was no linear relationship between the response to temperature treatments (starch digestibility; P=0.701, AME; P=0.804). There was a decrease in CTTAD of starch of wheat variety C treated at 85 °C, compared to 5 of the 7 other treatments (including controls) (P=0.020). Variety E dried at 85 °C had decreased starch digestibility compared to the control of the same variety and all variety C samples (P=0.020). In Experiment 2, the CDAD and CIAD of starch were increased with the 100 °C drying regime (P<0.001), suggesting increased efficiency of digestion and absorption. The effects of increased grain moisture and temperature on starch digestibility are difficult to predict. Based on the results described above, there should be no detrimental effect on starch digestibility or AME of drying wheat at temperatures of up to 100 °C, with starting moisture contents of up to 270 g/kg. In fact, there may even be an increase in digestibility, compared to equivalent non-temperature treated wheat.