Lack of relationship between either specific weight or presence of the 1B1R gene and nutritive value of wheat in broiler diets

1. A study of the relationship between specific weight (SW) and nutritive value of wheat in diets offered to broilers up to 4 weeks of age was carried out using 10 varieties of wheat grown at three different locations; this gave rise to a range of specific weights from 59 to 76 kg/hl. 2. Because five varieties contained the 1B1R translocation and 5 did not, this allowed the effects of 1B1R on the performance of birds given high concentrations of dietary wheat to be further investigated. 3. The diets, which contained wheat at 744 g/kg with casein (142 g/kg) as the sole protein supplement, were heat-treated and pelleted. They were fed ad libitum to male, Ross broiler chicks from 7 to 28 d, in individual cages in a controlled environment room. 4. A 7 d excreta collection was made from 14 to 21 d for determination of apparent metabolisable energy (AME) content and wheat AME was calculated from the diet values. A separate determination of true metabolisable energy (TME) was undertaken using the method of McNab and Blair (British Poultry Science, 29: 697-707, 1988). 5. Wheat samples were analysed for proximate constituents, starch, total and soluble non-starch polysaccharides (NSP), amino acid content and gross energy and specific weight, thousand grain weight and in vitro viscosity were recorded. They were also subjected to near infrared reflectance spectroscopy (NIRS) with a view to establishing relationships with chemical constituents and performance parameters. 6. At 28 d the birds were humanely killed and in vivo viscosity was determined on the jejunal digesta supernatant. 7. There was quite a narrow range of crude protein content (N 5·83) across the 30 wheat samples, the means for the three locations (Crossnacreevy, Downpatrick and Limavady, respectively) being 112, 121 and 122 g/kg. Starch contents of individual samples ranged from 604 to 679 g/kg, total NSP from 92·1 to 122·6 g/kg, soluble NSP from 17·6 to 32·6 g/kg and in vitro viscosity from 10·6 to 26·3 cps. 8. There were no significant varietal effects for dry matter intake (DMI), live weight gain (LWG) or gain:feed ratio. Apparent metabolisability of energy (ME:GE) for the diets ranged from 0·738 (Hussar) to 0·778 (Harrier) the effect being significant (P < 0·05) and there was a significant (P < 0·001) relationship (r2 = 0·46) between ME:GE and gain:feed ratios. Calculated wheat AME for individual varieties ranged from 14·1 to 15·0 MJ/kg DM (P > 0·05). TME values ranged from 16·3 to 16·6 MJ/kg DM (NS). 9. In vivo viscosity ranged from 12·3 (Ritmo) to 23·7 cps (Hussar), the varietal effects being significant (P < 0·001). Viscosity was significantly higher for the 1B1R varieties than for the non-1B1R (22·7 vs 16·3 cps) but there were no significant effects on mean DM intake, LWG, gain:feed, ME:GE, calculated wheat AME, ME:gain (MJ AME per kg gain) or TME. 10. Wheat AME was negatively correlated (P < 0·05) with total NSP (slope -0·035) but was significantly affected by variety, constants ranging from 16·74 to 17·79. For wheat TME there were significant variety*total NSP interactions with slopes ranging from 0·29 to -0·07. There was no significant relationship between wheat AME and soluble NSP and there were significant variety*soluble NSP interactions for TME with slopes ranging from 0·09 to -0·14. 11. Despite the relatively wide range of in vitro viscosity there was no significant correlation of either calculated wheat AME or wheat TME with in vitro viscosity. For TME there were significant variety*viscosity interactions with slopes ranging from 0·21 to -0·13 (P < 0·001). There was also no significant correlation between soluble NSP and in vivo viscosity. 12. AME values for the NIRS calibration set varied from 13·7 to 15·6 MJ/kg and averaged 14·6 MJ/kg, whereas for the validation set, the values ranged from 14·0 to 16·1 MJ/kg with the mean value being 14·7 MJ/kg. The best correlation coefficient (r2) of 0·90 was obtained when the calibration sample set was subjected to SMLR using the second-order derivatised data. However, moving to validation, the outcome was extremely poor with 1-VR values being 0·12 and 0·09, respectively, for the external validation and an internal validation subset. 13. In summary, none of the production characteristics showed a good correlation with specific weight. Using the linear estimates for wheat AME and TME the effect of a 10 kg/hl change in SW equates to a 3 or 1·4% change, respectively. The estimate for AME is likely to be a worst-case value due to the high inclusion level of wheat. The range of calculated wheat AME (approximately 1 MJ) across the 30 wheat samples is similar to that seen in previous studies for wheat grown in Northern Ireland and much lower than in some other studies. 14. None of the other parameters examined gave any better relationship with nutritive value (assessed as AME or TME) than SW. This is a disappointing outcome, particularly in view of some previous studies which suggested that in vitro viscosity provides a good prediction of nutritive value.