Feed access to, and inclusions of fishmeal and corn starch in, sorghum-based broiler diets influence growth performance and nutrient utilisation as assessed by the Box-Behnken response surface design

• Box-Behnken design was used to understand digestive dynamics in chickens.
• Rapidly digestible protein is more important than starch.
• Nitrogen retention was influenced by duration of feed access only.
• Over-consumption was prompted in birds by restricting feed access.
• Both feed access and corn starch influenced energy utilisation.

A feeding study with a Box-Behnken response surface design was completed to investigate the influence of starch and protein digestive dynamics on growth performance and nutrient utilisation under different durations of feed access. The design comprised three feed access durations (6, 15 and 24 h), three fishmeal inclusion rates (0, 87.5, 175 g/kg) and three corn starch inclusion rates (0, 100, 200 g/kg) in broiler diets based on sorghum and soybean meal. Each of thirteen dietary treatments was offered to 5 replicates of 6 birds per replicate or a total of 65 cages and 390 Ross 308 male and female chicks (50-50, feather-sexed) from 15 to 28 days post-hatch. Duration of feed access had the greatest impact on weight gain and feed conversion ratio (FCR). Both weight gain and feed conversion efficiency were improved with increasing feed access duration. At each level of feed access duration, FCR was improved with increasing fishmeal inclusion, whereas corn starch inclusion had little impact on FCR. Nitrogen retention was influenced by duration of feed access but not by fishmeal or corn starch. The predicted minimum N retention 52.38% equated to a feed access of 17.45 h duration. N retention decreased with feed access duration when feed access was less than 17.45 h and then improved with feed access duration when feed access was longer than 17.45 h. The predicted maximum feed intake of 1720 g/bird equated to a feed access of 22.00 h duration. It is noteworthy that birds with 24 h access to feed had significantly higher total feed intake but significantly lower hourly intake (P < 0.0001) than birds with 6 h feed access from 15 to 28 days post-hatch. Although birds were not offered comparable diets, the overall average hourly intake for birds with 15 h feed access (7.66 g/bird) is higher than birds with 24 h feed access (5.09 g/bird) while the total intakes were relatively similar (1608 versus 1711 g/bird). This may suggest over-consumption was prompted in birds by restricting feed access. Apparent metabolisable energy (AME) decreased with increased duration of feed access and there was an overall quadratic effect of corn starch inclusion on AME, which means the highest AME equated to a corn starch inclusion of 102 g/kg for each level of feed access duration. Corn starch inclusion higher or lower than 102 g/kg compromised AME in broiler chickens. The present study confirms that digestive dynamics of protein are of more importance than starch as increasing fishmeal improved FCR, which was not the case with increasing corn starch.