Effect of Reduced Early Growth Rate on the Protein Requirement of Roaster Chickens

How does early growth rate alter the response to the crude protein level in roaster diets? How is the mortality of birds kept to roaster weight affected by early body weight gain? A study was conducted by Hank Classen and Carlyle Bennett at the University of Saskatchewan to answer these questions.

A total of 1440 broiler cockerels, 720 each of two different broiler strains, were placed at one day of age in 48 pens, 30 birds per pen. The two broiler strains were housed in separate pens. All birds were reared under an increasing lighting program. Feed formulation was changed at 0 to 15 days, 15 to 29 days, 29 to 43 days, 43 to 57 days, and 57 to 64 days (Tables 1 and 2). Up to 43 days of age, half of the birds of each strain were fed either high (Trt 1 to 3) or low protein diets (Trt 4 to 6). After 43 days of age, the birds in the low and high protein feeding programs were divided between high, medium or low protein feeding regimes. While the birds reared on low and high protein feeding programs for the first 6 weeks were both split between three levels of protein, the birds reared on the lower protein rations were fed slightly lower levels of protein from 6 to 9 weeks than those reared on high protein rations. The combination of the two early feeding regimes and three different protein levels after 6 weeks of age, resulted in a total of six dietary treatments in the trial.

Table 1. Nutrient specifications of diets fed from 0 to 43 days of age

Feeding low protein diets from 0 to 43 days of age reduced body weight gain at 0 to 15, 15 to 29 and 29 to 43 days of age and increased feed:gain from 0 to 29 days of age. After 43 days of age, the situation was reversed with those reared on the low protein rations having higher body weight gain and lower feed:gain. Mortality for the birds reared on the low protein regime from 0 to 42 days was lower at almost all ages. The positive response after 6 weeks of age to slowing early growth rate occurred regardless of the level of protein fed after 6 weeks of age. No benefit to feeding higher levels of protein was observed after 6 weeks.

The two strains of broilers demonstrated remarkably different growth patterns and these differences were not affected by feeding program. The Strain B broilers grew slower up to 43 days of age but faster afterwards. The result was that the Strain B birds weighed 171 g less at 43 days but only 16 g less at 64 days than the Strain A. The Strain B broilers also had a lower feed:gain and mortality after 6 weeks of age. Using a broiler with slower early growth produced a similar response in bird performance to using low protein diets to slow early growth. Combining a slower growing breed with a low protein feeding program in the first six weeks produced the lowest overall mortality.

Table 3. Effect of feeding regime and broiler strain on body weight gain (kg)