Question:  query regarding decrease  body weight in some broiler 
Answer: told him to observe closely the managemental practice including the disinfection  of the farm house/premises .decrease growth rate is due to the difference in feed conversion ratio[ fcr ], the fcr is different for different bird ,so the variation may occur ,in 30 days old broiler the average body weight should be approximately 2-2.2 kg , under proper care & management ,some factors are responsible for decrease growth -like  genetic , temperature , feed composition ,managemental condition, timely use of   medicine& vaccine ,use of growth promoter  also plays an important role in body weight gain ;the decrease  growth of broiler may also occur  due to the process of incubation ( either natural or artificial) & due to the disease embryo at the time of selection off egg throuh candling