GROWTH, FEED CONVERSION AND MORTALITY OF COMMERCIAL MEAT-TYPE CHICKENDURING A 20 WEEK GROWING PERIOD

In the present study two commercial broiler strains A (Arbor Acres Reg ular) and B (Arbor Acres Yield Master) were reared on litter floor til l 20 weeks of age. From strain A 5 replicate pens from each sex, from strain B two replicate pens from each sex were used, each pen containi ng 120 chicks. Till 8 weeks of age 13 birds mere housed per m(2) floor space, after 8 weeks of age density was reduced to 6,5 birds/m(2) flo or space by doubling the number of pens in the trial. Management of th e birds was similar in all other respects to commercial practice. Till 8 weeks of age the diet fed contained 12.86 MJ ME energy/kg dry matte r and 241,6 g protein, the diet fed from 8 till 20 weeks of age contai ned 11.34 MJ/kg ME energy and 144,9 g protein respectively. Body weigh t was significantly different between sexes during the entire rearing period, the differences between genotypes were significant in most of the recorded periods. Liveweight of the broilers at 4, 8, 12, 16 and 2 0 weeks of age respectively were: Strain A males: 1074 g, 3050 g, 4374 g 5036 g, 5806 g, females: 994 g, 2613 g, 3642 g, 4144 g and 4757 g. Strain B males: 998 g, 2875 g, 4529 g, 5650 g, 6388 g, females 1013 g, 2539 g, 3624 g, 4285 g and 4982 g. Present day male commercial broile rs (strain B) needed 5 weeks less to reach 6 kg live weight compared t o broilers 15 years ago, in females the increase in growth capacity wa s even more pronounced. The coefficient of variation of live body weig ht increased in males considerably in the second part of the rearing p eriod, whereas in females this phenomenon was less apparent. Cumulativ e feed conversion and fed efficiency showed the expected age and growt h intensity related patterns till 10 weeks of age. After 10 weeks of a ge the increased mortality of males in both genotypes markedly decreas ed feed efficiency of the male populations compared to females. Total mortality during the 20 week growing period was 24.3 and 33.3 in males , and 7.9 and 11.7 percent in females for A and B broilers respectivel y. Vaccination against Marek's disease reduced mortality of males to 1 7.3 and that of females to 3.9 percent in A broilers tested. The signi ficantly lower mortality of female broilers compared to males was appa rent during all phases of rearing.