Experiment 1 was performed with 600 Sonali chickens (Rhode Island Red x Fayoumi) which were divided into two groups and reared with three farmers in rural area of Bangladesh for 10 weeks. Group 1 received the basal diet without added vitamin A until 42 days of age, when chickens were treated orally with 500 IU vitamin A for 5 days and thereafter received 500 IU vitamin A kg -1 feed until the end of the experiment. Group 2 received 1500 IU vitamin A kg -1 feed during the whole experiment.
Experiment 2 was carried out at the Department of Veterinary Pathology, Bangladesh Agricultural University , Mymensingh , Bangladesh . This experiment consisted of 720 day-old Sonali chickens, which were divided into four groups initially. Group 1 and 2 were left unvaccinated with IBDV vaccine and Group 3 and 4 were vaccinated with infectious bursal disease virus (IBDV) vaccine on day 14 and 21 of age. All groups were vaccinated with Newcastle disease vaccine at day 4, 28 and 60 of age. Group 1 and 3 received 1500 IU vitamin A kg -1 feed and Group 2 and 4 received 500 IU vitamin A kg -1 feed for 10 weeks.At day 32, each group was subdivided into Group A and B. All group A birds were challenged at day 35 with IBDV. All group B birds remained as control animals. The influence of vitamin A on humoral immune response ( IgG) to Newcastle disease vaccine and IBDV vaccine was determined in some of the chickens by using ELISA test kits specific for each disease.
The results of experiment 1 showed that birds performed poorly without supplemented vitamin A. Typical vitamin A deficiency symptoms appeared in Group 1 within day 18-39 at different time and severity for the three farmers. Overall mortality was 29% with 26.2% in Group 1 and 2.8% in Group 2. Feed consumption, body weight gain and feed conversion ratio were significantly poorer in Group 1 than Group 2. Mortality was much higher when deficient birds were affected with coccidiosis and response to treatment was poor. Retinol levels in blood plasma and liver at 42 days of age in Group 1 were very variable and not detectable in 10 out of 12 birds. The oral supplementation of vitamin A to Group 1 increased the liver concentration of vitamin A to a higher level than for Group 2, while the retinol concentration in blood plasma was similar in the two groups ( 0.16-0.39 µg ml-1). It is concluded that the stability of both natural occurring vitamin A in feed ingredients and retinol acetate in premix has to be investigated under the warm and humid conditions occurring in Bangladesh. Furthermore , the requirement of vitamin A and the vitamin A activity of b-carotene in local feed ingredients have to be determined under relevant production conditions in Bangladesh with local poultry breeds.
The results of experiment 2 showed that the low level of dietary vitamin A caused typical deficiency symptoms. Increased morbidity and mortality and slow recovery were observed when vitamin A deficient birds were challenged with IBDV. Body weight gain and feed conversion ratio were also inferior in the deficient challenged birds. Humoral immune response to ND and IBDV vaccines was lower in the birds receiving 500 than 1500 IU vitamin A kg -1 feed. The high level of vitamin A prevented a decline of maternal derived IBD antibodies. The chickens fed 500 IU vitamin A kg -1 feed were highly susceptible to ND and responded poorly to coccidiostats. Caecal core formation was a common finding in these birds. At the end of the experiment the retinol concentration was generally low in plasma ( 0.0 -0.06 µg ml -1 at the low vitamin A level and 0.02 - 0.16 µg ml -1 at the high level ), and in the liver tissue it was below detection limits. It is concluded that 500 IU vitamin A kg -1 feed is below the requirement of Sonali chickens, and under the hot and humid conditions in Bangladesh 1500 IU vitamin A is on the borderline of sufficiency. Supplementation of the feed with vitamin A along with vaccination against ND and IBD will certainly improve the performance of Sonali chickens. The optimum dietary levels of vitamin A, however, have to be determined under relevant production conditions in Bangladesh with local poultry breeds.
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