Effect of dietary supplementation of selenium and levothyroxine on performance, carcass characteristics, and thyroid hormone concentration in broiler chickens fed diets containing rapeseed meal

Soybean meal (SBM) is the main protein source in diets for poultry due to its high protein and excellent amino acid quality and profile, in addition to its high availability of amino acids. However, in some countries, other protein sources are available at lower prices and can be included in least-cost formulations instead of SBM. Rapeseed meal (RM) is the second most important oil-producing crop after soybean. Recently, there has been increasing interest in rapeseed production in countries around the world including Iran. Although RM has a good amino acid profile, its crude protein content (36% to 39%) is lower than that of SBM (44% to 48%). Despite higher levels of methionine and cysteine in RM, its lysine content is lower than SBM. The use of RM has been limited by its low available protein and energy contents relative to SBM. The main factor affecting the levels of available crude protein and energy content is the fibrous hull of RM. RM also contains non-starch polysaccharides and glucosinolates. Excessive levels of RM and thus high dietary glucosinolate content could lead to abnormalities in thyroid function activities and consequently a retardation in growth performance. Therefore, the objectives of the current study were to determine the effects of dietary selenium (Se) and levothyroxine (LT) supplementation on performance, carcass characteristics, and thyroid hormone concentration in broiler chickens fed diets containing RM.
This experiment was performed to evaluate the effect of dietary Se and LT supplementation on performance, carcass characteristics, weight of internal organs, and thyroid hormone concentration in broiler chickens of commercial strain Ross 308. The experimental treatments were: 1) diet based on SBM and corn grain (positive control), 2) diet with 10% RM (negative control 1, NG1), 3) diet with 20% RM (negative control 2, NG2), 4) NG1+ 0.3 mg Se/kg diet, 5) NG2+ 0.3 mg Se/kg diet, 6) NG1+ 80 µg LT/kg diet, 7) NG2+ 80 µg LT/kg diet, 8) NG1+ 0.3 mg Se and 80 µg LT/kg diet, and 9) NG2+ 0.3 mg Se and 80 µg LT/kg diet. Nine dietary treatments with four replications and 15 chickens in each replication were allocated to the experimental pens in a completely randomized design. During the experimental period (1 to 42 days), average feed intake, body weight gain, and feed conversion ratio were measured. At day 38, two birds from each pen were selected and blood was taken from their axillary vein for measuring thyroid hormones. At the end of the experiment (day 42), two birds from each replicate were selected, weighed, and slaughtered for carcass and internal organs weight determination.
The results showed that there were no significant differences between the treatments receiving RM and the positive control treatment in terms of growth performance. The percentage of abdominal fat and the relative weight of the internal organs, except the gizzard, were not significant between the dietary treatments. The treatment of 10% RM supplemented with Se and LT had the highest relative weight of the thymus. The treatment with 20% RM had the highest and the treatment with 10% RM + LT and 10% RM + LT and SE had the lowest relative weight of bursa Fabricius. The relative weight of the spleen was not affected by the experimental treatments. The treatment containing 20% RM showed the lowest carcass weight and the positive control treatment had the highest percentage of breast muscle. The use of RM at 20% caused a significant increase in thyroid weight compared to the positive control group (group without RM). The thyroid weight was reduced by SE and LT supplementation to diets containing RM. Rapeseed meal had no significant effect on T4 hormone levels, but treatments supplemented with SE increased and treatments supplemented with LT decreased T3 hormone levels. Also, the level of TSH hormone in the treatments receiving RM was lower than that in the treatments without RM. Chickens receiving RM with SE and LT had the highest levels of antioxidant enzymes of glutathione peroxidase and superoxide dismutase.
According to the results of the present experiment, supplementing RM with SE and LT decreased the weight of the thyroid gland and increased the level of antioxidant enzymes, which suggests the positive effects of the use of these compounds in the diet based on RM on the function of the thyroid gland.