The Effect of Dietary Supplemental Methionine Source and Betaine Replacement on the Growth Performance and Carcass Characteristics of Normal and Heat-Stressed Broiler Chickens

Heat stress is considered as one of the most important stressors accompanied by economic losses to the poultry industry. It causes reductions in weight gain and a series of metabolic disorders in broiler farms. Methionine is one of the most limiting amino acids, playing a crucial role in body protein synthesis, and therefore it would be beneficial to spare its function as a methyl donor. Broilers can utilize the isomers and analogs of methionine for protein synthesis, because of the unique enzymatic pathways to convert methionine isomers to L-methionine in the liver and kidney. Betaine is a common term for trimethylglycine, a substrate for betaine-homocysteine methyl transferase in the liver and kidney that acts as a methyl donor during methionine synthesis from homocysteine. The present study aimed to evaluate the effects of supplemental methionine sources and betaine replacement on growth performance and carcass characteristics of normal and heat-stressed broiler chickens.  This experiment was carried out in two adjoining poultry houses (n=1200, Ross 308). The experiment was designed in a 2 (Met sources)×2 (temperature)×3(Met levels)×2 (betaine levels) split-plot form, with two poultry houses (60 pens each) as the main plot and 12 different diets as the subplot, with 5 replicates of 10 birds each. Mash corn-soybean meal basal diets were prepared for starter (1–10 d), grower (11–24 d) and finisher (25–42 d) periods to meet 2014 Ross 308 nutrient recommendations, except for Met. Methionine levels in basal diets were adjusted at 30% lower than recommendation. They were increased to recommendation and/or 30% more than recommendation by supplementing DL-Met and/or L-Met. Betaine was substituted with 30% of supplemental DL-Met and/or L-Met. The betaine levels were calculated according to a molecular weight basis. Betaine contains about 3.75 times the methyl groups compared with Met. The temperature of both houses was set to thermal comfort temperature until 10 d of age. Then, the temperature of one house was gradually decreased by approximately 3°C/week until reached to the basal temperature (23°C) at d 28 and remained constant thereafter. In the other house, the temperature was gradually increased to 32°C between 0800 and 0930 and this high temperature was maintained for 6 h (until 1530). After that, the temperature was gradually decreased to the basal level by 1700. Body weight gain (WG) and feed consumption (FI) were recorded periodically, and feed conversion ratio (FCR) was calculated for each period by dividing feed intake by weight gain taking into account the mortality weights. Feed intake in broilers fed diet with 30% higher methionine was significantly lower than the other two groups. Body weight gain was higher in birds fed diets with recommended or 30% higher than recommended methionine compare to those fed diets with 30% less than recommended. It is tendentiously due to the inciting effect of Met on growth by means of growth factors and its influence on protein synthesis and breakdown. FCR in diet of 30% less than recommended methionine and containing L-methionine was significantly improved in comparison to diet containing DL-methionine in the same level. Chicks required 138 or 141 units of DL-methionine to achieve the same daily weight gain and G: F of birds receiving 100 units of L-methionine. Also, a number of studies have shown that the effectiveness of DL-methionine is similar to that of L-methionine in chicks. In this study, replacing of betaine with 30% of supplemental methionine, showed no significant differences on performance parameters, which implies the sparing effect of betaine for methionine. It appears that methionine and betaine supplementation to slightly methionine deficient broiler diets could result in an equivalent growth response and that methionine could be slightly spared by betaine. The production efficiency factor improved by elevated levels of DL-methionine, but no difference was found between highest level of methionine and its standard level. Carcass yield, breast yield and tights yield had significant increase in birds fed diets with recommended or 30% less than recommended methionine. This may be due to increased muscle protein deposition induced by methionine. Birds fed diets with recommended or 30% less than recommended methionine had the lowest and highest percentage of abdominal fat, respectively. The improvement in carcass lean percentage may be attributed to a higher availability of recommended and cystine for protein deposition. This is because an enhanced utilization of dietary amino acids for protein synthesis may result in fewer amino acids available for deamination and eventual synthesis of adipose tissue. In the present study, replacing betaine with 30% methionine showed similar responses to methionine. Changes in hormone levels and growth factors involved in the regulation of fat synthesis and degradation, as well as lower activities of lipogenic enzymes, have been observed following dietary betaine supplementation. Heat stress reduced performance and carcass yield and increased mortality and abdominal fat content. HS disturbs the intestinal flora balance and thereby diminishes nutrient digestibility and absorption. These results indicated that replacing 30% of methionine with betaine resulted same result in broiler performance. The use of methionine below the recommended level, reduces performance in broiler chickens. L-methionine appears to be more effective methionine source in improving the FCR than DL-methionine.